following modification permits obtaining spectra free of solvent, absorption. I n this case, the cell with solvent is transferred to a small desiccator, the cell cover is removed and the solvent allowed to evaporate until the meniscus can no longer be observed (15 minutes is usually sufficient). ~h~ cell is then removed and the spectrum Obtained as Previously described.
( 6 ) Hoffman, R. L., Silveira, A., ANAL. CHEM.36, 447 (1964). (,) Leggon, H. Ibid., 33, 1295 (1961). (8)Lohr, L. J., Kaier, R. J., Facts & Methods 2, 1 (1961). (9) Swoboda, P. A. T., Nature 199, 31
LITERATURE CITED
(1) Beroza, bl., Gas Chromatog. 2, 330 (1964). (2) C h a w s. s.3 BrobSt, K. Me, Ireland, (1962). C. E., Tai, H., ‘pectr*
w.,
(1963).
(3) Chang, S. S., Ireland, C. E. Tai, H., ANAL. CHEW 33, 479(1961). (4) Drew, C. M., Johnson, J. H., J. Chrom. 9,264 (1961). ( 5 ) Grasseli, J. G., Snavely, M. K., A p p l . Spectr. 16, 190 (1962).
THISinvestigation was supported in part
by Public Health Service Research Grant EF-00099 from the Division of Environmental Engineering and Food Protect ion
Absolute Method for Measuring l o w Gas Flow Rates with High Accuracy F. W. Noble, Kenneth Abel, and P. W. Cook, Laboratory of Technical Development, National Heart Institute, National Institutes of Health, Bethesda, Md.
HE MOST commonly used equipment ‘items for measuring lorn gas flow rates are rotameters, soap film flowmeters, and devices which measure the pressure differential across a fixed orifice. Of these, only the soap film flowmeter is absolute-Le., does not require calibration against a primary standard. It has been shown (3) that the soap film flowmeter can, under carefully defined conditions, attain an accuracy of i1/4% for low flow rates of nonreactive, nonsoluble gases. The addition of conductometric ( I ) and photoelectric ( 2 ) relays to start and stop electrical timers can increase the accuracy slightly. An entirely theoretical design for a flowmeter similar in several respects to the one to be described has been published previously ( 4 ) . At the time of publication, the instrument had not been built so that it is impossible to compare the performance. Briefly, the device previously described is a continuous flowmeter which will indicate instantaneous as well as cumulative flow “on line.” The device to be described here is a discontinuous flowmeter which measures the average volume flow rate in an interval of time. This system is much simpler, less costly, and probably more reliable, although certainly not so flexible in application. The flowmeter described beloiv is absolute and, in addition, requires no vapor pressure correction, no operator attention, and can be used with any gas compatible with stainless steel and Teflon. The principle of operation can be seen from Figure 1. The flowing gas is connected to the unit by appropriate means. I n the “R” (return) position, the gas passes through a normally open valve and is vented to atmospheric pressure. Closing the switch to the “M” (measure) position closes the valve thereby diverting the gas to a gas-tight, motor-driven syringe, and turns on the drive motor. A volume displacement switch actuates
2001 4
CLUTCH
NORMALLY OPEN SOLENOID VALVE-
TIMER
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Figure 1.
Overall system diagram
a magnetic clutch through a transistorized clutch driver to withdraw the syringe piston for each incremental accumulation of gas. The switch contacts are adjustable and will provide more than 1000 increments for full syringe travel a t a pressure differential of about 0.1 mm. H20. The time required for the piston to be withdrawn through a known fixed volume is elect,rically timed. Microswitch “A” provides a reproducible starting point for the piston while microswitch “BJ’shuts off the timer a t a reproducible terminal piston position. Microswitch “B” also shuts off the drive motor and opens the diversion valve. Opening of the diversion valve deactuates the clutch driver. With the switch in the “R” position, the motor is reversed and the clutch drive actuated. The piston is driven back
toward its starting position until halted by microswitch “A,” A rotameter is placed in series with the gas connection point and the diversion valve t o ensure that the flow rate does not exceed the capability of the drive mechanism. A full electrical schematic is shown in Figure 2. The displacement switch was made in ou shop. The diaphragm is aluminum, free diameter 1 inch, thickness 0.35 mil. The contact is stainless steel, threaded to allow adjustment of initial gap. The solenoid valve is stainless steel with a 3/32-in~horiface operating on 115 volts, 60-cycle ax., Skinner Yo. V51D42126. The clutch is a Warner SFC-160,1-25512 with a 90-volt d.c. coil operated between 8 and 22 volts d.c. The motor is a Bodine No. 803PC035 Type KYC-23R8, 150 VOL. 37, NO. 12, NOVEMBER 1965
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Electrical schematic of switching and sensing circuit
rpm, 2.6 in.-oz. 115 volts 60-cycle a.c. The relay is a Potter and Brumfield K R P 11 AG, D.P.D.T., 115 volts 60cycle a.c. The timer is a CramerGiannini Controls Corp. Type 6333A08-E 1/10 second manually resettable unit. The microswit,ches are S.P.D.T., 5A, 250 volts ax., Model 1SR.Il-MS-
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ANALYTICAL CHEMISTRY
25085-1 made by Micro, Freeport, Ill. The piston withdrawal screw thread pitch is '/4-20. EXPERIMENTAL
The syringe lrolume between microswitches was determined by using the drive mechanism to fill and empty the
syringe with water. The displaced water was measured gravimetrically and the volume calculated from the density of water a t the syringe temperature. This volume was 101.43 i 0.03 ml. To determine reproducibility for various gas flow rates, a flow regulation of better than 0.03% was required. Commercial two-stage diaphragm-type pressure regulators normally found in laboratories are not capable of this degree of regulation. To achieve the desired flow regulation, high value capillary restrictors were prepared and connected directly to the high pressure shut-off valves of size 1A compressed gas cylinders filled to various pressures from 15 to 100 atm. The relative standard deviation for each flow rate measured between 1 and 100 ml./min. was found to be O.O3%-i.e., equal to the calibration reproducibility. The calibration reproducibility is dependent in this system primarily on the repeatability of the snap action of the two microswitches and not, except indirectly, on the number of incremental actuations of the volume displacement switch. The quantity measured by this device is the average volume flow rate of gas during the measurement interval. From the ideal gas laws it is clear that if one wishes to obtain other types of information such as, for example, the mass flow rate of a gas mixture of constant composition, it is necessary to know the temperature and pressure. In situations where the time required to collect 100 nil. of gas is large compared with the thermal time constant of the collecting cylinder, the temperature of the cylinder must be controlled. We have not investigated this effect experimentally. The repeatability figures given above were obtained under normal laboratory conditions where the room temperature fluctuations are estimated to be of the order of i1" C. LITERATURE CITED
J.. Chemist-Analust 54, 56 (i965j. (2) Hunter, J. J., J. Sci. Instr. 42, 175 (1965). (3) Levy, A.,Zbid., 41,449 (1964). (4) Swaay, van, >I., J. Chromatog. 12, 99 (1963). (1) G. ~, Frisone.
