Title: Differential permeometer
Abstract: Permeability of a porous, sheet-like sample is determined using a device that is designed to measure the pressure differential between a first stream of fluid applied across an entire thickness of a test sample and a second stream of fluid applied across an entire thickness of a reference sample. The flow rate for both the test fluid flow system and the reference fluid flow system is determined independently, by measuring a pressure drop throughout the flow system. Data obtained from pressure and flow rate for test and reference fluids are used to calculate percent change in permeability and/or actual permeability using Darcy's Law.
Patent Number: 6,843,106 Issued on 01/18/2005 to Swersey, et al.
| Inventors:
|
Swersey; Burt (Stephentown, NY);
Harvey; Marcie J. (Pearland, TX);
Kaplan; Elias (Redmond, WA);
Lamana; Jennifer (Troy, NY);
Howard; Stephen J. (Troy, NY);
Meloney; Dean (Troy, NY);
Weed; John P. (Port St. Lucie, FL);
Novak; Timothy (Troy, MI)
|
| Assignee:
|
Rensselaer Polytechnic Institute (Troy, NY)
|
| Appl. No.:
|
800872 |
| Filed:
|
March 7, 2001 |
| Current U.S. Class: |
73/38 |
| Intern'l Class: |
G01N 015/08 |
| Field of Search: |
73/38
|
References Cited [Referenced By]
U.S. Patent Documents
| 3618361 | Nov., 1971 | Stephens et al. | 73/38.
|
| 4191046 | Mar., 1980 | Baker et al. | 73/38.
|
| 4384474 | May., 1983 | Kowalski | 73/38.
|
| 4566326 | Jan., 1986 | Lowell | 73/865.
|
| 4856967 | Aug., 1989 | Jones | 417/342.
|
| 5088316 | Feb., 1992 | McKelvey et al. | 73/38.
|
| 5107696 | Apr., 1992 | Mayer et al. | 73/38.
|
| 5503001 | Apr., 1996 | Wong | 73/38.
|
| 5544520 | Aug., 1996 | Graf et al. | 73/38.
|
| 5906743 | May., 1999 | Cohen et al. | 210/502.
|
| 5968312 | Oct., 1999 | Sephton | 159/47.
|
| Foreign Patent Documents |
| 518250 | Dec., 1992 | EP | .
|
| 787958 | Dec., 1980 | SU.
| |
| WO 9428393 | Dec., 1994 | WO | .
|
Primary Examiner: Williams; Hezron
Assistant Examiner: Fitzgerald; John
Attorney, Agent or Firm: Hand; Francis C.
Carella, Byrne, Bain
Parent Case Text
This application claims the benefit of U.S. Provisional Application No.
60/187,931, now filed on Mar. 8, 2000.
This invention relates to a differential permeameter. More particularly,
this invention relates to a differential permeameter for the measurement
of fluid permeability through a porous, sheet-like sample.
Claims
What is claimed is:
1. A differential permeameter comprising;
a pair of flow systems, each said flow system being disposed for a flow of
fluid there through;
a reservoir connected in common to said flow systems to receive a flow of
fluid from each said system;
a fan for drawing fluid through said systems into said reservoir;
a pair of clamping devices, each said clamping device being disposed in a
respective flow system to hold a porous sheet-like material sample across
said respective system for a flow of fluid therethrough; and
a pair of orifice plates, each said plate being disposed in a respective
flow system between said clamping device and said reservoir to create a
measurable pressure drop in a fluid passing therethrough.
2. A differential permeameter as set forth in claim 1 wherein each said
plate is disposed in a respective flow system between said clamping device
and said reservoir therein.
3. A differential permeameter as set forth in claim 1 which further
comprises a pair of honeycomb structures, each said structure being
disposed in a respective one of said systems between said orifice plate
therein and said reservoir for passage of a fluid therethrough to effect a
laminar andf steady flow of fluid therethrough.
4. A differential permeameter as set forth in claim 1 further comprising a
dual motorized screw drive disposed between said systems and connected to
and between said pair of orifice plates for simultaneous movement thereof
to adjust an orifice size thereof.
5. A differential permeameter as set forth in claim 1 further comprising a
first pressure transducer positioned in one of said systems to measure a
pressure drop across a test sample in said one system, a second pressure
transducer positioned in said one system to measure a pressure drop across
said orifice plate therein, a third pressure transducer positioned in the
other of said systems to measure a pressure drop across said orifice plate
therein, a fourth pressure transducer for measuring a differential
pressure between said systems and a computer connected to each said
transducer to receive a signal therefrom indicative of the pressure
measured thereby and to calculate a differential permeability value of a
sample in said one system in dependence on said signals.
6. A method for determining the permeability of a test sample comprising
the steps of;
placing a sheet-like reference sample of known permeability in
communication with a reference fluid flow system;
placing a sheet-like test sample of unknown permeability in communication
with a test fluid flow system;
supplying a fluid flow stream across both test and reference samples;
adjusting the pressure drop of the fluid flow stream across one or both
samples to a fixed standard;
measuring the fluid flow through the test fluid flow system by measuring
the pressure drop across a flow device within the test fluid flow system;
measuring the fluid flow through the reference fluid flow system by
measuring the pressure drop across a flow device within the reference
fluid flow system;
measuring a pressure differential between the test fluid system and the
reference fluid system; and
calculating the permeability of the test sample by using the pressure
differential across the test fluid flow system, the known permeability of
the test sample, and the fluid flow through both the test fluid flow
system and the reference fluid flow system.
Description
BACKGROUND OF THE ART
Material fluid permeability is an essential quality measurement in a
variety of industries including textiles and papermaking. Permeability in
itself is related to the porosity, density, and thickness of a material.
Consistency of these material properties over time is required within a
process as an indication of the quality. The purpose of permeability
measurement is to accurately indicate the quality and consistency of a
material product.
Historically, airflow permeability measurement devices have followed one of
two basic genres: series or bridge. The bridge method, exemplified by
Gurley Precision Instruments Co. [of Troy, N.Y.] Permeometer, compares
pressure drops across two streams with a single vacuum source. One flow
stream passes through a variable valve, comparator chamber, and fixed
orifice to the reservoir, while the second passes through the unknown
sample material, test chamber, and variable micrometer orifice into the
reservoir. Orifices are varied until the pressure drop across the variable
orifice is fixed at 0.5 inches of water and the pressures in both the test
chamber and comparator chamber are equal, thus the pressure drop across
the unknown sample is also 0.5 inches of water.
Among the many assumptions necessary for this measurement is the standard
environment. Conditions such as temperature or relative humidity affect
various components of permeability measurements. In 1856, Henry Darcy
published an equation for the basic relationship of flow through porous
media. He discovered that discharge varies directly with head loss over
distance, for small discharges. Although recent modifications have been
made to the coefficients, the relation has remained the same. Darcy's
equation is:
##EQU1##
(Albertson, et al. Fluid Mechanics, p.211-212). Where h.sub.f is head loss,
V is the mean velocity of flow, .mu. is the fluid absolute viscosity,
.gamma. is the fluid specific weight, d is the characteristic grain
diameter of the porous material, and c is the dimensionless coefficient
which describes the porous media by including the size and distribution of
grains, the porosity, and the orientation and arrangement of the grains.
This is referred to as the coefficient of permeability and is equal to the
pressure drop over specific weight. Note that the new flow coefficient
K.sub.D if d.sup.2 over coefficient c. Rewriting for volumetric flow equal
to bulk velocity times area gives:
##EQU2##
It should be noted that density does not enter into the equation of laminar
flow through a porous material. For laminar flow, the forces of inertia,
which depend on density, are negligible and the forces of viscosity are in
complete control. Since viscosity is a fluid property, it does not change
with pressure or location within the flow. Flow through a porous material
can be characterized by low velocity, high-pressure drop, and very small
pore diameter, so the conditions for laminar flow, such as a small
Reynolds number, is consistent.
Normalizing the flow constant per unit length, this dependence on viscosity
is an inherent dependence on temperature. According to the Handbook of
Chemistry and Physics, for air, absolute viscosity can be expressed solely
as a known function of temperature, linear in the region from 20 to 60
degrees Celsius.
##EQU3##
However, air not only flows through this permeable membrane, but also
various orifices. Flow through a fixed orifice is generally expressed in
the Bernoulli corrected form as
V=(2gh).sup.1/2
(Binder, Fluid Mechanics, p. 99). Where h is a head loss, commonly replaced
by .DELTA.P over .gamma., and .gamma. is the specific weight or fluid
density times gravitational constant. Expressed in terms of volumetric
flow rate,
##EQU4##
Where K is a new flow constant, A is the orifice area and .rho. is the
fluid density. Coefficient K is required because the cross-sectional are A
is inconsistent in the flow on fluid through an orifice. Density, however,
is much more difficult to specify than absolute viscosity. It requires
knowledge of atmospheric pressure, vapor pressure, relative humidity,
temperature and precise compressibility. Flow through an orifice is one of
the oldest, yet most reliable, methods of measuring and controlling the
flow of fluids (Binder), which most likely explains the historical use in
permeability measuring devices, however the limitation is in the accurate
specification of fluid density.
A permeameter sold by Frazier, Inc. [of Hagerstown, Md.] benchmarks the
series method. The device draws a variable suction across the permeable
membrane and a fixed but alterable orifice. Pressure drop across the
porous sheet-like material is held to a standard, while the pressure drop
across the fixed orifice is measured and compared with calibrated results.
Once again, problems arise with changes in atmosphere. Changes in
temperature, pressure, humidity, et cetera, between the conditions at
calibration and the conditions at measurement will cause error in results.
The simple series device above is governed by Darcy's Law and flow through
an orifice. Equating, the normalized permeability constant for a
particular sample test section may then be determined as follows
##EQU5##
Solving and combining with Darcy's Law at standardized conditions yields
the industrial standard permeability. The result is, once again, dependent
upon temperature, through viscosity (.mu.) and further atmospheric
conditions such as humidity, through density (.rho.).
Permeability measurement has been a necessary quality control measurement
in industry, including textile and paper industries. The measurement
issued as a fault detection platform across a web product span and between
successive products or webs. The main goal is to detect errors or
inconsistencies in a product or web, indicating process malfunction or
necessary web replacement due to use. For example, U.S. Pat. No. 4,495,796
uses an ad hoc permeability measurement as mechanical error detection
following a cigarette paper perforation device. U.S. Pat. No. 5,436,971
describes a device for measuring air permeability across a textile to find
manufactured, woven inconsistencies.
Single chamber designs have been developed as well, Such as described in
U.S. Pat. Nos. 4,756,183 and 4,991,425, both of which are single chamber
devices that ignore the change in permeability due to temperature change.
Most devices patented to this point ignore flow changes due to atmospheric
conditions. These devices assume that all measurements are taken at
standard conditions, which though desirable, is neither consistently
practiced nor universally practical for industrial use.
U.S. Pat. No. 4,649,738 takes atmospheric changes into consideration while
integrating high-speed permeability measurements in an industrial process.
The sample focused on is cotton at various stages of the cotton ginning
process. The device measures differentially over a measurement stream and
reference stream. The device does not, however, measure across an entire
sample, use a reference sample, or provide an accuracy level that is
needed in most applications. The device is also specific to the
measurement of a continuous flow of cotton, and sheet-like materials
cannot be measured using the present cofiguration.
It is clear that changes in atmospheric conditions will cause alteration of
standard expected flows, in differing amounts between an orifice and a
permeable membrane. Thus, measured pressure drop for a single material
will change as atmospheric conditions change. Removal of the dependence of
these conditions on the measurement of permeability will therefore vastly
improve the accuracy of measurement.
It is an object of the invention is to provide a method and device of
measuring differential permeability that eliminates environmental factors
and measures permeability accurately by measuring the differential
pressure drop across a fluid flow after flowing through a test sample and
the fluid flow after flowing through a reference sample.
It is another object of this invention to increase the limits of
permeability measurement accuracy.
It is another object of this invention to introduce the theory of
differential measurement across two samples to determine the permeability
of a porous material.
It is another object of this invention to eliminate variations in results
of permeability measurements due to a changing environment.
It is another object of this invention to increase permeability measurement
accuracy by changing the required range of gauge measurement.
SUMMARY OF THE INVENTION
Briefly, the invention extends from the basic concept of flaw detection.
This method of measurement compares two porous sheet-like samples across
their entire thickness in order to detect flaw, or difference, between the
two samples.
The invention provides a permeameter, which is comprised of:
a. A test head having a surface in communication with the test material;
b. A reference head having a surface in communication with the reference
material;
c. A clamping device for both the test sample and reference sample;
d. At least one flow measurement device, such as an orifice plate in the
test fluid flow system;
e. At least one flow measurement device, such as an orifice plate in the
reference fluid flow system which is identical to the test orifice plate;
f. An applied fluid supply;
g. A means for measuring the pressure differential between the test fluid
stream and the reference fluid stream;
h. A means for measuring the fluid flow in both the test fluid flow system
and the reference fluid flow system;
i. A honeycomb-type device placed in each flow system to promote laminar
flow and eliminate swirl;
The invention further provides a method for determining data to calculate
permeability of a test sample comprised of the following steps:
a. Place the sheet-like reference sample of known or desirable permeability
in the reference-clamping device and in communication with the reference
fluid flow system;
b. Supply a fluid flow stream across both test and reference samples, so
that the pressure drop across both samples is (very near to) a fixed
standard;
c. Measure the fluid flow through the test fluid flow system by measuring
the pressure drop across a flow device, such as an orifice plate; within
the test fluid flow system.
d. Measure the fluid flow through the reference fluid flow system by
measuring the pressure drop across a flow device, such as an orifice
plate; within the reference fluid flow system.
e. Measure the pressure differential between the test fluid stream and the
reference fluid stream and calculate the permeability of the test sample
by using the differential pressure across the test fluid flow stream, the
known permeability of the reference sample, and the air flow through both
the test fluid flow system and the reference fluid flow system.
Further objects and advantages of our invention will become apparent from a
consideration of the ensuing description taken in conjunction with the
accompanying drawings wherein:
FIG. 1 is a simplified schematic illustration of a permeameter constructed
in accordance with the invention;
FIG. 2 is an isometric sketch of the permeameter of FIG. 1;
FIG. 3 illustrates a side view of a clamping device employed in the
permeameter of FIG. 1;
FIG. 3a illustrates a perspective view of the clamping device of FIG. 3;
FIG. 4 illustrates a side view of a modified clamping device in accordance
with the invention;
FIG. 4a illustrates a perspective view of the clamping device of FIG. 4;
FIG. 5 illustrates a part cross-sectional side view of a magnetic clamping
device in accordance with the invention;
FIG. 6 is a simplified schematic of a variable orifice system in accordance
with the invention;
FIG. 7 is a top view of the variable orifice system of FIG. 6;
FIG. 8 is a simplified schematic of a Pitot tube construction in accordance
with the invention;
FIG. 9 illustrates an algorithm for a differential permeability control
calculation in accordance with the invention.
FIG. 10 illustrates an algorithm for fan speed control in accordance with
the Invention;
FIG. 11 illustrates an algorithm for fan speed control with a variable
orifice in accordance with the invention; and
DESCRIPTION
Referring to FIG. 1, the permeometer includes a test fluid flow system 10
and a reference fluid flow system 12 which are in the form of tubes and
are in common communication with a reservoir system 16. Fluid flow is
initiated by a fluid flow initiator 18, for example, a speed-controlled
centrifugal fan. The applied fluid used in this embodiment of the
apparatus is air. The cross section of each of the test fluid flow system
10, the reference fluid flow system 12 and the reservoir system 16 is
circular.
The airflow is very similar through the test fluid flow system 10 and the
reference fluid flow system 12 by the symmetry in diameter between both
systems. The cross-sectional area of the joining reservoir system 16 is
greater than the sum of the cross-sectional area of system 10 and the
cross-sectional area of system 12. Honeycomb structures 14a and 14b are
located at the base of both the test fluid flow system 10 and reference
fluid flow system 12. Each honeycomb structure consists of 1/4-inch
diameter pipes in a cluster that fills the inner diameter of both systems
10,12. Both honeycomb diameter and length can vary.
Upstream from the honeycomb structure 14a in the test fluid flow system 10
is an orifice plate 20a. Upstream from the honeycomb structure 14b in the
reference fluid flow system 12 is an orifice plate 20b. Each orifice plate
creates a measurable pressure drop in the respective fluid flow system
10,12. The measured pressure drop in each fluid flow 22 and 24 is directly
proportional to the velocity of that flow, and is used to compute
permeability of the test sample. The hole diameters in the respective
orifice plates 20a, 20b are always exactly identical. However, both plates
can be made to vary in hole size, either by interchanging a pair of fixed,
identical orifice plates of one hole diameter for a new pair of fixed,
identical orifice plates of a different hole diameter, or by means of the
continuously variable orifice system 34 as described below with respect to
FIGS. 6 and 7.
Referring to FIGS. 6 and 7, a change in orifice diameter is often needed to
ensure that the pressure measurements stay within the operational range of
the pressure gauges or required standard measurement range. Continuous
variation in orifice size is accomplished by sliding an orifice plate 67a,
67b over the plate 20a, 2b using a dual motorized screw drive 64 that is
mounted on a bracket 68 in the space between the flow systems 10,12. The
sliding action changes the total area of each orifice hole.
In order to use the permeameter, a sheet-like test sample 26 is required. A
sheet-like reference sample 28 is also required for percent difference in
permeability measurement. The reference sample should have a known
permeability or have known desirable characteristics. Samples 26 and 28
can also be similar, yet both unknown, in which case exact percent change
in permeability will be measured as a quantified quality/consistency
indication. If absolute permeability is the desired measurement, the
reference sample 28 should be omitted. The differential pressure
difference will read the absolute pressure drop across the test sample 26,
and the absolute permeability can be measured.
Referring to FIGS. 3 and 3a, a test-clamping device 40 is mounted at the
upper end of the test fluid flow system 10 so that the entire opening of
the system 10 is covered. Likewise, a reference-clamping device 40 is
mounted at the upper end of the reference fluid flow system 12 so that the
entire opening of the system 12 is covered. For any particular choice of
clamping method, the test and reference clamping devices are identical.
Each clamping device 40 is referred to as a direct weight clamping system
and is composed of two parts. The first part is a bottom flange 48, the
second part is a top flange 50. The bottom flange 48 fits tightly at the
entrance of the fluid flow system, and restricts airflow through the outer
diameter of the system using an o-ring. The top flange 50 is an unattached
piece that serves to apply downward clamping pressure on the test (or
reference) sample that is placed in between the flanges 48 and 50. The top
flange 50 consists of a lower contact ring 52 with the same outer and
inner diameter as the bottom flange 48 and an upper shelf 54 raised three
inches The shelf 54 has a purpose of holding accurate weight. This allows
for variability of clamping force. The clamping force minimizes lateral
fluid leakage through the sample and the flow entrance of each fluid flow
system 10 and 12, which can affect the pressure reading and therefore
alter the permeability measurement.
The top flange 50 is placed on top of the sample such that the outer
diameter of the lower contact ring 52 and the outer diameter of bottom
flange 48 are aligned. Alternatively, as shown in FIGS. 4 and 4a, a
clamping device 42 also referred to as an O-ring clamping system may be
used to hold a sample. As shown, the clamping device 42 is composed of two
parts. The first part is the bottom flange 48, which is identical to that
used in the direct weight clamping system 40, and the second part is a top
flange 56. The top flange 56 is an unattached piece that serves to apply
downward pressure on a primary O-ring seal 59, which lies in a groove
between the flanges 48 and 56.
A screw-down sample holder 58 is a tube that is threaded on its outer
surface with the same inner diameter as the bottom flange 48. The lower
end of the holder 58 comes in direct contact with the disk-like sample and
serves to hold the sample in place. The upper end of the holder 58 has an
annular shelf for the purpose of rotating the holder 58 with respect to
the top flange 56 so as to adjust the vertical position of the holder 58
and also for holding accurate weight.
The primary O-ring seal 59 eliminates lateral fluid leakage through the
circumference of the disk-like sample and thereby makes permeability
measurement independent of applied clamping pressure.
Referring to FIG. 5, a clamping device 44 also referred to as a magnetic
clamping system may also be used to hold a sample. This clamping device 44
is composed of two parts. The first part is a bottom flange composed of an
electromagnet 46 and a fabric guard 60, and the second part is a magnetic
clamping ring 47. The magnetic clamping ring 47 is an unattached piece
that serves to apply downward pressure on the test sample by means of a
magnetic attraction toward the electromagnet 46. The magnetic clamping
ring 47 consists of either a lightweight hollow ferrous structure, or a
lightweight nonferrous structure that contains internal permanent magnets.
The lower surface of ring 47 comes in direct contact with the sheet-like
sample and serves to apply clamping pressure that minimizes lateral fluid
leakage through the sample. Ring 47 and contacting surfaces of 46 and 60
may be coated with a protective, nondestructive material.
The electromagnet 46 is the source of the magnetic clamping force on the
ring 47. By adjusting the electric currents put through the electromagnet,
the resulting clamping pressure is thereby varied.
By recording the changing value of measured permeability while
simultaneously varying the magnetic clamping pressure in a known way, on a
fixed sample, the measurement of permeability in the limit of infinite
clamping pressure can be calculated by means of asymptotic analysis. This
limiting value is equal to the true permeability of the sheet-like test
sample in the ideal case of zero lateral fluid leakage.
Operation (Standard Operation)
The method of operation of the permeameter is completed with the use of
four pressure transducers mounted in a common housing 30 (see FIG. 2).
After the test sample 26 and reference sample 28 are manually placed in
the corresponding clamping devices such as those described by 40, 42 or
44, the speed of the fluid flow initiator 18 is manually or automatically
adjusted by a computer or other data/control system 32, so that the
pressure drop across the reference sample is 0.5 inches of water, measured
using pressure transducer PT1. The flow is similar through both the test
fluid flow system 10 and the reference fluid flow system 12, and therefore
the pressure drop across test sample 26 is similar to 0.5 inches of water.
When pressure drop across both samples is steady at approximately a desired
standard, the airflow is measured. This is accomplished by measuring the
pressure drop across the test orifice plate 20a and the reference orifice
plate 20b, due to the fact that air flow is proportional to pressure drop.
Pressure transducer PT2 is used to measure the pressure drop (P.sub.12
minus P.sub.13) across orifice 20a. Pressure transducer PT3 is used to
measure the pressure drop (P.sub.22 minus P.sub.23) across orifice plate
20b. The pressure measurement locations P.sub.11, P.sub.12, P.sub.13,
P.sub.21, P.sub.22, P.sub.23 are relative locations outlined in FIG. 1.
The small differential pressure between the test fluid flow system 10 and
the reference fluid flow system 12 (P.sub.11 minus P.sub.21) is measured
with high precision using pressure transducer PT4.
The permeability of test sample 26 and the percent difference in
permeability between test sample 26 and reference sample 28 are calculated
by the data acquisition system 32 using the measurements taken from
pressure transducers PT1, PT2, PT3, and PT4, which are sent to the
computer as analog signal 33.
As shown, a monitor is connected with the computer 32 to provide a visual
display of sample analysis and resultant readings.
Standard Algorithms for Adjusting Fan Speed and Variable Orifice Size and
Computing Differential Permeability
The fan control algorithm begins with an approximate value input by the
user, either in the form of a number or in the form of a material quality
such as relative strength, material type, and similar information.
Beginning with the base value, (which is estimated from user input) the
fan is adjusted by adding or subtracting speed until the measured pressure
drop between atmospheric pressure and the pressure within the reference
tube measures 0.5 inches of water. A basic representation of the Fan
Control Algorithm is represented in FIG. 9.
In models with automated variable orifices, a resultant differential
permeability value smaller than an acceptable range or an inability to
settle on a fan speed due to a lack of a pressure drop would result in an
appropriate adjustment of orifice size to form a measurable pressure drop.
An example of this is illustrated in FIG. 10.
To determine the test differential permeability quickly, an adjusting
algorithm is necessary. First, the algorithm takes a repetition of X
permeability readings where X is a set value that is small relative to the
overall number of tests to arrive at an initial average reading. The
values are then averaged, and the average set as the first half of a
number Y of tests. For example, out of Y=300 total tests, the average
value would be repeated for the first 150 values. The average of the Y
values is then taken and it represents the average differential
permeability value for the test. The Variables X and Y are relative to the
desired accuracy for test purposes, where Y is the total number of
averaging cycles and X is a small percentage of Y. If possible, the
algorithm should eliminate the rouge permeability values that naturally
occur in the testing process by comparing them to an expected value. For
instance, if in comparison to the initial average value, the measured
value during testing is of an opposite sign or much larger or smaller (by
a order of magnitude) it should be replaced with the initial average value
to minimize erroneous readings. The Flowchart representation of the
measurement algorithm is illustrated in FIG. 11.
Alternate Operation Procedures
The method for operation of the Pitot tube permeameter is completed with
the use of three pressure transducers. After the test sample 26 and
reference sample 28 are manually placed in the corresponding clamping
devices, the speed of the fluid flow initiator 18 is manually or
automatically adjusted so the pressure drop across the reference sample
and the atmosphere is 0.5 inches of water, measured using pressure
transducer PT1. The airflow is similar through both the test fluid flow
system 10 and the reference fluid flow system 12, therefore the pressure
drop across test sample 26 is similar to 0.5 inches of water.
Once the pressure drop across sample 28 is at 0.5 inches of water, the
airflow in each system is measured. This is accomplished by measuring the
difference in pressure (P.sub.21 minus P.sub.22) between the reference
fluid flow system 12 and the Pitot tube 62b with pressure transducer PT2.
Then, the small difference in pressure between the Pitot tubes 62a, 62b
(P.sub.12 minus P.sub.22) is measured with high precision using pressure
transducer PT4.
The permeability of the test sample 26 and the percent difference between
the test sample 26 and the reference sample 28 are calculated by the data
acquisition computer 32 using the measurements taken from pressure
transducers PT1, PT2, and PT4, which are sent to the computer 32 as analog
signal 33 (see FIG. 2).
Referring to FIG. 8, use may be made of a Pitot tube 62a, 62b to measure
fluid flow rate inside the test fluid flow system 10 and the reference
fluid flow system 12. As shown, each Pitot tube 62a, 62b is positioned
above the respective honeycomb structure 14a, 14b in the respective flow
system 10,12 to measure the total pressure in each respective system
10,12.
The fluid flow is initiated by the fluid flow initiator 18 that, in this
embodiment, is a speed-controlled fan.
A pressure transducer measures the pressure differential between the Pitot
tube 62a and the Pitot tube 62b, yielding P.sub.12 minus P.sub.22. The
difference in pressure shows a relationship in airflow between the two
systems 10 and 12, and is used to compute permeability of the test sample.
Beyond simple air permeability testing, the differential permeameter allows
accurate testing with almost any fluid flow, assuming the relative
viscosity is low enough. To perform low-viscosity fluid permeability
tests, minor device modifications should be considered. While background
theories hold for most low viscosity fluids, certain special conditions
may apply to fluids that are denser than air. In order to maintain even
distribution, the flow systems 10,12 may need to remain in a vertical
position to maintain evenly distributed laminar flow (to prevent pooling
in areas of the machine) though with most fluids this is unnecessary after
proper pressure is generated by the pumping device. In addition, in low
viscosity, lower-density fluids such as water; the test fluid can be
recycled via a reservoir.
All of the permeameter parts should be appropriate for (non-air) fluid
testing, for example, the pressure sensors should be approved for other
fluid testing and the pressure fan should be replaced with a variable
speed fluid pump. Further special considerations should be taken when
working with fluids that are potentially damaging to the apparatus (for
example acidic and basic fluids) and appropriate care and or replacements
should be practiced.
The fluid immersion differential permeability testing allows for the
examination and testing of a variety of materials beyond the capabilities
of air permeability, such as soil samples, wet filters, permeability to
different fluids (e.g. N2 or O2), and the like.
The operation of a fluid permeameter should be identical to the operation
of the standard construction of the permeameter. The minor operational
changes primarily govern fluid flow, specifically maintaining the level of
feed fluid either from a recycling reservoir or from a reserve source. In
addition, the pressure of the fluid against the flow surface of the
samples (external to testing tubes) should be maintained constant to
prevent erroneous differential permeability values.
In situations where permeability samples cannot be tested in a laboratory
environment, and where samples are restricted by dimensions of extensive
distance, a Large-Scale permeability measurement is applicable.
Large-Scale air/fluid permeability testing, which might apply to more
permanent-type production line or manufacturing process based testing,
requires attention to be paid to the even distribution of pressure at the
entrance (bottom in illustrations) of the testing tube. In order to ensure
the even distribution of flow, the source "reservoir" pipes need to be
wide enough that the pressure drop from friction along the outside is
negligible. This would call for larger pipes as the distance between tubes
increases.
In addition, the entrance to the testing tube should be near the center of
the tube where the distribution will be equal. The use of a plenum similar
to industrial heating and cooling methods would also be sufficient for
testing. Further adjustments might be made by multiplying the data
readings on the lower pressure tube by a factor of the change in pressure
between the entrance points on the reference and test pipes.
In Large-Scale permeability testing, a number greater than two test pipes
may exist. The permeometer will continue to function as long as the tubes
are arranged in a manner that ensures even distribution of pressure.
Approaches include the comparison of tubes in pairs (and preventing flow
in the idle testing tubes) to minimize the required pressure, or the
management of a large and even pressure reservoir to guarantee equal
pressure at all test points.
General operation of a large-scale permeability measurement system should
be generally identical to the standard method. The major difference is
that active management is needed to monitor and adjust the tubes being
utilized for testing purposes. Additional attention needs to be paid to
the even flow of pressure at the entrance point to each testing tube, and
software or hardware adjustments might need to made in order to ensure a
accurate experimental reading.
The invention thus provides a permeameter and method wherein environmental
factors are eliminated in the testing of a sheet-like permeable membrane
sample by either providing a known sheet-like permeable membrane reference
sample to provide an accurate permeability measurement or measuring the
percent change between test and reference samples.
While the above description contains much specificity, these should not be
construed as limitations on the scope of the invention, but rather as
exemplifications of one preferred embodiment thereof. Many other
variations are possible. For example, eliminating the orifice plates,
adding multiple orifice plates and changing the clamping device.
The invention provides a method of measuring permeability of a sheet-like
permeable membrane sample such that all environmental factors are
eliminated. The method is such that a change in local temperature does not
change the measurement accuracy of permeability and that changes in air
density, and the factors controlling air density, such as relative
humidity, do not affect the accuracy of measurement.
The method may be used to measure the change in permeability between
samples such as, a standard sample to a random sample; a particular area
on a cloth or web to other spots on the same cloth or web; a particular
area on a cloth or web to areas on another cloth or web; and two random
samples.
The permeameter may be operated to maintain a pressure differential applied
to gauges within their operational limits while increasing the distance
between testing tubes by manipulating air flow transmission pipes and
plenums.
The relative calculation time required for determining a reading of
relative accuracy may be decreased by estimating a large portion of test
values from a portion of small measurements;
The time required to reach an optimum fan speed for testing purposes may be
reduced by using a value estimated by the user in a variety of forms, or
by remembering the last value used to implement as the initial value,
beginning with a fixed value upon start-up.
The Honeycomb method may be used for maintaining quasi-laminar flow
throughout the permeometer;
For numerically integrating the measurement of differential permeability
over time to obtain a final measurement of differential permeability that
is precise to an arbitrarily high number of significant digits;
And for calculating the permeability of a sheet-like test sample at the
limit of infinite clamping pressure by measuring the change in
permeability while the applied clamping pressure is varied through a range
of pressures.
*
