Differential pressure type flowmeter and differential pressure type flow controller Number:7,367,241 from the United States Patent and Trademark Office (PTO) owispatent

Title: Differential pressure type flowmeter and differential pressure type flow controller

Abstract: A differential pressure type flowmeter comprises an orifice, a detector to detect a fluid pressure P.sub.1 on the upstream side of an orifice, a detector to detect a fluid pressure P.sub.2 on the downstream side of an orifice, a detector to detect a fluid temperature T on the upstream side of an orifice, and a control computation circuit to compute a fluid's flow rate Q passing through an orifice by using the pressure P.sub.1, where P.sub.2 and temperature T detected with the aforementioned detectors, and the aforementioned fluid's flow rate Q is computed with the equation Q=C.sub.1.cndot.P.sub.1/ T.cndot.((P.sub.2/P.sub.1).sup.m-(P.sub.2/P.sub.1).sup.m).sup.1/2 (where C.sub.1 is a proportional constant, and m and n are constants).

Patent Number: 7,367,241 Issued on 05/06/2008 to Ohmi,   et al.

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Inventors:	Ohmi; Tadahiro (Komegahukuro 2-chome, Aoba-ku, Sendai-shi, Miyagi 9800813, JP), Sugiyama; Kazuhiko (Yamanashi, JP), Uno; Tomio (Osaka, JP), Ikeda; Nobukazu (Osaka, JP), Nishino; Kouji (Osaka, JP), Nakamura; Osamu (Osaka, JP), Dohi; Ryousuke (Osaka, JP), Matsumoto; Atsushi (Osaka, JP)
Assignee:	Fujikin Incorporated (Osaka-shi, JP) Ohmi; Tadahiro (Sendai-shi, JP)
Appl. No.:	10/563,226
Filed:	June 18, 2004
PCT Filed:	June 18, 2004
PCT No.:	PCT/JP2004/008596
371(c)(1),(2),(4) Date:	June 09, 2006
PCT Pub. No.:	WO2005/003694
PCT Pub. Date:	June 18, 2004

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Foreign Application Priority Data

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Jul 03, 2003 [JP]			2003-190988

Current U.S. Class:	73/861.52
Current International Class:	G01F 1/37 (20060101)
Field of Search:	73/861.52

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References Cited [Referenced By]

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U.S. Patent Documents

2549622	April 1951	Nash et al.
3701280	October 1972	Stroman
4542993	September 1985	Mims et al.
4576036	March 1986	Huang et al.

Foreign Patent Documents

	53-88757		Aug., 1978		JP
	59-19365		May., 1984		JP
	59-19366		May., 1984		JP
	59-52450		Dec., 1984		JP
	61-202120		Sep., 1986		JP
	6-201417		Jul., 1994		JP
	10-055218		Feb., 1998		JP
	2001-201414		Jul., 2001		JP
	2003-195948		Jul., 2003		JP

Primary Examiner: Patel; Harshad
Attorney, Agent or Firm: Griffin & Szipl, P.C.

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Claims

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The invention claimed is:

1. A differential pressure type flowmeter characterized in that flow rate measurements are performed with high accuracy over a wide flow rate range by combining a first differential pressure type flowmeter for measuring a flow rate range of 100%-10% of the maximum flow rate range and a second differential pressure type flowmeter for measuring a flow rate range of 10%-1% of the maximum flow rate range, and by switching fluid to be measured in accordance with the aforementioned flow rate ranges using a switching valve, to supply fluid to the first and second differential pressure type flowmeters, wherein one of the first and second differential pressure type flowmeters is a differential pressure type flowmeter comprising: an orifice; a detector disposed to detect fluid pressure P1 on an upstream side of the orifice; a detector disposed to detect fluid pressure P2 on a downstream side of the orifice; a detector disposed to detect fluid temperature T on the upstream side of the orifice; and a control computation circuit that computes a fluid's flow rate passing through the orifice with the pressure P1, wherein pressure P2 and temperature T are detected by the aforementioned detectors, and the fluid's flow rate Q is computed using equation Q=C1.cndot.P1 T.cndot.((P2/P1).sup.m-(P2/P1).sup.m).sup.1/2, where C1 is a proportional constant, and m and n are constant.

2. A differential pressure type flowmeter as claimed in claim 1, wherein the switching valve comprises a No.1 switching valve including a first driving cylinder and a No.2 switching valve including a second driving cylinder, wherein either one of the No.1 switching valve and the No.2 switching valve is a normal/close type valve and the other one is a normal/open type valve, and operating fluid is supplied from one control electromagnetic valve to the first driving cylinder and the second driving cylinder.

3. A differential pressure type flowmeter as claimed in claim 2, wherein the detector disposed to detect pressure on the upstream side of the orifice, the pressure detector disposed to detect pressure on the downstream side of the orifice, and the detector disposed to detect temperature on the upstream side of the orifice, are made sharable by both the first and second differential pressure type flowmeters.

4. A differential pressure type flowmeter comprising: an orifice; a detector disposed to detect fluid pressure P1 on an upstream side of the orifice; a detector disposed to detect fluid pressure P2 on a downstream side of the orifice; a detector disposed to detect fluid temperature T on the upstream side of the orifice; and a control computation circuit that computes a fluid's flow rate Q passing through the orifice with the pressure P1 and pressure P2 and temperature T detected by the aforementioned detectors, wherein the control computation circuit is installed with a flow rate computation circuit employing an equation Q=C1.cndot.P1 T.cndot.((P2/P1).sup.m-(P2/P1).sup.m).sup.1/2 where C1 is a proportional constant, and m and n are constant, to compute fluid's flow rate Q, and further comprising a correction data memory circuit wherein changes in the pressure P2 on the downstream side of the orifice are obtained by actual measurements by the detector disposed to detect fluid pressure P2 on the downstream side of the orifice and flow rate errors of the fluid's flow rate are storable in the correction data memory circuit, and a flow rate correction computation circuit corrects computed fluid's flow rate Q with correction data from the correction data memory circuit, thus the computed fluid's flow rate Q is corrected depending on changes of pressure P2 on the downstream side of the orifice to output a corrected flow rate value.

5. A differential pressure type flowmeter as claimed in claim 4, wherein said control computation circuit further comprises a pressure ratio computation circuit that computes a ratio of the fluid pressure P1 on the upstream side of the orifice and the fluid pressure P2 on the downstream side of the orifice; a critical condition judgment circuit that judges a state of fluid by comparing the computed pressure ratio and a fluid's critical pressure ratio; and a No.2 flow rate computation circuit that computes fluid's flow rate Q by using equation Q=KP1, where K is a proportional constant, when fluid is under critical conditions, thus outputting the fluid's flow rate Q computed with the No.2 flow rate computation circuit when fluid is under critical conditions, and also outputting a fluid's flow rate value Q' corrected using the flow rate correction computation circuit when fluid is under non-critical conditions.

6. A differential pressure type flowmeter characterized in that flow rate measurements are performed with high accuracy over a wide flow rate range by combining a first differential pressure type flowmeter for measuring a flow rate range of 100%-10% of the maximum flow rate range and a second differential pressure type flowmeter for measuring a flow rate range of 10%-1% of the maximum flow rate range, and by switching a fluid to be measured in accordance with the aforementioned flow rate ranges using a switching valve, to supply the fluid to the first and second differential pressure type flowmeters, wherein one or both of the first and second differential pressure type flowmeters is a differential pressure type flowmeter according to claim 5.

7. A differential pressure type flowmeter characterized in that flow rate measurements are performed with high accuracy over a wide flow rate range by combining a first differential pressure type flowmeter for measuring a flow rate range of 100%-10% of the maximum flow rate range and a second differential pressure type flowmeter for measuring a flow rate range of 10%-1% of the maximum flow rate range, and by switching fluid to be measured in accordance with the aforementioned flow rate ranges using a switching valve, to supply fluid to the first and second differential pressure type flowmeters, wherein one or both of the first and second differential pressure type flowmeters is a differential pressure type flow meter according to claim 4.

8. A differential pressure type flowmeter comprising (a) a valve body comprising i. a fluid inlet; ii. a fluid outlet; iii. a first mounting hole, for a No.1 switching valve, formed in the valve body; iv. a second mounting hole, for a No.2 switching valve 11, formed in the valve body; v. a third mounting hole, for a fluid pressure detector disposed on an upstream side of an orifice, formed in the valve body; vi. a fourth mounting hole for a fluid pressure detector disposed on a downstream side of the orifice, formed in the valve body; vii. a fifth mounting hole, for a fluid temperature detector disposed on the upstream side of the orifice, formed in the valve body; viii. first, second and third fluid passages disposed to pass fluid from the fluid inlet through the valve body, wherein undersides of the first mounting hole for the No.1 switching valve 10, the third mounting hole for the fluid pressure detector disposed on the upstream side of the orifice and the second mounting hole for the No.2 switching valve are made in an interior of the valve body 12; ix. a fourth fluid passage disposed to communicate with the underside of the first mounting hole for the No.1 switching valve and the underside of the second mounting hole for the No.2 switching valve; x. a fifth fluid passage disposed to communicate with the underside of the second mounting hole for the No.2 switching valve and the underside of the fourth mounting hole for the fluid pressure detector disposed on the downstream side of the orifice; and xi. a sixth fluid passage disposed to communicate with the underside of the fourth mounting hole for the pressure detector disposed on the downstream side of the orifice and the fluid outlet; (b) the fluid pressure detector disposed on the upstream side of the orifice and fixed to the third mounting hole; (c) the fluid pressure detector disposed on the downstream side of the orifice and fixed to the fourth mounting hole; (d) the fluid temperature detector disposed on the upstream side of the orifice and fixed to the fifth mounting hole; (e) the No.1 switching valve fixed to the first mounting hole, wherein the No.1 switching valve conducts opening and closing between the third fluid passage and the fourth fluid passage; (f) the No.2 switching valve fixed to the second mounting hole, wherein the No.2 switching valve conducts opening and closing between the second fluid passage and the fifth fluid passage, wherein the orifice comprises an orifice for small flow quantity installed halfway on the fourth fluid passage and an orifice for large flow quantity installed on the first fluid passage or on the second fluid passage, and a control computation circuit computes fluid's flow rate Q passing through the orifice for small flow quantity and the orifice for large flow quantity depending on pressure P1, pressure P2 and temperature T detected by the aforementioned fluid pressure detectors and the temperature detector 4, respectively using equation Q-C1.cndot.P1/ T.cndot.((Pb 2/P1).sup.m-(P2/P1).sup.n).sup.1/2 where C1 is a proportional constant, and m and n are constants, so that when making it possible a flow rate measured by the differential pressure type flowmeter is in a large flow quantity range the No.1 switching valve is closed and the No.2 switching valve is opened, and when a flow rate measured by the differential pressure type flowmeter is in a small quantity range the No.1 switching valve is opened and the No.2 switching valve is closed.

9. A differential pressure type flowmeter as claimed in claim 8, wherein when the differential pressure type flowmeter measures flow rate range up to 100%-10% of the maximum flow rate then the No.1 switching valve 10 is closed and the No.2 switching valve is opened, and when the differential pressure type flowmeter measures a flow rate up to 10%-1% of the maximum flow rate then the No.1 switching valve is opened and the No.2 switching valve is closed.

10. A differential pressure type flowmeter as claimed in claim 8, wherein either one of the No.1 switching valve and the No.2 switching valve is a normal/close type valve comprising a first driving cylinder and the other one is a normal/open type valve comprising a second driving cylinder, and operating fluid is supplied from one control electromagnetic valve to the first driving cylinder and the second driving cylinder.

11. A differential pressure type flow controller comprising: a control valve part equipped with a valve driving part; an orifice installed on a downstream side of the control valve part; a detector disposed to detect fluid pressure P1 on an upstream side of the orifice; a detector disposed to detect fluid pressure P2 on a downstream side of the orifice; a detector disposed to detect fluid temperature T on the upstream side of the orifice; and a control computation circuit equipped with a flow rate comparison circuit that computes fluid's flow rate Q using pressure P1, pressure P2 and temperature T detected by the aforementioned detectors, and a difference between the computed flow rate Q and a set flow rate Qs is computed by the control computation circuit, and the control computation circuit is equipped with a flow rate computation circuit that computes fluid's flow rate Q using equation Q=C1.cndot.P T.cndot.((P2/P1).sup.m.cndot.(P2/P1).sup.n).sup.1/2, where C1 is a proportional constant, and m and n are constants, and further comprising a correction data memory circuit wherein changes of the pressure P2 on the downstream side of the orifice are obtained by actual measurements by the detector disposed to detect pressure P2 on the downstream side of the orifice and flow rate errors of the fluid's flow rate Q are storable in the correction data memory circuit, and the flow rate correction computation circuit corrects the computed fluid's flow rate Q using correction data from the correction data memory circuit, thus the fluid's flow rate Q is corrected depending on changes of pressure P2 on the downstream side of the orifice, and the corrected flow rate value Q' is inputted to the flow rate comparison circuit to compute a difference of flow rates .DELTA.Q=Q'-Qs.

12. A differential pressure type flow controller as claimed in claim 11, wherein the control computation circuit further comprises a pressure ratio computation circuit that computes a pressure ratio of fluid pressure P1 on the upstream side of the orifice and fluid pressure P2 on the downstream side of the orifice; a critical conditions judgment circuit that judges a state of fluid by comparing the computed pressure ratio and a fluid's critical pressure ratio; and a No.2 flow rate computation circuit that computes fluid's flow rate Q using equation Q=KP1, where K is a proportional constant, when fluid is under critical conditions, and then the fluid's flow rate Q computed by the No.2 fluid computation circuit when the fluid is under critical conditions and a fluid's corrected flow rate value Q' from the flow rate correction computation circuit when the fluid is under non-critical conditions are inputted respectively to the flow rate computation circuit.

13. A differential pressure type flowmeter characterized in that flow rate measurements are performed with high accuracy over a wide flow rate range by combining a first differential pressure type flowmeter for measuring a flow rate range of 100%-10% of the maximum flow rate range and a second differential pressure type flowmeter for measuring a flow rate range of 10%-1% of the maximum flow rate range, and by switching fluid to be measured in accordance with the aforementioned flow rate ranges using a switching valve, to supply fluid to the first and second differential pressure type flowmeters, wherein each of the first and second differential pressure type flowmeter comprises: an orifice; a detector disposed to detect fluid pressure P1 on an upstream side of the orifice; a detector disposed to detect fluid pressure P2 on a downstream side of the orifice; a detector disposed to detect fluid temperature T on the upstream side of the orifice; and a control computation circuit that computes a fluid's flow rate by using pressure P1, pressure P2, and temperature T detected by the aforementioned detectors, and the fluid's flow rate Q is computed by the control computation circuit using equation Q=C1.cndot.P1 T.cndot.((P2/P1).sup.m-(P2/P1).sup.m).sup.1/2 where C1 is a proportional constant, and m and n are constants.

14. A differential pressure type flowmeter as claimed in claim 13, wherein the detector disposed to detect pressure on the upstream side of the orifice, the detector disposed to detect pressure on the downstream side of the orifice, and the detector disposed to detect a temperature on the upstream side of the orifice, are made sharable by both the first and the second differential pressure type flowmeters.

15. A differential pressure type flowmeter characterized in that flow rate measurements are performed with high accuracy over a wide flow rate range by combining a first differential pressure type flow meter for measuring a flow rate range of 100%-10% of the maximum flow rate range and a second differential pressure type flowmeter for measuring a flow rate range of 10%-1% of the maximum flow rate range, and by switching fluid to be measured in accordance with the aforementioned flow rate ranges using a switching valve, to supply fluid to the first and second differential pressure type flowmeters, wherein each of the first and second differential pressure type flowmeters is a differential pressure type flowmeter comprising: an orifice; a detector disposed to detect fluid pressure P1 on an upstream side of the orifice; a detector disposed to detect fluid pressure P2 on a downstream side of the orifice; a detector disposed to detect fluid temperature T on the upstream side of the orifice; and a control computation circuit that computes a fluid's flow rate using pressure P1, pressure P2, and temperature T detected by the aforementioned detectors, and the control computation circuit is equipped with a flow rate computation circuit that computes fluid's flow rate Q using equation Q=C1.cndot.P1 T.cndot.((P2/P1).sup.m-(P2/P1).sup.m).sup.1/2 where C1 is a proportional constant, and m and n are constants, and further comprising a correction data memory circuit wherein changes of pressure P2 on the downstream side of the orifice are obtained by actual measurement by the detector disposed to detect fluid pressure P2 on the downstream side of the orifice, and flow rate errors of the fluid's flow rate are storable in the correction data memory circuit, and a flow rate correction computation circuit corrects the computed fluid's flow rate Q using correction data from the correction data memory circuit, thus computed fluid's flow rate Q is corrected depending on changes of pressure P2 on the downstream side of the orifice to output a corrected flow rate Q'.

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Description

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This is a National Phase Application in the United States of International Patent Application No. PCT/JP2004/008596 filed Jun. 18, 2004, which claims priority on Japanese Patent Application No. 2003-190988, filed Jul. 3, 2003. The entire disclosures of the above patent applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention is concerned with improvements in or relating to a differential pressure type flowmeter and differential pressure type flow controller (together hereinafter called a differential pressure type flowmeter and the like) employed for semiconductor manufacturing facilities, at chemical plants, food-products processing plants and the like. The invention may be manufactured at a low cost and with a structural simplicity, and used in a state of so-called inline, and at the same time, make it possible that a flow rate of a fluid either under of criticality or non-criticality is measured or controlled with accuracy and in real time even in a small flow quantity range under a vacuum.

BACKGROUND OF THE INVENTION

Heretofore, for semiconductor manufacturing facilities and at chemical plants, a differential pressure flowmeter and the like such as a mass flow type flowmeter (a thermal type mass flow meter) and the like and a buildup type flowmeter and the like have been widely used to measure or control a flow rate of process gases, raw material gases and the like.

However, with a thermal type mass flowmeter and the like, there have been seen a number of difficulties such as a low responsivity, a poor measuring accuracy in a small flow quantity range, a large number of operational problems, a limited variety of gases subject to control, and being easily influenced by pressure changes and the like.

Similarly, with a buildup type flowmeter and the like, there have been seen some difficulties such as being difficult in measuring or controlling a flow rate in real time, not being able to use in a state of inline, an existence of restraints of a pressure of gases subject to control, a separate line required for measurements and the like.

On the other hand, a differential pressure type flowmeter and the like for which an orifice and a manometer are employed demonstrates excellent effects such as having nearly no restraints of the type of gases subject to control, being usable in a state of incline, and also measuring and controlling a flow rate being able to be performed in real time.

However, this type of a differential pressure type flowmeter and the like uses an equation for a flow rate computation derived from Bernoulli's theorem with the assumption that the fluid is non-compressible, and then the flow rate of the fluid is computed by giving some corrections to it. Therefore, if large pressure changes of the fluid arise (that is, when approximations that the fluid is non-compressible break), a substantial drop in accuracy of measuring and controlling a flow rate cannot be avoided, thus resulting in a failure of accurate flow rate measurements and controls.

To solve these difficulties with the aforementioned differential pressure type flowmeter and the like, a pressure type flowmeter and the like has been developed and disclosed (TOKU-KAI-HEI No. 10-55218 and others) wherewith critical conditions of a fluid passing through an orifice, that is, a pressure P.sub.1 on the upstream side of an orifice and a pressure P.sub.2 on the downstream side of an orifice being forcibly set to make the velocity of a fluid to be the velocity of sound, and the flow rate of a liquid Q is computed by the theoretical equation Q=KP.sub.1 under the critical conditions.

However, even with the pressure type flowmeter and the like, non-critical conditions occur when a fluid is in a small flow quantity range (that is, when a pressure P.sub.1 on the upstream side of an orifice and a pressure P.sub.2 on the downstream side to an orifice are in a state of being close), thus resulting in large errors in a flow rate measurement value Q or a flow rate control value Q.

Namely, with a conventional differential pressure flowmeter (or a pressure type flowmeter) and the like, it is so made that an equation for a flow rate computation derived from Bernoulli's theorem with the assumption that a fluid is non-compressible is used, and under non-critical conditions before a fluid reaches the velocity of sound (a non-sound velocity range), a flow rate on the downstream side is determined by the equation Qc=SC(P.sub.2(P.sub.1-P.sub.2)).sup.1/2/T.sup.1/2, where under critical conditions after having reached the velocity of sound (a sound velocity range), a flow rate Q is computed by an equation Qc=SCP.sub.1/T.sup.1/2 (where T is an absolute temperature of a fluid passing through an orifice, S is a cross-sectional area of an orifice and C is a proportional factor).

Critical conditions for the velocity of a fluid to reach the velocity of sound is given by a critical value r.sub.c of a pressure ratio P.sub.2/P.sub.1. The critical value r.sub.c is determined by the equation P.sub.2/P.sub.1=r.sub.c=(2//(n+1)).sup.n/(n-1) using a specific heat ratio n of a gas.

Furthermore, a specific heat ratio n is given by the equation n=Cp/Cv where Cp is a constant pressure specific heat and Cv is a constant volume specific heat. With biatom-molecular gases, n is n=7/5=1.4, and r.sub.c is R.sub.c=0.53, while with non-linear type triatom-molecular gases, n is n=8/6=1.33, and r.sub.c is R.sub.c=0.54.

To solve problems with the aforementioned conventional differential pressure type flowmeter (or a pressure type flowmeter), a flow rate value computed with the previous theoretical flow rate equation derived from the assumption that a fluid to be used under non-critical conditions is non-compressible is compared with the actually measured flow rate value, to derive an empirical flow rate equation having a plurality of parameters Qc'=SC/T.sup.1/2P.sub.2.sup.m(P.sub.1-P.sub.2).sup.n=KP.sub.2.sup.m(P.sub- .1-P.sub.2).sup.n from a previous theoretical flow rate equation Qc=SC/T.sup.1/2(P.sub.2(P.sub.1-P.sub.2)).sup.1/2, and to determine the aforementioned parameters m and n to make a flow rate value computed by the empirical flow rate equation Qc' equal to a measured value, thus an empirical flow rate equation Qc' that suitably matches with the fluid of compressibility being introduced by inventors of the present invention, and disclosed in TOKU-GAN No. 2001-399433.

With the aforementioned empirical flow rate equation Qc', a proportional constant K is given by SC/T.sup.1/2 and computed from conditions of substance and absolute temperature T. P.sub.1 designates a pressure on the upstream side of an orifice and P.sub.2 a pressure on the downstream side of an orifice. kPaA (kilo Pascal Absolute pressure) is the unit. Further, in the measured flow rate range of 10-30 sccm (a unit of a flow rate in a normal state), it has been found that parameters m and n are m=0.47152 and n=0.59492 respectively.

The values of the aforementioned 2 parameters m and n have a dependence on the range of a flow rate to be measured and the type of a gas. The aforementioned values m=0.4715 and n=0.59492 are values that hold true when the flow rate is in the range of 10-30 sccm. So, m and n don't hold true when the range of a flow rate is 10-100 sccm or 100-1000 sccm, and accordingly they deviate from these values.

FIG. 14 is a block diagram of an improved pressure flow controller for which the aforementioned empirical flow rate equation Qc'. This was previously disclosed by inventors of the present invention in the TOKU-GAN No. 2001-399433. The controller in the FIG. 14 is constituted as a flow controller. However, it is easily understood that it can be turned to be a differential pressure type flowmeter by eliminating a control valve 21, a valve driving part 22, and a flow rate comparison part 23e.

Referring to FIG. 14, 20 designates an orifice, 21 designates a control valve, 22 designates a valve driving part, 23 designates a control circuit, 23a designates a pressure ratio computation part, 23b designates a pressure ration computation part, 23c designates a flow rate computation part, 23d designates a flow rate computation part, 23e designates a flow rate comparison part, P.sub.1 designates a fluid pressure detector on the upstream side of an orifice, P.sub.2 designates a fluid pressure detector on the downstream side of an orifice, T designates a fluid temperature detector, Qs designates a flow rate setting value signal, .DELTA.Q designates a flow rate difference signal, and Qc' designates a flow rate computation value.

With the controller, firstly a pressure ratio P.sub.2/P.sub.1 is computed with the detected upstream side pressure P.sub.1 and downstream side pressure P.sub.2 (23a), a judgment is made continually to find if the fluid is under critical conditions or non-critical conditions (23b), and the flow rate is computed with a flow rate equation Qc=KP when under critical conditions (23c), while the flow rate is computed with an empirical flow rate equation Qc'=KP.sub.2.sup.m(P.sub.1-P.sub.2).sup.n when under non-critical conditions.

As stated above, the value of critically rc is given by an equation (2/(n+1)).sup.n/(n-1), (where n is the specific heat ratio of a gas). With bi-atom molecular gases, rc is rc=0.53 and with non-linear tri-atom molecular gases, rc is rc=0.54. Therefore, rc is written as rc=approx. 0.5.

A flow rate difference .DELTA.Q between a set flow rate Qs and a computed flow rate Qc is computed with a flow rate comparison part 23e to operate a valve driving part 22 to control valve 21 so that the flow rate difference .DELTA.Q reaches zero. However, when it is used as a flow meter, as stated above, a flow rate comparison part 23e, a control valve 21 and a valve driving part 22 can be eliminated.

Curve A in FIG. 15 shows flow rate measurements or flow rate control characteristics with an improved pressure type flowmeter and the like, while Curve B shows flow rate measurements or flow rate control characteristics with an conventional pressure type flowmeter and the like when an equation Qc=KP.sub.1 is used under non-critical conditions. As apparent from FIG. 15, with the improved pressure type flowmeter and the like, a flow rate equation Qc=KP.sub.1 is used when under critical conditions, while an empirical flow rate equation Qc'=KP.sub.2.sup.m(P.sub.1-P.sub.2).sup.n is used when under non-critical conditions, thus an accurate flow rate Q in proportion to the set flow rate being able to be computed, linearity to a set % of a flow rate being held as shown by Curve A in FIG. 15, and the comparatively accurate flow rate measurement and control being ensured even in the range of a small flow quantity.

Patent Literature 1: TOKU-KOU-SHO No. 59-19365 Public Bulletin

Patent Literature 2: TOKU-KOU-SHO No. 59-19366 Public Bulletin

Patent Literature 3: TOKU-KAI-HEI No. 10-55218 Public Bulletin

DISCLOSURE OF THE INVENTION

Object of the Invention

With the aforementioned pressure type flowmeter and the like shown in FIG. 14, flow rate measurements or control can be achieved with comparatively high accuracy if the flow rate is in a small flow quantity range up to approximately 10% of the maximum flow rate as shown by Curve A in FIG. 15, thus excellent practical effects being attained.

However, when a flow rate is in a small flow quantity range of less than approximately 10% of the maximum flow rate, problems that a practical flow rate measurements or flow rate control cannot be achieved with accuracy arise in reality.

Furthermore, with the improved pressure type flowmeter and the like, a measurement error (% SP or %FS) becomes comparatively larger to the reference set flow rate when a pressure P.sub.2 on the downstream side of an orifice becomes vacuum of less than approximately 200 Torr, thus resulting in occurrence of difficulties in practical use.

It is an object of the present invention to provide a differential pressure type flowmeter and the like at a low cost and simple in structure to solve the problems with an improved pressure type flowmeter and the like previously developed by inventors of the present invention, thus making it possible to achieve highly accurate flow rate measurements or control over a wide flow rate range from the maximum flow rate (100%) to approximately 1% of the maximum flow rate, and even when a pressure P.sub.2 on the downstream side of an orifice is in the vacuum and makes changes, by storing the error data obtained by actual measurements beforehand in the memory device, and then by correcting flow rate computation values in reference with the correction data.

Means to Achieve the Object

The present invention, according to a first embodiment, is fundamentally so constituted, with a differential pressure type flowmeter comprising an orifice, a detector to detect a fluid pressure P.sub.1 on the upstream side of an orifice, a detector to detect a fluid pressure P.sub.2 on the downstream side of an orifice, a detector to detect a fluid temperature T on the upstream side of an orifice, and a control computation circuit to compute a fluid's flow rate passing through an orifice with the pressure P.sub.1, pressure P.sub.2 and temperature T detected by the aforementioned detectors, the aforementioned fluid's flow rate Q is computed with an equation Q=C.sub.iP.sub.i/ T((P.sub.2/P.sub.1).sup.m-(P.sub.2/P.sub.1).sup.m).sup.1/2 (where C.sub.1 is a proportional constant, and m and n are constants).

The present invention, according to a second embodiment, is fundamentally so constituted, with a differential pressure type flowmeter comprising an orifice, a detector to detect a fluid pressure P.sub.1 on the upstream side of an orifice, a detector to detect a fluid pressure P.sub.2 on the downstream side of an orifice, a detector to detect a fluid temperature T on the upstream side of an orifice, and a control computation circuit to compute a fluid's flow rate Q passing through an orifice with the pressure P.sub.1 and pressure P.sub.2 and temperature T detected by the aforementioned detectors, the aforementioned control computation circuit is installed with a flow rate computation circuit wherewith a fluid's flow rate Q is computed with an equation Q=C.sub.1P.sub.1/ T((P.sub.2/P.sub.1).sup.m-(P.sub.2/P.sub.1).sup.n).sup.1/2 (where C.sub.1 is a proportional constant, and m and n are constants), a correction data memory circuit wherein changes in the pressure P.sub.2 on the downstream side of an orifice obtained by actual measurements beforehand and flow rate errors of the aforementioned fluid's flow rate Q are storable, and a flow rate correction computation circuit to correct the aforementioned computed fluid's flow rate Q with the correction data from the correction data memory circuit, thus the computed fluid's flow rate Q being corrected depending on changes of a pressure P.sub.2 on the downstream side of an orifice, to output a corrected flow rate value Q'.

The present invention, according to a third embodiment, further modifies the second embodiment so that, by installing, with a control computation circuit, a pressure ratio computation circuit to compute the ratio of a fluid pressure P.sub.1 on the upstream side of an orifice and a fluid pressure P.sub.2 on the downstream side of an orifice, a critical condition judgment circuit to judge a state of the fluid by comparing the aforementioned computed pressure ratio and a fluid's critical pressure ratio, and a No. 2 flow rate computation circuit to compute the fluid's flow rate Q by using the equation Q=KP.sub.1 (where K is a proportional constant) when the fluid is under critical conditions, thus outputting a fluid's flow rate Q computed with the aforementioned No.2 flow rate computation circuit when the fluids is under critical conditions, and also outputting a fluid's flow rate value Q' corrected with the flow rate correction computation circuit when the fluid is under non-critical conditions.

The present invention, according to a fourth embodiment, is fundamentally so constituted that flow rate measurements can be performed with high accuracy over the wide flow rate range by combining a differential pressure type flowmeter for measuring a flow rate range of 100%-10% of the maximum flow rate range and a differential pressure type flowmeter for measuring a flow rate range of 10%-1% of the maximum flow rate range and by switching a fluid to be measured in accordance with the aforementioned flow rate ranges using a switching valve, to supply the fluid to the aforementioned differential pressure type flowmeters.

The present invention, according to a fifth embodiment, further modifies the fourth embodiment so that both differential pressure type flowmeters are made to be differential pressure type flowmeters comprising an orifice, a detector to detect a fluid pressure P.sub.1 on the upstream side of an orifice, a detector to detect a fluid pressure P.sub.2 on the downstream side of an orifice, a detector to detect a fluid temperature T on the upstream side of an orifice, and a control computation circuit to compute a fluid's flow rate by using the pressure P.sub.1, pressure P.sub.2, and temperature T detected by the aforementioned detectors, and the aforementioned fluid's flow rate Q is computed with the equation Q=C.sub.1P.sub.1/ T((P.sub.2/P.sub.1).sup.m-(P.sub.2/P.sub.1).sup.n).sup.1/2 (where C.sub.1 is a proportional constant, and m and n are constants).

The present invention, according to a sixth embodiment, further modifies the fourth embodiment so so that both differential pressure type flowmeters are made to be differential pressure type flowmeters comprising an orifice, a detector to detect a fluid pressure P.sub.1 on the upstream side of an orifice, a detector to detect a fluid pressure P.sub.2 on the downstream side of an orifice, a detector to detect a fluid temperature T on the upstream side of an orifice, and a control computation circuit to compute a fluid's flow rate by using the pressure P.sub.1, pressure P.sub.2, and temperature T detected by the aforementioned detectors; and the aforementioned control computation circuit is equipped with a flow rate computation circuit wherewith a fluid's flow rate Q is computed with the equation Q=C.sub.1P.sub.1/ T((P.sub.2/P.sub.1).sup.m-(P.sub.2/P.sub.1).sup.n).sup.1/2 (where C.sub.1 is a proportional constant, and m and n are constants), a correction data memory circuit wherein changes of a pressure P.sub.2 on the downstream side of an orifice obtained by actual measurements beforehand and flow rate errors of the aforementioned fluid's flow rate Q is stored, and a flow rate correction computation circuit to correct the aforementioned computed fluid's flow rate Q with the correction data from the correction data memory circuit, thus computed fluid's flow rate Q being corrected depending on changes of a pressure P.sub.2 on the downstream side of an orifice to output a corrected flow rate value Q'.

The present invention, according to a seventh embodiment, is fundamentally so constituted by forming it with a valve body 12 provided with a fluid inlet a, a fluid outlet b, a mounting hole 17a for the No. 1 switching valve 10, a mounting hole 17b for the No.2 switching valve 11, a mounting hole 18a for a fluid pressure detector 2 on the upstream side of an orifice, a mounting hole 18b for a fluid pressure detector 3 on the downstream side of an orifice, a mounting hole for a fluid temperature detector 4 on the upstream side of an orifice, fluid passages 16a, 16b and 16e for directly passing through a fluid inlet a, the undersides of a mounting hole 17a for the No.1 switching valve 10, a mounting hole 18a for a fluid pressure detector 2 on the upstream side of an orifice and a mounting hole 17b for the No.2 switching valve 11 which are made in the interior of the aforementioned valve body 12, a fluid passage 16f for communication of the underside of a mounting hole 17a for the No.1 switching valve 10 and the underside of a mounting hole 17b for the No.2 switching valve 11, a fluid passage 16c for communication of the underside of a mounting hole 17b for the No.2 switching valve 11 and the underside of a mounting hole 18b for the fluid pressure detector 3 on the downstream side of an orifice, a fluid passage 16d for communication of the underside of a mounting hole 18b for the fluid pressure detector 3 on the downstream side of an orifice and a fluid outlet b, a fluid pressure detector 2 on the upstream side of an orifice and a fluid pressure detector 3 fixed to the aforementioned mounting holes 18a and 18b respectively, a fluid temperature detector 4 on the upstream side of an orifice, the No.1 switching valve 10 wherewith opening and closing are conducted between the aforementioned fluid passage 16e and fluid passage 16f, the No.2 switching valve 11 wherewith opening and closing are conducted between the aforementioned fluid passage 16b and fluid passage 16c, an orifice 1' for a small flow quantity installed halfway to the aforementioned fluid passage 16f, an orifice 1'' for a large flow quantity installed on the aforementioned fluid passage 16a or fluid passage 16b, and a control computation circuit to compute a fluid's flow rate Q passing through an orifice 1' for a small flow quantity and an orifice 1'' for a large flow quantity depending on the pressure P.sub.1, pressure P.sub.2 and temperature T detected by the aforementioned pressure detectors 2 and 3 and temperature detector 4 respectively by using the equation Q=C.sub.1P.sub.1/ T((P.sub.2/P.sub.1).sup.m-(P.sub.2/P.sub.1).sup.n).sup.1/2, thus making it possible to measure a flow rate in a large flow quantity range by closing the aforementioned No.1 switching valve 10 and opening the No.2 switching valve 1, while a flow rate in a small quantity range is measured by opening the aforementioned No.1 switching valve 10 and closing the No.2 switching valve 11.

The present invention, according to an eight embodiment, further modifies the seventh embodiment so that a flow rate range up to 100%-10% of the maximum flow rate is measured by closing the No.1 switching valve 10 and opening the No.2 switching valve 11, while a flow rate range up to 10%-1% of the maximum flow rate is measured by opening the No1 switching valve 10 and closing the No.2 switching valve 11.

The present invention, according to a ninth embodiment, further modifies the fourth or seventh embodiments so that either one of the No.1 switching valve 10 or No.2 switching valve 11 is made to be a normal/close type valve and the other a normal/open type valve, and a operating fluid is supplied from one control electromagnetic valve Mv to driving cylinders 10a and 10b of both switching valves.

The present invention, according to a tenth embodiment, further modifies the seventh or eighth embodiments so that a pressure detector 2 to detect a pressure on the upstream side of an orifice, a pressure detector 3 to detect a pressure on the down stream side of an orifice, and a temperature detector 4 to detect a temperature on the upstream side of an orifice are made sharable with both differential pressure type flowmeters.

The present invention, according to an eleventh embodiment, is characterized in that, with a differential pressure type flow controller comprising a control valve part equipped with a valve driving part, an orifice installed on the downstream side thereof, a detector to detect a fluid pressure P.sub.1 on the upstream side of an orifice, a detector to detect a fluid pressure P.sub.2 on the downstream side of an orifice, a detector to detect a fluid temperature T on the upstream side of an orifice, and a control computation circuit equipped with a flow rate comparison circuit wherewith a fluid's flow rate Q passing through an orifice is computed by using the pressure P.sub.1, pressure P.sub.2 and temperature T detected by the aforementioned detectors, and the different between a computed flow rate Q and a set flow rate Qs is computed, the aforementioned fluid's flow rate Q is computed by the equation Q=C.sub.1P.sub.1/ T((P.sub.2/P.sub.1).sup.m-(P.sub.2/P.sub.1).sup.n).sup.1/2 (where C is a proportional constant, and m and n are constants).

The present invention, according to a twelfth embodiment, is characterized in that, with a differential pressure type flow controller comprising a control valve part equipped with a valve driving part, an orifice installed on the downstream side thereof, a detector to detect a fluid pressure P.sub.1 on the upstream side of an orifice, a detector to detect a fluid pressure P.sub.2 on the downstream side of an orifice, a detector to detect a temperature T on the upstream side of an orifice, and a control computation circuit equipped with a flow rate comparison circuit wherewith the fluid's flow rate Q is computed by using the pressure P.sub.1, pressure P.sub.2 and temperature T detected by the aforementioned detectors, and the difference between a computed flow rate Q and set flow rate Qs is computed, the aforementioned control computation circuit is equipped with a flow rate computation circuit wherewith a fluid's flow rate Q is computed by the equation Q=C.sub.1P.sub.1/ T((P.sub.2/P.sub.1).sup.m-(P.sub.2/P.sub.1).sup.n).sup.1/2 (where C.sub.1 is a proportional constant, and m and n are constants), a correction data memory circuit wherein changes of the pressure P.sub.2 on the downstream side of an orifice obtained by actual measurements beforehand and flow rate errors of the aforementioned fluid's flow rate Q are stored, a flow rate correction computation circuit to correct the aforementioned computed fluid's flow rate Q with the correction data from the correction data memory circuit, thus a fluid's flow rate Q being corrected depending on the changes of a pressure P.sub.2 on the downstream side of an orifice, and the corrected flow rate value Q' being inputted to the aforementioned flow rate comparison circuit to compute the difference of flow rates .DELTA.Q=Q'-Qs

The present invention, according to a thirteenth embodiment, further modifies a twelfth embodiment so that, by installing with a control computation circuit a pressure ratio computation circuit to compute the ratio of a fluid pressure P.sub.1 on the upstream side of an orifice and a fluid pressure P.sub.2 on the downstream side of an orifice, a critical conditions judgment circuit to judge a state of a fluid by comparing the aforementioned computed pressure ratio and a fluid's critical pressure ratio, the No.2 flow rate computation circuit to compute a fluid's flow rate Q by using the equation Q=KP.sub.1 (where K is a proportional constant) when the fluid is under critical conditions, a fluid's flow rate Q computed by the aforementioned No.2 fluid computation circuit when the fluid is under critical conditions and a fluid's flow rate value Q' corrected from the flow rate correction computation circuit when the fluid is under non-critical conditions are inputted respectively to the aforementioned flow rate computation circuit.

Effects of the Invention

With the present invention, the structure of a differential pressure type flowmeter and the like is remarkably simplified, and it is so constituted that a flow rate computation is performed by using a novel empirical flow rate computation equation which makes it possible to obtain a computation flow rate value corresponding with the measured value with high accuracy, thus allowing the flowmeter and the like to be manufactured at low cost, and moreover, they take an inline form, and can be used without constraints of fitting positions, and a control flow rate is not influenced nearly at all by pressure changes, enabling highly accurate flow rate measurements or flow rate control in real time.

Also, with the present invention, it is so made that a control computation circuit is equipped with a correction data memory circuit for pressure changes, and a correction circuit for a computation flow rate, thus enabling easy correction even when pressure changes arise on the secondary side of an orifice. Therefore, highly accurate flow rate measurements or flow rate control can be achieved virtually without being influenced by pressure changes even a pressure P.sub.2 on the secondary side of an orifice is under a vacuum (a low pressure of less than 50 Torr).

Furthermore, with the present invention, it is so made that a differential pressure type flowmeter for a small flow quantity and a differential pressure type flowmeter for a large flow quantity are organically and integrally assembled. Therefore, highly accurate flow rate measurements with errors (% SP) of less than 1 (% SP) can be performed continuously over a wide flow rate range from the rated flow rate (100%) to a small flow quantity (1%) or approximately 1% of the rated flow rate, by both differential pressure type flowmeters being switched.

In addition, with the present invention, the control system can be further simplified by making the switching operation, for both differential pressure type flowmeter for a small quantity and differential pressure type flowmeter for a large quantity, automatic with a single-system control signal Sc.

As stated above, the present invention achieves excellent, practical effects that all types of gases are measured or controlled over the wide flow rate range with high accuracy even when a gas of less than 100 Torr is used although the differential pressure type flowmeter and the like are structured simply and at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a basic block diagram of a differential pressure flowmeter according to the first embodiment of the present invention.

FIG. 2 is a diagram to show error characteristics of a differential pressure type flowmeter in FIG. 1.

FIG. 3 is a diagram to show the relationships of "a flow rate, the secondary side pressure and error" in the event that the pipe resistance on the secondary side is changed at the time when a pressure P.sub.2 on the downstream side of an orifice is the vacuum.

FIG. 4 shows a measurement circuit utilizes to obtain data in FIG. 3.

FIG. 5 is a basic block diagram of a differential pressure flowmeter according to the second embodiment of the present invention.

FIG. 6 is a basic block diagram of a differential pressure flowmeter according to the third embodiment of the present invention.

FIG. 7 is a system diagram to show the whole configuration of a differential pressure type flowmeter according to the fourth embodiment of the present invention.

FIG. 8 is a cross-sectional schematic diagram of a major part of a differential pressure type flowmeter according to the fourth embodiment of the present invention.

FIG. 9 is an explanatory drawing of a switching operation system of a differential pressure type flowmeter for which a normal open type switching valve and a normal close type switching valve are employed according to the present invention.

FIG. 10 is a basic block diagram of the first embodiment of a differential pressure type flow controller according to the present invention.

FIG. 11 is a basic block diagram of the second embodiment of a differential pressure type flow controller according to the present invention.

FIG. 12 is a basic block diagram of the third embodiment of a differential pressure type flow controller according to the present invention.

FIG. 13 is a basic block diagram of the fourth embodiment of a differential pressure type flow controller according to the present invention.

FIG. 14 is a block diagram of an improved pressure type flow controller disclosed previously.

FIG. 15 is a diagram to show flow characteristics of an improved pressure type flow controller disclosed previously.

LIST OF REFERENCE CHARACTERS AND NUMERALS

Q designates Empirical flow rate computation equation

Q' designates Corrected flow rate

Qs designates Set flow rate

SF designates a Standard flow controller (a pressure type flow controller)

A designates a Differential pressure type flowmeter

V.sub.21-V.sub.23 designate a Control valve on the secondary side

VP designates a Vacuum pump

a designates a Gas inlet

b designates a Gas outlet

1 designates an Orifice

1' designates an Orifice for a small quantity

1'' designates an Orifice for a large quantity

2 designates an Absolute pressure type pressure detector on the upstream side of an orifice

3 designates an Absolute pressure type pressure detector on the downstream side of an orifice

4 designates Gas absolute temperature detector on the upstream side of an orifice

5 designates a Control computation circuit

5a designates a Flow rate computation circuit

5b designates a Correction data memory circuit

5c designates a Flow rate correction computation circuit

5e designates a Critical condition judgment circuit

5f designates a Second flow rate computation circuit for computing a flow rate under critical conditions

5g designates a Comparison circuit for a set flow rate and computed flow rate

5' designates a First control computation circuit

5'' designates a Second control computation circuit

6 designates a Flow rate output terminal

7 designates a Power input terminal

8 designates a Gas supply facility

9 designates a Gas use facility (a chamber)

10 designates a No.1 switching valve

10a designates a Driving cylinder

11 designates a No.2 switching valve

11a designates a Driving cylinder

12 designates a Body

12a designates a Gas inlet part element

12b designates a Gas outlet part element

12c designates a No.1 body element

12d designates a No.2 body element

13a-13b designate Seals

14a-14b designate Mounting bolts for pressure detectors

15a-15b designate Diaphragm valve mechanisms

16a-16f designate Passages

17a designates a Mounting hole for the No.1 switching valve

17b designates a Mounting hole for the No.2 switching valve

18a designates a Mounting hole for a pressure detector on the upstream side of an orifice

18b designates a Mounting hole for a pressure detector on the downstream side of an orifice

21 designates a Control valve

22 designates a Valve driving part

Mv designates a Control electromagnetic valve

Sc designates a Control signal

BEST MODE TO CARRY OUT THE INVENTION

The following embodiments of the present invention are described hereunder with reference to the drawings.

FIG. 1 is a basic block diagram of a differential pressure type flowmeter according to the first embodiment of the present invention. The differential pressure type flowmeter comprises an orifice 1, an absolute pressure type pressure detector 2 on the upstream side of an orifice, an absolute pressure type pressure detector 3 on the downstream side of an orifice, a gas absolute temperature detector 4 on the upstream side of an orifice, a control computation circuit 5, an output terminal 6, an input terminal 7, and the like. And, 8 designates a gas supply facility and 9 a gas use facility (a chamber).

With the differential pressure type flowmeter according to the present invention, a gas flow rate Q passing through an orifice 1 under differential pressure conditions (that is, under non-critical conditions) is computed by an empirical flow rate equation as the below-stated equation (1), and the computed value is outputted to the outside through the output terminal 6. Q=C.sub.1P.sub.1/T((P.sub.2/P.sub.1).sup.m-(P.sub.2/P.sub.1).sup.n).sup.1- /2 (1)

The aforementioned empirical flow rate equation Q is what is newly introduced by inventors of the present invention based on the following flow rate equation (2) based on the previously known continuous equation.

.delta..times..kappa..kappa..times..kappa..kappa..kappa. ##EQU00001##

With the equation (2), .delta. designates a gas density, .kappa. a specific ratio of a gas, P.sub.1 a pressure on the upstream side of an orifice, P.sub.2 a pressure on the downstream side of an orifice, T a gas temperature, R a gas constant, and S a cross-sectional area of an orifice. The equation (2) has been publicly known.

With the aforementioned equation (1) according to the present invention, Q designates a volume flow rate (SCCM) converted to a standard state, C a coefficient including a cross-sectional area S of an orifice 1, P.sub.1 an absolute pressure (Pa) on the upstream side of an orifice, P.sub.2 an absolute pressure (Pa) on the downstream side of an orifice, and T an absolute temperature (K) on the upstream side of an orifice.

m and n are constants determined by computing .kappa. of N.sub.2 gas .kappa.=1.40 with the equation (2). With a flowmeter having an orifice measuring 2.0 mm .phi. in diameter .phi. and a maximum flow rate of 2000 sccm, C.sub.1, m and n in the equation (1) become C.sub.1=2680, m=1.4286 and n=1.7143 respectively.

There is no need to say that the constants C.sub.1, m and n change depending on the measurable gas type. In the case of the N.sub.2 gas, it has been known as m=1.4286 and n=1.7143.

FIG. 2 is the measured values to show relationships of a set flow rate value (%), pressure P.sub.1 and P.sub.2 (Torr) and error (% SP) of a differential pressure type flowmeter (100% set value 2000 sccm) in FIG. 1. Flow rate errors are found to be of extremely minimal values (less than 1% SP) to a set flow rate value (%) if the set flow rate is around 10(%) (10% of the maximum flow rate=200 sccm) even when gas pressures P.sub.1 and P.sub.2 are under the vacuum of less than 50 Torr.

However, when a set flow rate value is less than 10(%), difficulties arise impractical use because a flow rate error becomes more than -1(% SP) when a set flow rate value is less than 10(%).

FIG. 3 is a diagram to show relationships of a pressure P.sub.2 (Torr) on the secondary side of an orifice, a set flow rate (%), errors (% SP) and piping conditions on the secondary side of a differential pressure type flowmeter according to the present invention, where 9a is in the event that a set flow rate (%) is 100 sccm, 9b 200 sccm, 9c 400 sccm, 9d 600 sccm, 9g 1200 sccm, 9j 1800 scm, and 9k 2000 sccm (100%) respectively, The maximum flow rate (100%) of a differential pressure type flowmeter employed herewith is 2000 sccm.

Among set flow rate values (%), as shown in FIG. 3, square marks show errors (% SP) in the event that a valve and the like are not installed on the pipe passage (4.35 mm .phi.100 mm) on the outlet side of a differential pressure type flowmeter, rhombus marks show errors (% SP) in the event that a control valve with a Cv value of 0.3 on the outlet side of a differential pressure type flowmeter is installed, and triangle marks show errors (% SP) in the event that a control valve with a Cv value of 0.2 is installed, and circle marks show errors (% SP) in the event that a control valve with a Cv value of 0.1 is installed respectively.

Namely, as apparent from FIG. 3, when pressure conditions in use are in the vacuum (less than 50 Torr), relationships of a pressure P.sub.2 and a flow rate Q change substantially depending on piping conditions on the secondary side (the downstream side of an orifice), thus resulting in changing errors (% SP).

Therefore, at the time of adjustments of a differential pressure type flowmeter, flow rate errors (% SP) are measured in advance for cases wherein the secondary side pipe resistance (a conductance is changed (with 4 conditions and at 11 points in the case of FIG. 3), and correction coefficients to cancel the errors are obtained in advance. Thus, by correcting flow rate values Q computed with the empirical flow rate equation (1) with the flow rate computation circuit 5 using the correction coefficients, highly accurate flow rate computation becomes possible even when a pressure P.sub.2 on the secondary side of a differential pressure type flowmeter changes under a vacuum.

FIG. 4 illustrates a measurement circuit to obtain error correction coefficients in FIG. 3, for which a pressure type flow controller is