A simple and accurate apparatus for the generation of a calibrated water vapor pressure

Transcription

1 Agricultural and Forest Meteorology, 57 ( 1991 ) Elsevier Science Publishers B.V., Amsterdam A simple and accurate apparatus for the generation of a calibrated water vapor pressure P.M. Cortes l, C.F. Reece and G.S. Campbell Grain Legume Research, 215 Johnson Hall, Washington State University, Pullman, 1~ USA (Received 28 March 1991; revision accepted 24 June 1991 ) ABSTRACT Cortes, P.M., Reece, C.F. and Campbell, G.S., A simple and accurate apparatus for the generation of a calibrated water vapor pressure. Agric, For. Meteorol., 57: Calibration of plant gas exchange equipment and atmospheric vapor sensors requires a gas stream of known vapor pressure. Previously used methods which rely on temperature control are slow and complicated. Here, we describe a simple apparatus for the production of a stream of air with a known concentration of water vapor. The system employs the "two pressure" method in which air with a known water vapor pressure is released to a lower pressure with an accompanying dilution in vapor concentration. The calculated vapor pressure of the generated stream was compared with measurements using two types of dewpoint meters and found to have errors of less than _+ 1.5% over the range kpa. Less than 3 min were required to achieve stable vapor pressures after a step change. The apparatus described is inexpensive and can be easily made using commonly available materials. INTRODUCTION Water vapor pressure or concentration is frequently measured in meteorology and in connection with the characterization of plant gas exchange. For these measurements, periodic calibration of the measuring instruments is necessary. A calibration stream of air with a known water vapor concentration is usually obtained by cooling saturated air in a controlled temperature bath. In order to generate different concentrations, the water bath temperature must be changed. This requires considerable time for a calibration. In this paper we describe an accurate, fast, simple and inexpensive instrument for generating a stream of air with water vapor at a desired concentration. t Present address: USDA-ARS, Cropping Systems Research Laboratory, Route 3, Box 215, Lubbock, TX 79401, USA /91/$ Elsevier Science Publishers B.V. All rights reserved.

2 28 P.M. CORTES ET AL MATERIALS AND METHODS The instrument uses the "two-pressure" method for generating water vapor streams (Weaver and Riley, 1948; Amdur and White, 1963; Challa, 1976; Cortes et al., 1988 ). Saturated air from a chamber with an elevated total pressure Pc and a vapor pressure e is released to a lower total pressure Pa. Assuming ideality, the vapor pressure at the lower pressure is given by ea=e Pa/Pc (both pressures absolute) (1) By adjusting Pc, the desired vapor pressure, ea, can be achieved. The air inside the chamber is assumed to be in equilibrium with the water and its vapor pressure can be calculated using the formula (Buck, 1981 ) e =fx exp [( Tc/227.3)Td(Tc )] (2) where e is in kilopascals, Tc is the temperature of the chamber water ( C), andfis an enhancement factor. The nature and partial pressures of gases other than water vapor present in the chamber influence e. Equation (2) contains an enhancement factor to account for the increase in vapor pressure caused by the additional gases in the moist air. Buck ( 1981 ) provides values for the enhancement factor and several formulae by which it can be computed as a function of temperature and total pressure. Although Buck's formulae are optimized for pressures below 0.1 MPa, his relationship f= pc (3) where Pc is given in megapascals, is in reasonable agreement with Webster's (1950) data for higher pressures. Weaver and Riley (1948) account for the increased chamber pressure by multiplying eqn. ( 1 ) by (1 -cpa)/(1 -cpc) where c is MPa- ~ for air. The apparatus illustrated in Fig. 1 was constructed to produce saturated air at an elevated pressure. The outer housing is a cylindrical stainless steel chamber able to withstand high pressure. In order to facilitate saturation, air entering the cnamber passes through a sintered glass block which produces a stream of fine bubbles. Before exiting the chamber, saturated air from the space above the water in the chamber is drawn through an interior coil immersed in the water. The coil is necessary because, under normal operating conditions, the air above the water can be 1-2 C warmer than the water. The accuracy of the instrument is limited by the accuracy of measurement and control of the temperature of the air at the warmest point at which it is saturated before leaving the chamber. Without the coil this point is difficult to control. By cooling air from the top of the chamber to the water temperature (4)

3 APPARATUS FOR CALIBRATED WATER VAPOR PRESSURE GENERATION 29 ~g pier 220 ling 3 pie ~k Fig. I. Vapor generator chamber. All dimensions are in millimeters. and trapping condensed water vapor and droplets carried in the air stream, the temperature of the air trap can be reliably used for this warmest point of saturation. In a series of tests, there was no difference between the temperature of the air in the water trap and that of the surrounding water. The water temperature, measured with a copper-constantan thermocouple was therefore used in computing the saturated vapor pressure in the chamber. To measure its accuracy and performance, the vapor generator was connected as indicated schematically in Fig. 2. The pressure in the chamber was established using a two-stage regulator and measured using a mercury manometer. Flow through the system was controlled by a needle valve and was measured using a rotameter. The dewpoint of the air emerging from the system was determined using a dewpoint-based water activity meter (Model CX- 1, Decagon Devices, Pullman, WA) and a dewpoint hygrometer (Model 990, Cambridge Systems Inc., Newton, MA). The output from the Model 990 hygrometer and the thermocouple in the chamber water was recorded automatically using a datalogger (Model 21 X, Campbell Scientific, Logan, UT ). The pneumatic connnections were made using nylon tubing 6 mm in diameter. To prevent loss of water by adsorption and absorption, the tubing was

4 30 P.M. CORTES ET AL. Fig. 2. Experimental setup. CA, compressed air; VGC, vapor generator chamber; HgM, mercury manometer; NV, needle valve; FM, flow meter; DPM, dewpoint meter; TC, thermocouple; DP, dewpoint; DL, data logger. Pneumatic connections are indicated by solid lines, electrical conneclions by dashed lines. heated slightly by passing a small current through a high resistance wire wrapped around it. The flow-controlling needle valve was also heated to compensate for cooling owing to the expanding air. The pressure in the measuring chamber of the dewpoint meters (Pa) was determined, using a mercury manometer, to be at atmospheric pressure. RESULTS The chamber pressure (Pc) was varied so as to produce vapor pressures in the range of kpa. The generated vapor pressure, ea, was calculated using eqns. ( 1 )-(3 ) and the percent error of the calculated vapor pressure is shown in Fig. 3. The percent error was computed as Percent error= 100 (measured vapor pressure- ea)/ea (5) Over the range of vapor pressures produced, the error in calculated water vapor pressure was less than + 1.5%. Using Weaver and Riley's (1948) correction (eqn. (4)) instead of Buck's ( 1981 ) enhancement factor (eqn. (3)) yielded similar results for the percent error. The nature of the error is not clear. The errors are within the listed accuracy of the dewpoint meters so they could be the result of errors in either the calibration or the meters. However, the errors for both meters show similar trends, particularly at the humid end of the range. The disparity does not seem to be the result of errors in the enhancement or correction factors because they are

5 APPARATUS FOR CALIBRATED WATER VAPOR PRESSURE GENERATION b. I..U o 8 (3_ O o o 0 0 o O.S Calculated Vapor Pressure (kpa) 2.5 Fig. 3. Percent error of the calculated generated water vapor pressure plotted against the calculated value. Open circles, Model CX-I readings; solid circles, Model 990 readings ~ 1.1 ~L ~ 1.o (9 ~ 0.9 ~0~ 0.8 CL ~ 0.7" ~ O.6" ~ " lo " ~'0 ~'0 10 Time (min) Fig. 4. Time course of generated vapor pressure changes in response to chamber pressure step changes. Arrows indicate the times at which the chamber pressure was changed. Open circles, Model 990 readings; solid line, calculated vapor pressure. close to unity at the high vapor pressures, in which Pc was lowest and the percent error was the greatest. The response of the system to step changes in chamber pressure is shown in Fig. 4. One minute averages of the output from the dewpoint hygrometer and the vapor pressure calculated using eqns. ( 1 )- (3) are plotted as a function of time. After changing the pressure, the system achieved a stable reading within 3 min and no hysteresis was measured. The accuracy of the system as described was insensitive to flow rates up to 10 1 min-~, the highest rate measured. CONCLUSIONS The apparatus described is accurate, uncomplicated and easy to build. The system does not require the use of a controlled temperature water bath and, because the response to pressure changes is rapid, it will work well even if the chamber temperature is not maintained at a constant level. Modifications of

6 32 P.M. CORTES ET AL. the design presented can be readily incorporated in order to accommodate existing equipment or special applications. For example, a standard pressure chamber used to determine the water potential of plant tissue could be easily modified. The system can also be automated by using an electronic pressure transducer with a mass flow controller to adjust the chamber pressure (Cortes et al., 1988). Cooling water baths are needed in conventional unpressurized systems, as well as the design of Challa ( 1976 ), in order to produce dewpoints below the ambient temperature. In addition, vapor pressures corresponding to a dewpoint below 0 C (0.61 kpa) are difficult to achieve continuously in these systems because of the formation of ice. Under normal laboratory conditions, however, the vapor generating chamber operating at a chamber temperature, To, of 25 C and a chamber pressure, Pc, of 1.0 MPa, would have a saturated vapor pressure in the chamber, e',!, of 3.28 kpa. At an ambient pressure, Pa, of 0.1 MPa, it would produce a vapor pressure, ea, of 0.33 kpa, which corresponds to a dewpoint of -8.2 C. Vapor pressures below this range can be generated using higher chamber pressures or by cooling the chamber. Higher vapor pressures can be easily produced by heating the chamber, either electrically or with a water bath. Several factors were found to be important in the design. Tiny water droplets, present in the air above the water in the chamber, are carried with the air stream and it is important to ensure that they are eliminated from the air stream before it exits the chamber. Removing the droplets proved to be difficult in practice but the use of the internal cooling coil and the tangential attachment of the coil to the water trap, so as to swirl the entering air and force the droplets against the wall, was effective. Because of the sensitivity of the generated vapor pressure to the chamber pressure at low chamber pressures, it was necessary to use a mercury manometer for Pc below 0.1 MPa. A manometer was also used at higher pressures because of the hysteresis in output from gages. The water level in the chamber was not critical. However, the water level in the chamber was adjusted so that, at the working flow rates, all visible droplets were well below the air intake of the cooling coil. The simplicity and accuracy of the described system should make it useful for the routine calibration of equipment used for meteorological and gas exchange measurements. It may also be useful in generating air streams for physiological experiments in which response to rapid changes in humidity are important. REFERENCES Amdur, E.J. and White, R.W., Two-pressure relative humidity standards. In: A. Wexler and W.A. Wildhack (Editors), Humidity and Moisture. Measurement and Control in Science and Industry. Vol. 3. Fundamentals and Standards. Reinhold, New York, pp

7 APPARATUS FOR CALIBRATED WATER VAPOR PRESSURE GENERATION 33 Buck, A.L., New equations for computing vapor pressure and enhancement factor. J. Appl. Meteorol., 20: Challa, H., An analysis of the diurnal course of growth, carbon dioxide exchange and carbohydrate reserve content of cucumber. Versl. Landbouwkd. Onderz. 861 : Cortes, P.M., van der Beek, J. and Blom, J.H.G., Field gas exchange laboratory. Rijksinst. voor Onderz. in de Bos and Landschapsbouw "De Dorschkamp '', Technical Rep. No Wageningen, p. 24. Weaver, E.R. and Riley, R., Measurement of water in gases by electrical conduction in a film of hygroscopic material. Anal. Chem., 20" Webster, T.J., The effect on water vapour pressure of superimposed air pressure. J. Soc. Chem. Ind., 69: