US3699814A

US3699814A - Gas sampler

Info

Publication number: US3699814A
Application number: US233082A
Authority: US
Inventors: Warren F Kaufman
Original assignee: Philco Ford Corp
Current assignee: Space Systems Loral LLC
Priority date: 1972-03-09
Filing date: 1972-03-09
Publication date: 1972-10-24
Anticipated expiration: 1989-10-24
Also published as: DE2236972A1; JPS4924490A; DE2236972B2; CA963290A; BR7301725D0; GB1428213A; JPS5130794B2

Abstract

Apparatus for sampling gaseous exhaust emissions, on a constant flow basis. A centrifugal type extractor pump is used to draw gas through a critical flow venturi of predetermined flow rate. Exhaust emissions are diluted with air and the critical flow venturi acts as a constant volume flow device serving a metering function. Samples are taken and collected continuously during the test run, and are available to be analyzed for contaminants such for example as hydrocarbon, carbon monoxide and oxides of nitrogen.

Description

United States Patent Kaufman 1 Oct. 24, 1972 [54] GAS SAMPLER 3,603,155 9/1971 Morris ..73/42l.5

[72] Inventor: Warren F. Kaufman, Santa Ana,

Calif Przmary Examiner-Douglas Hart Attorney-Robert D. Sanborn et al. [73] Ass1gnee: Philco-Ford Corporation; Philadelphla, Pa. [57] ABSTRACT [22] Filed: March 1972 Apparatus for sampling gaseous exhaust emissions, on 2 App] 233,0 2 a constant flow basis. A centrifugal type extractor pump is used to draw gas through a critical flow venturi of predetermined flow rate. Exhaust emissions are Jll diluted with air and the Critical flow venturi acts as a a n s t l s s t t s s s s Q u t t Q I v t t n a [58] Field of Search ..73/42l.5, 23, 27, 60/276 tion. Samples are taken and collected continuously during the test run, and are available to be analyzed [56] References cued for contaminants such for example as hydrocarbon,

UNITED STATES PATENTS carbon monoxide and oxides of nitrogen.

3,593,023 7/ 197.1 Dodson ..73/42l.5 10 Claims, 6 Drawing Figures 34 I .L J

I PATENTEDom 24 m2 SHEET 1 BF 3 :x/uwr 70 Amanda?! l 1 ma M a L n M M t Awv M0 Z A r m UMIII' m GAS SAMPLER BACKGROUND OF THE INVENTION certain contaminants. For example the emissions may not exceed: hydrocarbons 3.4 grams per vehicle mile; carbon monoxide 39.0 grams per vehicle mile; and oxides of nitrogen 3.0 grams per vehicle mile.

The presence of such standards has made it imperative that the exhaust emissions from vehicle engines be tested and analyzed to determine the relative amounts of impurities therein. Much effort has gone into the development of equipment for use in this field of gas sampling and it is now known to pass gases from an internal combustion engine through test apparatus at an accurately controlled flow rate for purposes of determination, and analysis, of the relative amounts of impurities. Since the engine is breathing ambient air, and since such air is itself a source of contaminants, care is taken that the analysis accurately reflects the contaminants introduced by the engine only. The apparatus currently available for sampling provides for introduction of diluent air and combining of the same with the exhaust gases. A proportional part of the diluted exhaust emissions is collected continuously, for subsequent analysis, using a constant volume (although variable dilution) sampler. A system which satisfies these requirements, as set out by the Environmental Protection Agency, is shown in diagrammatic form and described in the identified portion of the Federal Register. (See also FIG. 1 hereof.)

The apparatus as illustrated and described utilizes a positive displacement pump, specifically a Roots blower, to provide the constant volume flow which is a prerequisite to successful operation of the equipment. The volumetric flow rate is determined as a function of the number of revolutions of the positive displacement pump.

Apparatus of this kind is subject to a number of difficulties and disadvantages among which are the following. Such equipment customarily operates in what can be termed a hostile environment. That is, the exhaust gases contain corrosive substances as well as particulate matter. Such matter tends to build up within the constant displacement pump, gradually diminishing the fine working clearances and causing variation in displacement, and even pump failure. Because of these problems periodic shutdowns of the apparatus are necessary for maintenance and cleaning purposes. In addition it has not been possible completely to eliminate leakage across the pump, which is a function of gas temperature and differential pressure. The leakage factor must be taken into account in conducting tests with the equipment known heretofore, and maintenance of this factor within predetermined limits requires relatively frequent calibration tests of the positive displacement pump, in order to insure continuing accuracy of flow measurement. Further, it has been found that when such pumps are used in the mentioned hostile environment pump damage results from the presence of excessive heat as well as the corrosive elements referred to above.

There is growing emphasis on turbine type engines and these engines must also be subjected to exhaust testing. As can readily be appreciated, a constant displacement pump system of the kind shown and described in the Federal Register would have to be of enormous proportions in order to deal with the output of such engines, particularly when it is recognized that the pump also must handle the dilution air admitted into the system.

SUMMARY OF THE INVENTION I have discovered that all of the foregoing difficulties and limitations can be eliminated in relatively simple manner by utilizing a different constant flow device in place of the positive displacement pump used heretofore. For this purpose I use a venturi device in combination with a pump of capacity sufficient to insure that the venturi operates at conditions of critical flow, that is, under conditions such that gases reach sonic speed in thethroat of the venturi. It will be recognized by those skilled in the art that the maximum gas coreelary it has been recognized that the gas, by

weight, reaches its maximum flow rate when passing through the restriction under the sonic, or critical, flow condition.

Since a venturi device operated in this way is inherently a constant volume flow device, I subject the system to sufficient pressure difference across the venturi to insure that the sonic condition is reached and maintained throughout the test. For this purpose I prefer to employ a simple centrifugal air pump, or blower, coupled to the expansion side of the venturi and of capacity sufficient to maintain critical flow of the mixture of exhaust gases and diluent air through the venturi. If desired it is possible, in accordance with the principles of my invention, to use a plurality of constant flow venturi gas samplers in parallel, each fed from a separate engine and preferably with a single centrifugal pump or blower.

With the foregoing in mind it is the general objective of this invention to provide improved sampling apparatus which will operate in a predetermined range of flow and volume in an entirely reliable and predictable manner, and with minimal deterioration of critical components and virtual elimination of problems due to particulate accumulation. More specifically, my improved apparatus operates with such accuracy of flow measurement that deviations of only 1 percent in flow rate are currently achieved, and this with a reduction of audible noise output and greatly prolonged life of the equipment. It is possible to scale flow rates up or down relatively easily, and I have built units which are rated at 100, 300 and 350 CFM (standard air).

The periodic calibration tests presently required on prior art systems are not necessary with my apparatus and, in one embodiment, temperature changes are no longer a limitation. Additionally, variations in leakage rate are no longer of consequence. A very important advantage of the improved system is that it is readily usable in the sampling of exhaust emissions from turbine engines.

In considering the nature and importance of the present improvement, it is desirable to record that while the critical flow characteristics of venturis have long been recognized, it has not previously been known to utilize this characteristic to achieve the advantages mentioned above. This fact is particularly significant when it is borne in mind that in recent years great effort and expense has been devoted to the production of emission sampling equipment, and yet those engaged in these efforts have not advanced the art beyond the system described in the Federal Register.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustration of the prior art exhaust gas sampling system illustrated and described 1 in the Federal Register;

FIG. 2 is a perspective view of the actual hardware comprising a sampling system constructed in accordance with the present invention;

FIG. 3 is a diagrammatic view of the'apparatus illustrated in FIG. 2;

FIG. 4 is a sectional view, on a larger scale, illustrating a critical flow venturi of the type used in the apparatus illustrated in FIGS. 2 and 3;

FIG. 5 is a diagrammatic view of a modified'form of the invention employing a plurality of constant flow of venturi devices arranged in parallel; and

FIG. 6 is a perspective illustration of modified apparatus in which a heat exchanger is not employed, and which apparatus is particularly suited for testing gas emissions from turbine engines.

DESCRIPTION OF THE PREFERRED EMBODIMENT during the entire test. It will be understood that when a vehicle engine is under load, a larger portion of the mixture will be exhaust gas. The approved equipment is so designed that only a very small portion of the mixture is comprised of contaminants. Small samples of the mixture are drawn off and diverted to analyzing equipment (not shown). In such apparatus, as long as flow through the equipment is maintained constant, the proportion of gas drawn off will always be uniform. Since all of the contaminants (after suitable correction for the atmospheric conditions prevailing on the day of the test; see probe 18 and bag 14) can be assumed to come from the vehicle exhaust, sampling of a constant fraction will permit calculation of the grams of contaminant included in the entire stream of exhaust. As noted above, a Roots blower has been used as the constant volume displacement device, and counting the number Reference is first made to FIG. 1, which illustrates present invention has elements in common with this system. However, as will now be understood, it incorporates a critical flow venturi and associated centrifugal blower in place of the positive displacement pump appearing in FIG. 1.

In operation of the known apparatus shown in FIG. 1 the total volumetric flow of exhaust gas, derived (throughout a simulated driving cycle) from tail pipe adaptor l0, and from air introduced through the dilution inlet 11, is metered by passing through a positive displacement pump identified with that term and bearing the reference numeral 12. Samples of the exhaust gas and diluent air flow through suitable conduits and are collected in the bags or receptacles shown, respectively, at 13 and 14. Filters are introduced in the conduits where required, see for example the three-part filter assembly 15 disposed in air inlet 11, and also the filters appearing at 16 and 17 in the gas sampling lines which are fed by

probes

18 and 19. Three-way solenoid valves 20 and 21 direct sample streams either to their

respective bags

14 or 13 or to discard. Leak-tight fittings and suitable shut-off devices are also employed, as are

counter devices

22 and 23 which count the revolutions of the constant displacement pump while tests are in progress and samples are being collected.

In approved apparatus, incorporating as it does a constant displacement pump, mixing of the entire stream of gas from an internal combustion engine with a quantity of diluted air, insures that a constant volume of the mixture will be translated through the equipment of revolutions of the pump or blower provides a direct indication-of the quantity of mixture displaced per unit of time. It is this blower which has the limitations, and

presents the difficulties, which my invention overcomes.

In a system of the kind shown in FIG. 1 the inlet gas temperature to the positive displacement pump 12 must be substantially constant, in order that the pump may serve as a constant volume metering device. A heat exchanger H is illustrated in FIG. 1, and this heat exchanger, as is apparent from the legends appearing on the drawing includes means for both heating and cooling the gases. This exchanger, which need not be described in further detail herein, meets the requirements of constant volume flow.

During an exhaust emission test, samples are collected in the receptacles, or bags, 13 and 14, and analytical tests are made to determine the hydrocarbon, carbon monoxide and oxides of nitrogen mass emissions. In making such determinations use is made of suitable analytical apparatus which is also shown diagrammatically and described in the Regulations. Such analytical equipment need not be illustrated or considered herein, since, per se, it forms no part of the present invention.

Under the present standards a sample-collecting test is conducted while simulating an average trip (e.g. 7.5 miles) in an urban area, assuming a cold start. A proportionate part of the diluted gas emissions is collected continuously for analysis, such proportionate part being diluted with air to a constant volume.

For further and more detailed description of the system of FIG. 1, and of the manner in which the apparatus thereof is controlled and energized, reference should be had to the Federal Register which includes a complete description of this system, including the apparatus (not shown) for conducting gas analyses.

Apparatus of this kind, which as noted constitutes the present standard, is subject to the difiiculties and disadvantages set forth above. In essence the present invention eliminates thesedifficulties by separating the air metering function from the air pumping function. This is done by utilizing a critical flow venturi, for the metering function, in combination with a simple and compact centrifugal blower. Apparatus of this improved type is shown in FIGS. 2 and 3 now to be described. Later in this specification reference will be made to FIG. 5, in which a plurality, for example two,

venturis are used in place of the single venturi device of the apparatus shown in FIG. 3.

Referring first to FIG. 2, there is illustrated apparatus which has yielded very good results. This apparatus is shown housed in a cabinet 24 and comprises an inlet conduit 25 adapted for connection to the exhaust pipe of an internal combustion engine (not shown). Environmental air flows down through the stack 28 into a chamber 27, and thence upwardly through a conduit 26 to join the exhaust gas inletted through 25. The combined gas and air pass through conduit 29 into a separator 30. This separator is of known centrifugal type and functions to remove from the air stream fine particles which are introduced either through the conduit 25 or the stack 28. After passing through the centrifugal separator the mixture of gas and air moves through a conduit 31 and enters a heat exchanger 32 which leads, through a passage 33, to a critical flow venturi shown generally at 34. The heat exchanger is of a kind shown diagrammatically in FIG. 1, and constitutes an element which was hitherto essential to air sampling systems. As explained later in this description, my invention also contemplates embodiments which omit the heat exchanger. This is particularly important where exhaust emissions from a turbine engine are to be sampled, since the heat exchanger would be excessive in size.

The improved apparatus of FIG. 2 also includes a centrifugal blower 35 which is coupled, by a sleeve and clamp assembly shown at 36, to the outlet or expansion side of the venturi device 34.

The system of FIG. 2 is shown diagrammatically in FIG. 3, which latter figure bears reference numerals corresponding to those appearing in FIG. 2. The centrifugal separator 30 is not illustrated in FIG. 3, but it should be understood that such a separator is preferably employed. It has the advantage of producing an intimate mixture of particulate matter, exhaust gas and air. As is the case with the positive displacement pump system of FIG. I, probes 18a and 18b are, respectively, disposed to sample the diluent air and the mix- The length of the venturi along its axis, measured from the inlet end at 37 to the outlet 38, which latter flares outwardly from the expansion portion 39, is equal to 14.25 inches. The throat of the venturi is defined by the narrow annular area represented in the drawing at 40 and having a dimension parallel to the axis of the venturi and equal to 0.100 inch. The inlet portion of the venturi can be considered as being measured from the reference line 40a to the plane of the inlet orifice 37 and is equal to 2.528 inches. The flare angle of this inlet portion is 30 with respect to the axis of the venturi.

The expansionportion of the device extends from the reference line 40b to a lefthand line of reference shown at 41, a distance equal to 9.062 inches and at a flare angle of 6. From the plane of termination of the ture of exhaust and diluent air prior to delivery of the samples to air bags, not shown. The blower 35 of the system of FIG. 2 also appears in FIG. 3, and it is believed that operation of the apparatus of this diagrammatic figure will now be understood without further description. It should, however, be recognized that a system in accordance with FIGS. 2 and 3 would also include the filters, valving and gas collection instrumentalities shown in the apparatus of FIG. 1, as well as the necessary switching apparatus. The revolution counters of FIG. 1 are, of course not required in a system of the kind diagrammatically represented in FIG. 3, since the system of the latter figure, as well as the other embodiments of this invention, employ a venturi device rather than a rotating positive displacement pump.

Turning now to FIG. 4 there is seen an enlarged cross sectional illustration of the venturi device 34. This device is specifically designed to achieve a proper flow rate, in this case 300 CFM, when operating under conditions of critical flow. The venturi is the result of mathematical analysis and development tests which require no treatment herein, since the nature thereof will be understood by those skilled in the art, it only being necessary to specify the desired rate (300 CFM at critical flow).

expansion portion of the venturi, i.e. reference line 41, the device flares at an angle of 279/minutes, with respect to the axisand terminates in the exit orifice located in the plane shown at 38 in FIG. 4.

The venturi device is fabricated of suitable corrosion resistant steel, and has a wall thickness of 0.25 inch. The tubing 33 which extends into adjacency with the inlet of the venturi terminates in a flanged portion 42 which confronts a similar portion 43, formed on the venturi inlet. A circular clamp 44 serves to secure the two flanges in confronting relation. At the opposite end of the venturi device the flared outlet is provided with a circumferential flange 45 which is shaped and disposed to cooperate with a sleeve 46 forming a part of the housing of centrifugal blower 35. As mentioned earlier in this description, a sleeve of resilient material surrounds the

portions

45 and 46 and is secured by clamping structure 36. i

As will be understood from the foregoing description, the venturi 34 serves as a means for metering the flow of gaseous mixture and maintains the flow rate of said mixture within very close limits. Since the mixture of exhaust gas and diluent air flows through the venturi throughout a test, and because the pump means coupled to the expansion side of the venturi restriction is of capacity sufficient to insure that the restriction operates at conditions of critical gas flow throughout any test period, the combined venturi and pump operate, inherently, as a constant volume flow control device.

FIG. 5 illustrates a modified embodiment of the invention in which two systems of the kind illustrated in FIG. 3 are operated in parallel, with common manifolding 47 adapted to receive the outflow of two venturi devices 34a and 34b. A single blower 35a, or other pressure reducing device, is coupled to the common manifold 47, with provision for final exhausting of the output at 48. Such a multiple system, which may include more than two venturi devices in parallel, is adapted to receive gaseous input from several internal combustion engines simultaneously, as is clear from the presence of exhaust pipe connections 25a and 25b. The

probes

18c and 18d would, of course, be coupled with gas analyzer equipment (not shown).

In constant volume gas sampling, using embodiments of the kind described thus far, the inlet gas temperature to the critical flow venturi must be substantially constant. For this reason a heat exchanger, which includes means for heating or cooling the gases, has been provided and illustrated in the preceding embodiments.

An alternate approach, which provides a constant percentage sample flow rate without the necessity of a heat exchanger, is illustrated in FIG. 6. in common with the systems already described, the mixture of exhaust gas and diluent air is fed to the inlet side of a venturi 34 which is coupled to an excess capacity centrifugal pump 35. Gas samples are derived through a probe comprising a small tube 19' which, in this embodiment, is provided with a venturi restriction as appears at R. A small blower B is provided. This blower is of sufficient capacity to maintain critical flow within the venturi R. it can readily be demonstrated that the quantity of flow taking place through the small venturi R, as a percentage of the primary stream flow through venturi 34', is constant and equal to the ratio of the areas of the two venturis. Since the flow through each venturi is a function of the temperature of the gas mixture, and since the same temperature prevails at each venturi, no temperature compensation is necessary and the ratio of the areas is solely determinative of the respective flows. As will be understood, samples of gas derived from the venturi R and the blower B would be fed to collection equipment and suitable analyzing apparatus. Apparatus of this kind is particularly useful in the testing of emissions from gas turbine engines. The term internal combustion, as used in the appended claims, should be understood as including such engines within its scope.

I claim:

1. In apparatus for sampling the gaseous emissions of internal combustion engines, for collection and analysis, a source of diluent air adapted for connection with a conduit carrying the exhaust of an engine, to produce a mixture of emissions and diluent air, means for deriving samples of said mixture for analysis, and means for regulating the flow of said mixture to maintain the flow rate substantially constant, said last means comprising: a venturi restriction having an inlet side and an expansion side, said inlet side being coupled in the apparatus to receive said mixture; and pump means coupled to the expansion side of said venturi restriction and effective to draw the mixture therethrough, said pump means being of capacity sufficient to insure that the venturi restriction operates at conditions of critical gas flow throughout test periods.

2. Apparatus in accordance with claim 1, and in which said means for deriving samples includes hollow probe means of restricted cross-section disposed in the stream of flowing mixture up-stream of said venturi restriction, and receptacle means coupled to receive the samples derived through said probe means.

3. Apparatus in accordance with claim 2, and further including second hollow probe means of restricted cross-section disposed in the stream of diluent air at a location to derive, for analysis, samples of such air at a region upstream of the point of connection of said conduit carrying the engine exhaust.

4. Apparatus in accordance with claim 1, and in which said pump means is of the centrifugal type.

5. Apparatus in accordance with claim 1, and further characterized by the inclusion of: means defining a plurality of flow paths disposed in parallel, each including its own source of diluent air and each adapted for connection to a separate engine; a plurality of venturi restrictions each individual to a correspondin one of said flow paths; and a single pump coupl in the system to draw the mixture flowing in each flow path through the venturi restriction included in that path.

6. Apparatus in accordance with claim 5, and in which said means for. deriving samples includes a plurality of probes of restricted cross-section, each probe being disposed to sample the mixture flowing in a corresponding one of said flow paths.

7. Apparatus in accordance with claim 1, and in which said means for deriving samples is disposed in the region of said venturi restriction and comprises: a conduit having a part disposed to receive a small portion only of said mixture; a part defining in said conduit an auxiliary venturi restriction; and means insuring that said auxiliary venturi restriction operates at conditions of critical gas flow during sampling.

8. Apparatus in accordance with claim 7, and further characterized in that the ratio of the minimum crosssectional areas of the two venturi restrictions is solely determinative of the quantity of flow through each.

9. In apparatus for sampling the gaseous emissions of internal combustion engines, for collection and analysis, a source of diluent air adapted for connection with a conduit carrying the exhaust of an engine, to produce a mixture of emissions and diluent air, means for deriving samples of said mixture for analysis, and means for regulating the flow of said mixture to maintain the flow rate substantially constant, said last means comprising: a venturi restriction coupled in the apparatus to receive said mixture; and pump means coupled to said venturi restriction and effective to cause flow of the mixture therethrough, said pump means being of a capacity sufficient to insure that the venturi restriction operates at conditions of critical gas flow throughout test periods.

10. In apparatus for sampling the gaseous emissions of internal combustion engines for analysis, means providing an inlet source of diluent air, means coupling said source with a conduit carrying the exhaust of an engine, to produce a mixture of emissions and diluent air, conduit means through which said mixture passes and including heat exchanger means effective to maintain the mixture at a substantially constant temperature throughout a test period, means coupled to said conduit means and effective to derive samples of said mixture after passage through said heat exchanger means and to collect said samples for analysis, and means for regulating the flow of said mixture through said apparatus to maintain the flow rate substantially constant, said last means comprising: means defining a venturi restriction having an inlet side and an expansion side, said inlet side being coupled to said conduit means to receive the mixture flowing therethrough; and a centrifugal pump coupled to the expansion side of said venturi restriction and effective to draw the mixture therethrough, said centrifugal pump being of capacity sufficient to insure that the venturi restriction operates at conditions of critical gas flow throughout test periods.

Claims

2. Apparatus in accordance with claim 1, and in which said means for deriving samples includes hollow probe means of restricted cross-section disposed in the stream of flowing mixture up-stream of said venturi restriction, and receptacle means coupled to receive thE samples derived through said probe means.
3. Apparatus in accordance with claim 2, and further including second hollow probe means of restricted cross-section disposed in the stream of diluent air at a location to derive, for analysis, samples of such air at a region upstream of the point of connection of said conduit carrying the engine exhaust.
4. Apparatus in accordance with claim 1, and in which said pump means is of the centrifugal type.
5. Apparatus in accordance with claim 1, and further characterized by the inclusion of: means defining a plurality of flow paths disposed in parallel, each including its own source of diluent air and each adapted for connection to a separate engine; a plurality of venturi restrictions each individual to a corresponding one of said flow paths; and a single pump coupled in the system to draw the mixture flowing in each flow path through the venturi restriction included in that path.
6. Apparatus in accordance with claim 5, and in which said means for deriving samples includes a plurality of probes of restricted cross-section, each probe being disposed to sample the mixture flowing in a corresponding one of said flow paths.
7. Apparatus in accordance with claim 1, and in which said means for deriving samples is disposed in the region of said venturi restriction and comprises: a conduit having a part disposed to receive a small portion only of said mixture; a part defining in said conduit an auxiliary venturi restriction; and means insuring that said auxiliary venturi restriction operates at conditions of critical gas flow during sampling.
8. Apparatus in accordance with claim 7, and further characterized in that the ratio of the minimum cross-sectional areas of the two venturi restrictions is solely determinative of the quantity of flow through each.
9. In apparatus for sampling the gaseous emissions of internal combustion engines, for collection and analysis, a source of diluent air adapted for connection with a conduit carrying the exhaust of an engine, to produce a mixture of emissions and diluent air, means for deriving samples of said mixture for analysis, and means for regulating the flow of said mixture to maintain the flow rate substantially constant, said last means comprising: a venturi restriction coupled in the apparatus to receive said mixture; and pump means coupled to said venturi restriction and effective to cause flow of the mixture therethrough, said pump means being of a capacity sufficient to insure that the venturi restriction operates at conditions of critical gas flow throughout test periods.
10. In apparatus for sampling the gaseous emissions of internal combustion engines for analysis, means providing an inlet source of diluent air, means coupling said source with a conduit carrying the exhaust of an engine, to produce a mixture of emissions and diluent air, conduit means through which said mixture passes and including heat exchanger means effective to maintain the mixture at a substantially constant temperature throughout a test period, means coupled to said conduit means and effective to derive samples of said mixture after passage through said heat exchanger means and to collect said samples for analysis, and means for regulating the flow of said mixture through said apparatus to maintain the flow rate substantially constant, said last means comprising: means defining a venturi restriction having an inlet side and an expansion side, said inlet side being coupled to said conduit means to receive the mixture flowing therethrough; and a centrifugal pump coupled to the expansion side of said venturi restriction and effective to draw the mixture therethrough, said centrifugal pump being of capacity sufficient to insure that the venturi restriction operates at conditions of critical gas flow throughout test periods.