promise between small volume and ease of use and construction, resulting in a cell that is practical for measuring infrared absorption spect,ra of neat volatile liquids of a few microliters volume and of neat nonvolatile liquids of a few tenths of a microliter volume.
ACKNOWLEDGMENT
LITERATURE CITED
The author acknoffledges the technical advice of R. s. LfClhmld and the helpful comments from Dorothy McClung, who used the cell for infrared measurements.
E.R.1 Parrish, N.,Bird, G . R., Abbate, M.J., J . Opt. SOC.A m . 42, 966
( 1 ) Blout,
(1952). ( 2 ) Cole, A. R. H., Jones, R. N., Zbid., p. 348. (3) J ~ R. Norman, ~ ~ Nadeau, ~ , Armand, Spectrochim. Acta 12, 183 (1958).
Pneumatic Dewpoint Meter William S. Pappas, Technical Division, O a k Ridge Gaseous Diffusion Plant, Union Carbide Corp., Nuclear Division, O a k Ridge, Tenn. PNEUMATIC response due to Condensation of moisture a t a restriction in a flowing system has been used in a family of moisture analyzers: ( I ) a simple portable dewpoint meter, ( 2 ) a highly sensitive, accurate instrument, (3) a n automatic dewpoint alarm, and (4)a sensitive and accurate continuous moisture analyzer. ‘I’hese instruments are characterized by their sensitivity, low cost and maintenance, and relative independence of operator judgment. The analyzers provide pneumatic signals to be applied directly to control systems. The principles are generally applicable t’o determination of condensables, including corrosive compounds. These low cost analyzers were developed to meet a need for intermittent and continuous measurement of dewpoints, especially a t low moisture levels. Most methods ( I ) for moistme determination have limited sensitivity and accuracy a t low concentrations and some are dependent upon operator judgment. Subsequent to the completion of this work, Keidel introduced the Electrolytic Hggrometer ( 2 ) which i;s applicable to a wide variety of gas media; however, it is subject to several interferences (2). Also, trace contaminants such as methanol and basic gases are detrimental to the detector. While the Electrolytic Hygrometer is sensitive to moisture as low as 1 to 2 p.p.m., commercial instruments generally have limited application at lower levels. This report describes the development of a simple analyzer for the determination of condensables, with emphasis on its application to measuring trace moist>urein air in the range of 0.02 to 10 p.p.m. THE
Time lndicolor
COOLING CURVE
Figure 1.
Model I-manual
pressure. The measured dewpoint temperature can be corrected to standard (atmospheric) pressure conditions. For continuous measurement, the pressure signal may be used to operate the restriction heater to maintain a partial condensation plug. Under these conditions the temperature of the restriction follows the dewpoint temperature of the gas being tested. MANUAL DEWPOINT METER
Two portable analyzers, differing slightly from each other, were developed for intermittent dewpoint measurement.
PRINCIPLE
Model I-Simple Portable Instrument. As shown in Figure 1, sample
Figure 1 represents a typical sensing system. For a gas passing at a controlled rate through a progressively cooled restriction, an abrupt increase in the differential pressure occurs when condensation causes plugging. The restriction temperature, when the pressure deflection occurs, corresponds closely to the dewpoint of the ?;as a t the system
gas enters the analyzer at constant pressure P I and leaves a t constant pressure P 3 . Pressure P p , the variable pressure, is dependent principally upon the size of the “detector” restriction which is subject to rapid change when condensation occurs. To make a dewpoint measurement, a cooling bath of liquid nitrogen is applied to the cold finger. With
dewpoint meter
sample gas flowing, the restriction is progressively cooled, resulting in a gradual decrease in pressure P , due to thermal effects on gas density and viscosity. An abrupt pressure change occurs a t the condensation point as shown in the cooling curve (Figure 1). The temperature a t the restriction when the pressure deflection occurs is taken as the dewpoint. DETECTING RESTRICTION.For dewpoint measurements above -90” F. a small capillary or porous plug restriction is satisfactory. More sensitive response is obtained with a slit-type restriction, prepared by four simple operations. I n the first, a 3/8- or inch copper tube is pinched with a standard pinch-off tool. In the second, the tube is sawed into two Iiieces, either one being useful as a restriction; in the third, the pinched end is smoothed with a file to seal the slit opening. In the final operation, the slit is opened by applying mechanical iiressure at the slit ends to allow about 15 cc. per minute flow a t 1 1i.s.i. pressure differential. For best sensitivity a slit of uniform rectangular cross section is desirable and is achieved with little VOL. 36, NO. 9, AUGUST 1964
0
1885
n Table 1.
Routine Dewpoint Measurements of Air
Pneumatic Dewpoint Meter (Model I),
RefrigeratedMirror-
Type Analyzer, O F .
O F .
-88
Sensitive Range
-90 -110 -110 - 102
- 105 - 105 - 100 - 104 - 110
- 108 - 110
-110 -110 -78 - 104 -110 - 110
O i l Densify
-110 -110 -78 - 106 -110 -110 -90 -88 -88 -98
-90 -88
-88
-95
0.87
Pressure Input
B
Figure 2.
practice. The slit-type restriction is sensitive to very low dewpoints and also to dewpoints above 32" F. (liquid condensation). PRESSURE INDICATOR. A suppressed range manometer (Figure 2) was designed for observing the variable pressure Pz. This manometer provides greater sensitivity than a mercury manometer, without the height of a simple oil manometer. The degree of suppression is adjusted by moving vertically the position of reservoir C; the sensitivity is a function of the relative diameters of tubes A , B, and C, as well as the angle of the sloped tube and the density of the oil. The design variables are related: Aa
=
AP
[$+ $ + (sin
6 - $)E]
Suppressed range manometer
where Aa = oil deflection along sloped tube, mm. AP=change in input pressure, mm. Hg A , B, and C = tube cross sectional areas 6 = included angle of inclination and R = density ratio of oil to mercury The bracketed denominator is a constant for a given configuration. For the design shown in Figure 2, a pressure change of 1 mm. Hg causes a 20-mm. shift in the oil meniscus. ACCURACY AND PRECISION. Dewpoint temperature values obtained in routine operation are shown in Table I, compared with those obtained with a standard refrigerated-mirror-type dewpoint instrument. Excellent agreement was obtained on most samples, using the first, visual indications on both instruments. Where disagreement occurred, the lower denpoints obtained with the
180
mirror instrument were attributed to difficulty in detecting the initial frost formation. The values obtained with the Pneumatic Dewpoint Meter were reproducible to within 1" F. upon remeasurement. The measurements shown in Table I were made with a positive pressure of about 50 mm. Hg at the detector inlet and were not corrected to atmospheric pressure conditions. For dewpoints in the region of -110" F., the corrected dewpoints would be 0 . 8 " F. lower than those shown in Table I, assuming initial frost, formation was a t the inlet of the detector restriction. Therefore the pressure bias is usually negligible when measuring low dewpoints. Contributing to the variance obtained with the simple portable model at low moisture levels is the manner in which pressure Pz responds to the temperature decrease a t the restriction. The decrease in pressure is primarily due to increased gas fluidity and density a t the cooled restriction. Figure 3 shows the gradual decrease in pressure, until the condensation temperature is reached, and the subsequent reversal
I50
e
=
120
-ue
-
pz
N
i
L
a
I
w 3
To Monameler
TIME
COOL ING CURVE
v)
Y)
K w
a
60
- 79
T E M P . , 'F.
-99
-111
-117
Defecfor Reslricfion
-120
30
0
Figure 3.
1886
I
2 3 TIME, minuter
4
Pressure vs. cold restriction temperature ANALYTICAL CHEMISTRY
5
Figure 4. Model Il-improved temperature compensation
dewpoint
meter
with
Figure 5 .
Pneumatic dewpoint meter (Model II) for moisture in air
Toble II. Sensitivity and Accuracy of Improved Portable Dewpoint Meter (Model II) Sample 1 2
Dewpoints, O F . Make-up Measured -125
-142
Moisture, P.P.M. H,O Make-up Measured
-123 to -130
-141 to -144
0.15 0.023
0.17 t o 0 . 0 9 0.027to0.019
Conlrol P0i"l
of the pressure. A discrepancy of about 2" F. occurs between the point A , where the pressure begins to change rapidly, and point B where the pressure is a t a minimum. A fairly accurate dewpoint temperature may be obtained a t the minimum pressure; however point A more nearly indicates the point of incipient condensation. The measurements in Table 1 were probably taken closer to point B than to point A , since the pressure signals were not recorded, the operator depending on first visual indications. Model 11-Improved Portable Instrument. An improved instrument, illustrated in Figure 4, has been designed for application where better sensitivity and accuracy are desired. By using two cooled restrictions and measuring the pressure between the restrictions, the variable pressure drift noted with the simple portable model prior to the "end-point" is greatly reduced. By design, the detector restriction cools slightly faster than the exit restriction, resulting in the small pressure rise before the dewpoint temperature is reached. I t should he noted that P, is measured after the coolest restriction in this instrument, whereas in the earlier model, Figure 1, P1 was measured before the coolest restriction. The changes in P, are therefore in opposite directions, so that plugging of the inlet restriction results in a pressure drop at P,. Since a much smaller range of pressures is observed, a more sensitive suppressed range manometer can be used. In practice more rapid cooling of the restriction is possible without "overshooting" the dewpoint, as might happen with the simple model. Less than 10 minutes time is required for a dewpoint measurement. The instrument, shown in Figure 5, continues to provide accurate analyses after several years of use, with little maintenance required. SENSITIVITY AND ACCURACY. This design gave excellent sensitivity and accuracy of moisture measurement in the parts per billion range. Presented in Table I1 are measurements made on dry air samples standardized by passing air slowly through a copper coil maintained at constant low temperature.
lndicoling Pressure Switch CONTINUOUS AND AUTOMATIC MOISTURE MONITORING
Two types of automatic analyzers have been developed: a simple alarm and a moisture monitor. Constonl Pressure 70 mm. Hg
+
E x i l Reslriclion
1
To Coolina Syslem Dewpoinl Recorder
Figure 6 . Automatic continuous moisture analyzer
Automatic Dewpoint Alarm. Where only a n indication of excess moisture in air is required-such as where dry air with a dewpoint below -90' F. is desired-a simple device suffices. The temperature of the cold restriction is controlled a t the desired level. When the dewpoint of the sample gas exceeds the controlled temperature, the pressure P1 (Figure 1) increases and activates an alarm through a transducer. Automatic Continuous Moisture VOL. 36. NO. 9, AUGUST 1964
1887
Monitor. For application to continuous monitoring such as on t h e output of a n air drying plant, a n automatic continuous analyzer, Figure 6, includes a temperature recorder and control system for maintaining the restriction temperature a t the dewpoint. The temperature adjusting system consists of a restriction heater controlled by a switch activated by pressure Pz. Pressure P1was set a t 70 mm. Hg and Pz a t 24 mm. Hg above atmospheric, when putting the analyzer into use. When condensation occurs a t the restriction, pressure PB decreases and activates the heater, which volatilizes the condensate from the restriction
and causes pressure P1 to increase. The cycling continues in this manner with the restriction partially plugged, providing a continuous recording of dewpoints. The heater is positioned so that it warms the exit restriction faster than the detector: this results, during the warming cycle, in a modest rise in pressure Pp due to thermal effects on the exit restriction. This effect gives proportional dampening control, reducing the amplitude of the detector temperature oscillations to less than 1 " F. Readings of -75" =t1 O F . were obtained on air which was delivered through a copper coil maintained a t -75" F. immediately upstream of the detector.
ACKNOWLEDGMENT
o. H. Ottinger
assisted in the design the improved portable evalUatedbY w. D. Cline. Of
LITERATURE CITED
(1) Considine, D. M., "Process Instruand controls ~ ~ ~ d Mcb ~ Graw-Hill, N~~ yo&, 1957. (2) Keidel, F. A., ANAL.CHEM.31, 2043 (1959). Division OfACS, Boston, Mass., 135th Meeting, April 1959. This work wa8 performed at the Oak Ridge G~~~~~ Diffusion Plant erated by Union Carbide Corp. for U. S. Atomic Energy Commission.
tfi
lntrascintillation Vial Reaction Tube G. G. Slater, Edward Geller, and Arthur Yuwiler, Neurobiochemistry Laboratory, Veterans Administration Center and Departments of Psychiatry and Biological Chemistry, U. C. L. A. Center for the Health Sciences, Los Angeles, Calif.
evolved isotopic carbon dioxide has been generally employed in applications varying from assessment of cellular respiration to quantitative analysis. I n our laboratory a simple, inexpensive, and versatile apparatus has been routinely employed for rapid collection of evolved C1402 by liquid scintillation. The apparatus (Figure 1) consists of a three-holed, flanged-top incubation chamber ( B ) made from 16-mm. borosilicate glass tubing which fits into a standard 28- X 57-mm. scintillation vial (A) containing Hyamine and the apparatus is sealed EASUREMEXT O f
Figure 1. Reaction tube mounted in scintillation vial A. 6. C.
Scintillation vial containing Hyomine Reaction tube Containing reaction mixture Rubber sleeve top serum stopper
1888
ANALYTICAL CHEMISTRY
with a rubber sleeve stopper (C), type E, Aloe Scientific Company. As used in our laboratory for measurement of decarboxylase activity, 0.2 to 1.0 ml. of Hyamine hydroxide in methanol [p-(diisobutylcresoxyethy1)dimethylbenzylammonium hydroxide] (2) is placed in the bottom of the scintillation vial; 1.5 ml. of reaction mixture containing enzyme and buffer is placed in the reaction chamber (a maximum of 2 ml. can be used); the apparatus is sealed, and placed in a Dubnoff metabolic shaker. The clamps intended for 25-ml. Erlenmeyer flasks hold the vials snugly in place and the use of both the gas and bath covers ensures constant internal temperature in the shaker. In experiments where the methanol may inhibit the enzyme the Hyamine can be injected a t the end of the incubation. Air can be removed by flushing with N2for 2 minutes; two hypodermic needles serve as inlet and outlet tubes. The reaction is initiated by injection of 0.1 ml. of C14substrate into the reaction chamber. After appropriate time the reaction is terminated by injection of 0.2 ml. of HzSOa and after another 30 minutes, absorption is virtually complete (Figure 2). The reaction tube is removed, scintillation solution added, a cap placed on the scintillation vial, and the number of counts determined in the scintillation counter. Baker, Feinburg, and Hill ( I ) suggested collecting the C 0 2 in a small inner tube or qide well, but their system still requires the transfer of the absorbing fluid to a counting tube. Our apparatus has the advantage, besides simplicity and economy, of eliminating liquid transfer before scintillation count-
'
2'0
4b & MINUTES
8'0
100
Figure 2. Variation of count rate with time after termination of the reaction with H2S04
ing. I t provides a short gas path to minimize diffusion time and requires relatively little space so that a large number can be conveniently used in one experiment. LITERATURE CITED
(1) Baker, Nome, Feinberg, Harold, H111, Robert, ANAL.CHEM.26, 1504 (1954). ( 2 ) Passmann, J. M., Radin, N. S., Cooper, J. A . D., Ibid., 2 8 , 484 (1956). WORK supported by the Veterans Administration, National Institutes of Health, Public Health Service Grant No.
B-3556, Sational Science Foundation Grant No. GB-651, California State Department of Mental Health Grant No. 60-2-15.
~
k
