Method and apparatus for control of flow through a filter chamber by measured chamber equilibration pressure

A method and apparatus for the controlled instrumentation processing of cells and other paricles with a filter device measures a parameter of the flow through the filter device of a fluid carrying the particles. A measure of the change of fluid flow through the filter device yields desired information for quantizing the particles and for quantizing the obstruction of the filter device by the particles. The method and apparatus typically operate automatically.

BACKGROUND 
This invention relates, in one instance, to measuring the quantity or 
concentration of cells in a biological sample. The invention is useful in 
anatomic pathology, which is a medical and laboratory specialty that makes 
diagnoses on findings in human tissues and cells. 
More broadly, the invention provides a method and apparatus for the 
controlled instrumented processing of particles with a filter device. The 
filter device is of the screen type, e.g. a membrane filter, that blocks 
particles larger than a threshold size and passes smaller particles. The 
particles of interest are carried in a fluid, and a change in the flow of 
the liquid carrying the particles, due to blockage of the filter device by 
the particles, provides information of interest both regarding the 
blockage of the filter and regarding the particles. 
The invention thus provides quantitative instrumentation information 
regarding particles, generally of unknown particles, by an indirect 
technique that measures a flow condition of a screen-type filter device in 
the flow path of a fluid that carries the particles. 
One application of the invention is in the pathological test, termed a Pap 
smear test, that examines cells for the presence of cancer. An established 
procedure for this test transfers a measured quantity of cells from a 
biological sample to a microscope slide for examination. One prior 
procedure for obtaining the desired measured quantity of cells from the 
sample employs a flow cytometer, such as a Coulter counter. Another prior 
cell-counting procedure employs a photometric technique in which light is 
directed through a fluid-suspension of the cells. Photodetectors 
responsive to the resultant scattered light provide signals that are a 
measure of the quantity of cells in the suspension. 
These known cytological procedures for quantizing cells have drawbacks, 
including requiring expensive equipment and having limited performance in 
terms of reliability, repeatability, accuracy and precision. They also 
present biohazard risks, including from the handling of biosamples. 
It is accordingly an object of this invention to provide an improved method 
and apparatus for quantizing cells and other particles carried in a fluid 
medium. Specific objects are to provide such a method and apparatus for 
implementation at a relatively low cost, and for controllable automated 
operation with relatively high reliability, repeatability, accuracy and 
precision. 
Other objects of the invention are to provide an improved method and 
apparatus for collecting a selected quantity of cells and other particles 
that are carried in a fluid medium, particularly in a liquid medium. 
It is also an object of the invention to provide an improved method and 
apparatus for determining a quantitative measure of the flow condition of 
a screen-type filter device subject to obstruction by particles larger 
than a known threshold size. 
Another object of the invention is to provide an improved method and 
apparatus for collecting a specified sample of cells for cytological 
examination. 
Other objects of the invention will in part be obvious and will in part 
appear hereinafter. 
GENERAL DESCRIPTION 
In accordance with the invention, a quantitative measure responsive to the 
number of particles, e.g. cells, in a fluid medium is obtained by 
providing a screen-type filter device in a flow path for the 
particle-carrying fluid. A flow condition, e.g. a selected pressure or 
flow velocity as a function of time, is imposed on the fluid medium. A 
measurement is made of a parameter responsive to the resultant flow, 
including change in flow, through the filter device due to the applied 
flow condition. The invention further provides for determining a selected 
change in that measured parameter responsive to the obstruction of the 
filter device by particles larger than a threshold size. 
As used herein, a screen-type filter device refers to a filter, such as a 
membrane filter, that blocks cells and other particles larger than a 
selected threshold size and that passes smaller particles essentially 
without obstruction. Such a filter device typically has a filtering 
surface that is progressively obstructed as it blocks an increasing number 
of particles above the threshold size. 
The applied fluid condition can be a pressure signal that causes fluid to 
flow through the filter device. Examples include an applied pressure 
signal that remains essentially constant over a selected time interval. 
Another example is a succession of pressure pulses, typically of known 
magnitude, duration and time spacing. The applied flow condition can also 
be an applied fluid velocity through the filter device. Examples are to 
maintain a selected constant flow through the filter device, over a 
selected time interval, and to apply a succession of flow pulses. 
The measurement of a parameter responsive to flow through the filter device 
due to the applied flow condition includes, in one practice of the 
invention, measuring the rate of flow in response to an applied uniform 
pressure signal. In another practice, it includes measuring the change in 
pressure across the filter device required to maintain a selected flow 
velocity. Another example is to measure the time required for the flow 
through the filter device to change by a selected amount, typically as 
measured by a change in flow rate for a given applied pressure or by a 
change in pressure to maintain a selected flow rate. 
In accordance with a further practice of the invention, the applied 
condition, or applied flow signal, is a succession of pulses, and the 
measured parameter monitors the equilibration of the flow following each 
applied pulse. In one illustrative instance, the measured parameter is the 
time for the flow velocity to equilibrate to a selected relative level 
following application of a selected pressure pulse. 
In one specific practice of the invention, a liquid suspension of cells 
flows under constant applied pressure through a membrane filter. The rate 
of fluid flow through the filter device is measured, typically either for 
a selected interval of time or until the flow rate decreases by a selected 
amount. The change in fluid flow through the filter device is directly 
responsive to the number or concentration of particles or cells in the 
liquid, because the filter blocks the cells of interest while passing 
smaller cells, and accordingly becomes increasingly blocked or clogged by 
the cells of interest. 
In another specific practice of the invention, a succession of known 
pressure pulses, which can be of positive pressure or of negative 
pressure, is applied to drive the cell-carrying fluid through the filter 
device, and the time is measured after each pulse for the pressure across 
the filter device to return to a selected relative level, i.e. to 
equilibrate a selected amount. 
When the measured equilibrate times have increased from the initial 
measure, i.e. for the initial pressure pulse, by a selected amount, a 
corresponding known quantity of cells has collected on the filter. 
The practice of the invention thus, in one aspect, measures the flow 
condition of a screen type filter as it becomes increasingly obstructed, 
to determine a measure of particles in the fluid medium directed through 
the filter. The particles being measured in this indirect way have a size 
larger than a selected value determined by the pore size or other porosity 
measure of the filter screen. 
The practice of the invention can thus provide a quantitative measure 
regarding particles in a fluid medium larger than a selected threshold 
value, and the measure is obtained indirectly, by applying the pressure 
and flow factors of Boyle's law with a screen-type filter. The 
quantitative measure can be of the number of particles, where the average 
size of the particles above the threshold value is known. Otherwise, the 
measure is of the relative area or portion of the filter surface that the 
particles cover. 
The practice of invention further provides for collecting a selected 
quantity of cells or other particles carried in a liquid or other fluid 
medium. For this practice of the invention, continued flow of the fluid 
deposits progressively more particles above the threshold value on the 
filter surface. Thus an increasing area of the filter surface collects and 
is obstructed by additional particles. This obstruction of a known 
relative portion of the filter surface area corresponds directly with the 
collection of a known quantity of particles having a known average size 
larger than the threshold size. This quantitatively known collection of 
particles, which typically is obtained from an unknown quantity of 
particles in a sample liquid or other fluid, can be further processed, 
typically by removal of the collected particles from the filter device, or 
in response to a reverse pressure or reverse flow. One illustrative 
practice is to collect a selected quantity of cells in this manner and to 
subject the collected quantized cell sample to cytological examination, 
using known cytological testing techniques. 
The invention further features a programmable control element that applies 
a selected flow signal for producing a flow of fluid that carries 
particles through the filter device, and for monitoring flow-responsive 
parameters, such as pressure across the filter or fluid flow through the 
filter. The programmable control element enables the practice of the 
invention to be automated, to have a controlled fluid flow or pressure 
change, and to stop or otherwise change the operation automatically, 
depending on the application. 
A further feature of the invention provides an apparatus having a container 
for the fluid medium that carries the particles and having a vessel closed 
at one wall portion with a filter device that blocks passage of cells or 
other particles of interest. The vessel is disposed with the filter device 
immersed in the fluid in the container. In one specific cytological 
embodiment, the vessel disposes the filter device immersed below the level 
of a cell-carrying liquid in the container. A fluid source applies a 
selected flow condition to the filter device, causing the fluid medium to 
flow through the filter device from the container to the vessel. One or 
more sensors are provided for measuring one or more parameters responsive 
to the fluid flow through the filter device. 
Another feature of the invention provides a chamber element in fluid 
communication with the vessel and container system for reducing the effect 
of changes in the height of the liquid therein. 
The apparatus also has a source that applies a selected flow condition to 
the container-vessel system. The source can apply a selected pressure 
condition upon the filter device, or impose a selected fluid flow through 
it. In accord with a further feature of the invention, a further chamber 
element is provided to decrease the time for the pressure across the 
filter device to equilibrate after an applied pressure pulse, and thereby 
to speed up the overall time for measurement in accordance with the 
invention. This further chamber element has a volume closed to the 
atmosphere and greater than a volume associated with the filter vessel. 
A further practice of the invention directs a flow of air carrying 
microscopic particles, for example, contaminants above a selected size, 
across a membrane filter that blocks particles of interest while passing 
smaller particles. When the average size of the particles above this 
threshold value is known, a measurement of the change of the fluid flow 
through the filter yields a precise and reliable measure of the quantity 
of the airborne particles. Further, this practice of the invention can 
collect a known quantity of the particles on the filter for further 
measurement or other processing. 
The invention thus provides a method and apparatus for determining 
quantitative information regarding particles above threshold size, 
including airborne particles and biological cells, present in a fluid -- 
either gaseous or liquid -- indirectly, by a measurement of the fluid 
flow. The practice of the invention can employ relatively inexpensive 
measuring equipment that operates with dependability and accuracy and 
precision, and on a controllable automatic basis. 
The invention accordingly comprises the several steps and the relation of 
one or more of such steps with respect to each of the others, and the 
apparatus embodying feature of construction, combinations of elements and 
arrangement of parts adapted to effect such steps, all as further 
exemplified in the following detailed disclosure, and the scope of the 
invention is indicated in the claims.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS 
FIG. 1 shows a system 10 according to one practice of the invention for 
controlled instrumented processing of biological cells. The illustrated 
system collects a selected quantity of cells onto a screen-type filter 12. 
The system 10 has a specimen container 14 that contains a liquid 16 that 
carries the cells. The filter 12 is on the bottom wall of a collection 
vessel 18. The collection vessel is fitted within the specimen container 
14 to immerse the filter 12 into the liquid 16 in the container 14. 
The illustrated specimen container 14, as shown in FIG. 2, is open at the 
top to the atmosphere and can be an open vessel such as a cup, vial, or 
beaker. The illustrated collection vessel 18 has a cylindrical tubular 
body 20 with the filter 12 spanning and closing a normally lower axial 
end. The body 20 of the collection vessel 18 is fitted with a cap 22 at 
the other, normally upper end. The screen-type filter 12 is preferably a 
membrane filter and hence is apertured with a uniform distribution of 
pores of substantially uniform size to block cells and other particles 
above a threshold size determined by the size of the pores, and to freely 
pass smaller particles. The filter has a filtering surface, illustrated as 
an essentially flat disc that has a surface area of known or readily 
determined size. 
The cap 22 that closes the top of the vessel 18, together with the body 20, 
renders the vessel pressure tight except at the filter 12 and at a port 24 
in the cap. As shown in FIG. 1, the illustrated cap 22 also mounts a 
pressure transducer 26 arranged for sensing the pressure within the 
collection vessel 18, preferably at its normally upper end. 
As further shown in FIG. 1, a pressure hose 28 connects the port 24 of the 
collection vessel 18 to a pressure unit 30, so that the pressure unit is 
in fluid communication with the interior of the collection vessel. An 
electronic control unit 32 connects with the pressure transducer 26 to 
receive a pressure-responsive electrical signal, and connects with the 
pressure unit 30. 
The pressure unit 30, typically in response to electrical control signals 
from the control unit 32, which can be microprocessor controlled, applies 
selected fluid conditions to the interior of the collection vessel 18. 
More particularly, the control unit 32 and pressure unit 30 operate the 
illustrated system 10 to collect a selected quantity of cells onto the 
underside of the filter 12, from a sample carried in the liquid 16 and 
wherein the cells have a known average size above the filter pore size, 
i.e. above a selected threshold size, and otherwise are of unknown 
quantity. 
For this operation, the pressure unit 30, typically in response to signals 
from the control unit 32, applies a flow condition to the interior of the 
collection vessel 18 to create a selected flow of liquid from the specimen 
container to the collection vessel, by way of the filter 12. This flow of 
liquid carries cells to the filter, which accordingly becomes 
progressively covered and hence blocked by the cells. The pressure unit 30 
applies the selected flow condition to the collection vessel until the 
filter becomes clogged by a selected amount, as determined at least in 
part by the pressure sensed within the vessel 18 by means of the 
transducer 26. 
FIG. 3 illustrates one such operating sequence in which the applied pulse 
signal from the pressure unit 30 is a constant selected negative pressure 
within the collection vessel 18. The constant applied pressure across the 
filter 12 produces a flow of liquid 16 from the container 14 into the 
vessel 18, through the filter 12. The flow decreases with time, due to 
progressive obstruction of the filter by cells in the liquid. A measure of 
a parameter responsive to the change in flow rate accordingly provides a 
quantitative measure of the surface of the filter clogged by cells in the 
liquid, i.e. of the increase in filter clogging by the cells, and of the 
number of cells above the filter threshold size, assuming the average size 
of such cells is known. 
For this illustrated embodiment, the pressure unit 30 can employ a 
displacement pump, such as a piston pump 36 driven by a stepping motor 38. 
The control unit 32 monitors the pressure in the collection vessel, by way 
of the transducer 26, and controls the stepping motor pulses required to 
maintain the constant applied pressure. When the timing of the stepping 
motor pulses slows by, for example, ten percent from the initial rate to 
maintain the selected applied pressure, the system 10 has collected a 
quantity of cells that covers ten percent of the filter surface. When the 
control unit terminates operation at this juncture, with a sample of cells 
having known average size above the filter threshold size, a 
correspondingly known quantity of cells is collected on the surface of the 
filter 12 and can if desired, be transferred from the filter to, for 
example, a microscope slide for image analysis either visually or by 
machine vision or both. The transfer of the collected cells from the 
filter 12 to a microscope slide can be carried out by applying a slight 
mechanical pressure within the vessel 18 against the filter 12, e.g. by 
pressing an alcohol-bearing sponge against the filter, after microscope 
slide is brought into contact with the filter 12, to essentially lift the 
cells off the filter 12 to adhere to the microscope slide. U.S. Pat. No. 
4,395,493 discloses one practice of this type of transfer of cells from a 
filter type object to a microscope slide. 
The embodiment of FIG. 3 thus operates with an applied pressure signal and 
measures a time parameter, i.e. the rate of stepping motor pulses, 
responsive to the resultant flow rate through the filter device, thereby 
to provide an indirect quantitative measure. 
FIG. 4 illustrates operation of the FIG. 1 system 10 with a pressure source 
30 and a control unit 32 arranged to apply a sequentially pulsed flow 
signal, illustrated by waveform 40, and to measure the time for the 
pressure within the vessel 18, i.e. across the filter 12, to equilibrate 
after each applied pulse to a selected level P.sub.E. The FIG. 4 waveform 
42 illustrates the pressure within the vessel 18 and hence across the 
filter 12 which the pressure sensor 26 senses. 
The signal waveforms in FIG. 4 do not reveal the changes in the pressure 
head P.sub.H, during this operation. The pressure head decreases during 
operation by a relatively small and significant amount, depending on the 
densities of the fluids in container 14 and in vessel 18. Accordingly, in 
the illustrated embodiment, the pressure head decreases as the levels of 
liquid 16 in the container and in the vessel change in response to each 
applied pressure pulse. 
The FIG. 4 waveform 40 of pressure pulses which the pressure unit 30 
applies to the collection vessel 18, and hence across the filter 12 since 
the collection container 14 is open to the atmosphere, shows that each 
pulse reduces the pressure within the vessel 18 from a positive pressure 
head value designated P.sub.H to a selected small negative value 
designated P.sub.A. Each illustrated applied pressure pulse has a negative 
P.sub.A value of 0.050 psi. This specific value, and others stated herein, 
are by way of example only and the invention can be practiced with other 
values as those skilled in the art will determine in accord with this 
description. The pulses repeat at a rate such that the pressure within the 
vessel 18, i.e. the vessel pressure, returns before each new pulse is 
applied to a value slightly below the value of the pressure head prior to 
the last applied pressure pulse. 
The waveform 42 of the vessel pressure that results from the applied flow 
condition is normally at the P.sub.H value, and drops to the P.sub.A value 
in response to each applied pulse. After each applied pulse terminates, 
the vessel pressure gradually returns to the P.sub.H value, as liquid 
flows from the sample container 14 through the filter 12 into the 
collection vessel 18. The pressure returns at an exponential rate. 
The control unit 32 monitors the time, after application of an applied 
pressure pulse, for the vessel pressure of waveform 42 to equilibrate from 
the P.sub.A value toward the P.sub.H value to a selected equilibrate level 
P.sub.E. The rate of pressure return is responsive to the degree of 
clogging of the filter, and accordingly gradually slows as an increasing 
portion of the filter surface becomes covered by and hence clogged by 
cells larger than the filter threshold size. The monitored equilibrate 
time t.sub.1, t.sub.2, . . . t.sub.n therefore increases in direct 
proportion to the rate at which the filter clogs with cells. 
The control unit 32 stops the pressure unit 30 from applying further 
pressure pulses when the equilibrate time has increased by a selected 
amount from the initial time t.sub.1. The increase in equilibrate time is 
selected to correspond to a selected increase in filter obstruction, which 
in turn corresponds to the collection of a selected quantity of cells from 
the sample liquid 16 onto the filter 12. By way of example, a system as 
shown in FIG. 1 and operating as described with reference to FIG. 4 with 
pressure pulses each having a value of 0.050 pound per square inch and 
with a filter 12 having surface area of approximately three square 
centimeters, transfers in the order of fifty microliters of liquid from 
the sample container 14 into the filter vessel 18 with each applied 
pressure pulse. The system accordingly measures the cells within the 
liquid at essentially a liquid drop at a time, for each applied pressure 
pulse. 
FIG. 5 shows a construction for the pressure unit 30 preferred for the 
foregoing pulsed operation of the system of FIG. 1 as illustrated in FIG. 
4. The illustrated pressure unit construction embodies two further 
features, one of which diminishes measuring errors due to a change in the 
pressure head, i.e. a decrease in the pressure head as liquid is drawn 
from the sample container 14 into the filter collection vessel 18. A 
second feature speeds the equilibration of the vessel pressure to the 
P.sub.H value, and thereby reduces the time required to attain a given 
collection of cells on the filter 12. 
In the illustrated pressure unit, a pump 50 is connected by way of a valve 
52 to maintain a selected negative pressure in a plenum 54, and is 
connected by way of a valve 56 to maintain a selected positive pressure in 
a plenum 58. A valve 60 connects the pressure in plenum 54 with the 
collection vessel 18, by way of a pressure line 64. A further valve 62 
applies the positive pressure in plenum 58 to the pressure line 64 leading 
to the collection vessel 18. Each plenum 54 and 58 preferably is fitted 
with a pressure transducer 54a and 58a, respectively, and electrical 
leads connect the pump 50, each valve 52, 56, 60 and 62 and each plenum 
pressure transducer to control circuits within the control unit 32. A 
primary auxiliary plenum 66 is coupled in communication with the pressure 
line 64, and a valve 68 selectively couples a secondary auxiliary plenum 
70 in communication with the pressure line 64, the valve 68 is operated by 
the control unit. 
The control unit 32 operates the pump and the valves 52 and 56 to maintain 
a selected negative pressure, e.g. -0.25 psi, in the plenum 54 and to 
maintain a selected positive pressure, e.g. +0.50 psi, in the plenum 58. 
The auxiliary plenum 66 is in direct communication by way of the pressure 
line 64 with the pressure vessel and accordingly is at the same pressure. 
The secondary auxiliary plenum 70 is coupled to be at the pressure of the 
collection vessel 18 when the valve 68 is open. 
In the illustrated embodiment, the combined fluid volume of collection 
vessel 18, when empty of liquid, and of the pressure line 64 between the 
vessel 18 and the valves 60, 62 and 68 is in the order of ten cubic 
centimeters. The volume of each plenum 54 and 58 is typically two orders 
of magnitude or more larger. The volumes of the primary plenum 66 and of 
the secondary plenum 70 are selected to balance one another and the 
combined volumes of the vessel 18 and pressure lines 64, for the pulsed 
and equilibrate operation described further below. In the illustrated 
embodiment, the primary plenum 66 has a volume of approximately twenty 
cubic centimeters and the secondary plenum 70 has a volume of 
approximately 250 cubic centimeters. 
One sequence for operating the system 10 of FIG. 1 as shown in FIG. 4 with 
the pressure unit 30 of FIG. 5 commences with opening the valve 60 to wet 
the filter 12 with a small volume of fluid from the sample container 14. 
(Unless stated otherwise, this and other operations described herein 
proceed with the valves 60, 62 and 68 in normally closed condition.) The 
valve 60 is then closed, and the valve 62 is opened to apply positive 
pressure to the collection vessel for driving liquid therein outward 
through the filter 12, to empty the collection vessel 18 and to remove any 
cells and other particles from the filter 12. 
With the filter 12 thus wet with the liquid in the container 14 and the 
collection vessel and the filter cleared of fluid and of cells, the 
operation illustrated in FIG. 4 commences with valve 60 open for brief 
intervals to apply the pressure pulses P.sub.A as shown in waveform 40. 
The control unit 32 continues this operation with the pressure unit and 
with monitoring the pressure in the collection vessel 18 to determine the 
equilibrate times, until the equilibrate time has increased by the 
selected margin relative to the initial value T.sub.1. The control unit 32 
then stops the operation, with a selected quantitative measure of cells 
collected on the filter 12. 
More particularly, prior to applying each pressure pulse and with the 
filter vessel 18 at the head pressure corresponding to the difference in 
liquid level between the vessel 18 and the container 16, the control unit 
32 opens valve 68 to allow the pressure in the secondary plenum 70 to 
equilibrate to that pressure head. The control unit 32 then closes the 
secondary plenum valve 68 and applies a pressure pulse to the filter 
chamber 18 by opening valve 60 and monitoring vessel pressure with the 
transducer 26. When the pressure transducer signal indicates that the 
vessel pressure has dropped from the head value, P.sub.H, to the desired 
applied pressure, P.sub.A, the control unit 32 closes valve 60. 
The resultant pressure differential across the filter 18 causes liquid to 
flow from the container 16 into the vessel 18 and hence across the filter 
12. The rate of flow, and correspondingly the pressure difference, 
diminish at an exponential rate. During this time, all three valves 60, 
62, and 68 are closed. 
When the control unit 32 senses that the vessel pressure has dropped to the 
selected equilibrate level and has measured the corresponding time 
interval T.sub.n, the control unit 32 opens the secondary plenum valve 68, 
to speed the return of the pressure vessel to the P.sub.H value 
immediately prior to the last applied pressure pulse. 
The volumes of the plenums 66 and 70 and of the collection vessel 18 and 
pressure line 64 are selected so that, when the valve 68 is opened after 
the filter vessel pressure has equilibrated to the P.sub.E value, the 
head-pressure value stored in the secondary plenum 70--corresponding to 
head pressure prior to the last applied pressure pulse--brings the vessel 
pressure to a level close to, yet less than the head pressure. A 
relatively small further liquid flow across the filter 12 accordingly 
fully equilibrates the vessel pressure to a new, slightly lesser, head 
pressure. 
Thus the secondary plenum 70, in essence, stores a pressure corresponding 
in value to the head pressure prior to the application of a pressure 
pulse, and speeds up return of the vessel pressure to a new, lesser head 
pressure corresponding to conditions after application of that pressure 
pulse. The illustrated system preferably avoids the condition where the 
stored pressure in a secondary plenum 70, upon opening a valve 68 after 
the vessel pressure is at the equilibrate value causes a reverse flow of 
liquid from the vessel 18 into the container 16. Such a reverse flow 
condition is considered disadvantageous for the illustrated operation and 
accordingly is avoided. 
The primary plenum 66 is in parallel with and augments the collection 
vessel pressure, to minimize the effect of changes in pressure head. This 
plenum also is sized to match, or balance, the desired dynamic range of 
volume of the filter vessel 18, between conditions of being empty and 
conversely filled with liquid from the container 16, for the foregoing 
speed-up operation with the selected volume of the secondary plenum 70. 
Thus, the primary plenum 66 preferably has the smallest volume that, 
relative to the volume of the collection vessel 18 and pressure line 64, 
accommodates the dynamic range of filter vessel capacity change, without 
back flow of liquid outward from the vessel 18 through the filter 12. The 
ratio of volume in the secondary plenum 70 to the combined volumes of the 
primary plenum 66 and the collection vessel 18 and pressure line 64 is 
preferably in the order of approximately ten to one. 
The foregoing arrangement of the pressure unit 30, as illustrated in FIG. 
5, thus enables the illustrated system to have a small collection vessel 
18 and to operate with relatively low sensitivity to changes in the height 
of liquid therein relative to the liquid height in the sample container 
14. This reduction in sensitivity to changes in the pressure head enhances 
measuring accuracy and precision. The arrangement also allows the system 
to employ a compact and relatively inexpensive collection vessel 18 that 
may be discarded after each measurement for precluding intersample 
contamination. 
It will thus be seen that the invention efficiently attains the objects set 
forth above, among those made apparent from the preceding description. It 
will also be understood that changes may be made in the above construction 
and in the foregoing sequences and operation without departing from the 
scope of the invention. It accordingly is intended that all matter shown 
in the accompanying drawings be interpreted as illustrative rather than in 
any limiting sense. 
It is also to be understood that the following claims are intended to cover 
all of the generic and specific features of the invention as described 
herein, and all statements of the scope of the invention which, as a 
matter of language, might be said to fall therebetween.