“Self-checking” psychrometer system for gradient and profile determinations near the ground

Agricultural Meteorology, 13(1974) 215--226 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

"SELF-CHECKING" PSYCHROMETER SYSTEM FOR GRADIENT AND PROFILE DETERMINATIONS NEAR THE GROUND*

NORMAN J. ROSENBERG and K. W. BROWN**

Agricultural Meteorology Section, Department of Horticulture and Forestry, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, Nebr. (U.S.A.) (Received February 18, 1974; accepted May 21, 1974)

ABSTRACT

Rosenberg, N. J. and Brown, K.W., 1974. "Self-checking" psychrometer system for gradient and profile determinations near the ground. Agric. Meteorol., 13: 215--226. An assembly of aspirated psychrometers for gradient and profile determinations using differential thermopiles of copper-constantan with temperature resolution (dry and wet) of 0.013°C is described. The design permits automatic checking of instrument function and accuracy by periodically moving all sensors to a horizontal position. Measurements of dry and wet thermocouple temperature are made in this position and deviations from uniformity between sensors caused by instrument imbalance are used to correct subsequent measurements when the psychrometers are vertically arranged. The sensors employ cotton wicks, aspiration rates are easily regulated and the assemblies are easily and cheaply constructed. In a field comparison with an Assmann psychrometer the thermocouple system underestimated absolute dry and wet bulb temperatures by about 0.5°C. Sources of the discrepancy are considered. INTRODUCTION M e a s u r e m e n t o f h u m i d i t y p r o f i l e s a n d g r a d i e n t s a b o v e n a t u r a l s u r f a c e s is a m o n g t h e m o s t d i f f i c u l t p r o b l e m s in m i c r o m e t e o r o l o g y . I n f r a - r e d gas analysis and electrical resistance networks based on the hygroscopicity of m a t e r i a l s have b e e n u s e d t o m e a s u r e h u m i d i t y p r o f i l e s . H o w e v e r , f o r r e a s o n s o f a c c u r a c y , s t a b i l i t y a n d s i m p l i c i t y p s y c h r o m e t r i c i n s t r u m e n t s have b e e n m o s t o f t e n u s e d in m i c r o m e t e o r o l o g y . S o m e m o d e l s p r o p o s e d : P a s q u i l l ( 1 9 4 9 ) , M c I l r o y ( 1 9 5 5 ) , R i d e r e t aI. ( 1 9 6 3 ) , S a r g e a n t a n d T a n n e r ( 1 9 6 7 ) and Lourence and Pruitt (1969). In a r e c e n t r e v i e w T a n n e r ( 1 9 7 1 ) discusses a d v a n t a g e s a n d d i s a d v a n t a g e s * Published with the approval of the Director as Paper Number 3300, Journal Series, Nebraska Agricultural Experiment Station. Research reported was conducted under Project Number 20--31. ** Present address: Dept. of Soil and Crop Sciences, Texas A and M University, College Station, Texas 77843, U.S.A.

216 of psychrometers for measurement of humidity features of the climate and the horizontal and vertical humidity gradients in the surface layers of the atmosphere. He also outlines stringent design criteria for psychrometers considering the causes of experimental error which theory and practice identify. We have designed, developed and tested in four years of field work an assembly of wet and dry thermocouple psychrometers which is useful in determining temperature and humidity profiles and gradients for any number of levels near the ground. Aerodynamic methods usually require profiles of 3, 4 or more levels for calculations of latent and sensible heat flux. Two levels of measurement are sufficient for Bowen-ratio calculations. Since agreement in Bowen-ratio between successive levels above the surface can be used as a measure of divergence, there may be great value in use of the full assembly to produce data on the profile of Bowen-ratio. A unique feature of our psychrometer assembly is a provision which permits automatic identification of imbalances and errors, whatever the cause. When the imbalances are found due to such serious failures as open circuits, impaired water flow to wet thermocouples, impeded ventilation, etc., repairs may be made quickly or new sensors substituted into the assembly. When imbalances are slight the instruments are n o t adjusted but, rather, measurements of the imbalance are used as correction factors in the computational process.

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Fig.1. Diagrammatic cross-section of the aspirated psychrometer. A, plastic tube to aspiration manifold; B, styrofoam pipe cover; C, chrome plated Mylar; D, glass tube conducting wick to wet bulb; E, cotton wick (shoestring); F, glass water reservoir; G, rubber stopper; H, dry bulb; I, lucite tube; J, lucite tube for friction fitting; K, plastic bolt; L, spacers; M, phenolic baffles; N, wet bulb; O, 30-gauge copper-constantan wires.

217 PSYCHROMETER UNIT SENSOR, DESIGN AND CONSTRUCTION T h e p s y c h r o m e t e r unit sensor is a m o d i f i c a t i o n of t h a t given b y B r o w n and Covey (1966). A s c h e m a t i c diagram is s h o w n in Fig.1. Two sets o f t h e r m o c o u p l e s (wiring diagrams s h o w n in Fig.2) are used t o provide a signal c o m p a t i b l e , in o u r application, with a p o t e n t i o m e t e r of + 5 m V range. T h e t h e r m o p i l e a r r a n g e m e n t s h o w n can provide a r e s o l u t i o n o f 0 . 0 2 5 ° C b e t w e e n d r y and w e t t h e r m o c o u p l e t e m p e r a t u r e s in t h e same sensor. T h e t h e r m o p i l e s m a y be m a d e m o r e or less sensitive according t o the application and t h e d a t a logging e q u i p m e n t available. DRY WET THERMOCOUPLES THERMOCOUPLES

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Fig.2. Schematic diagram of thermopile wiring for a set of four psychrorneters used to measure profiles of dry and wet thermocouple temperature. AT is temperature difference between dry (D) and wet (W) thermocouples numbered 1--4 in ascending order. T h e t h e r m o c o u p l e s are set with e p o x y c e m e n t into 25 m m long pieces o f t e f l o n rod, 6 m m in diameter. T o assure good t h e r m a l c o n t a c t , the holes are drilled t h r o u g h the r o d and the e p o x y is d r a w n in b y v a c u u m . T h e t h e r m o c o u p l e wires are lead t h r o u g h slits at b o t h ends o f a 15 m m ID, 18 m m OD lucite t u b e which serves as a housing and are t a p e d o n the outside to m a i n t a i n the desired position. T h i r t y gauge wire is sufficiently strong to s u p p o r t the bulbs in p r o p e r p o s i t i o n within the lucite t u b e . G o o d p s y c h r o m e t e r design requires t h a t the d r y t h e r m o m e t e r be ahead o f the w e t t h e r m o m e t e r in the v e n t i l a t i o n stream and t h a t the wicking be in i n t i m a t e t h e r m a l c o n t a c t w i t h the sensor. A c o t t o n wick (shoelace) e x t e n d s f r o m within the w a t e r reservoir t o the w e t bulb o n which it is fixed with c o t t o n thread. Water f r o m t h e t e s t - t u b e reservoir is fed t o the w e t bulb b y

218 capillarity with water tension less than 2 cm to the base of the bulb and 4 cm to the top. In practice the wicks, which are boiled before use in detergent and distilled water, always appear to glisten with moisture, a characteristic which T a n n e r {1971) accepts as evidence of adequate wetting. Less than 1 m m separates the top of the wick conducting t ube from the base o f the wet t h e r m o c o u p l e . This is done to avoid any significant input of vapor to the air stream. Water feed from above would eliminate the possibility of " p r e c o n d i t i o n i n g " error but in practice leads f r e q u e n t l y to dripping o n t o the dry t h e r m o c o u p l e -- a much m ore serious cause of error. It may be argued as well that the same source of error affects all wet bulbs in the assembly and should thus be negligible in gradient determinations. The problem can be avoided totally by redesign to permit separate airstreams to pass the dry and wet thermocouples. The p s y c h r o m e t e r is thermally insulated from its surroundings by a layer of 15 mm thick s t yr of oam . The outside of the s t y r o f o a m cover is lined with chrome-plated Mylar* for radiation shielding. Fuchs and Tanner {1965) suggest th at aluminized Mylar would be still more effective for the purpose. Air is drawn into the p s y c h r o m e t e r through baffles of 0.64 mm phenolicfiber board, painted white. The p s y c h r o m e t e r is aspirated from above at a flow rate of 35 1 min -1 past the axially oriented dry and wet thermocouples. Tanner (1971) shows evidence that t e m p e r a t u r e depression of a wet bulb 6 mm in diameter is nearly 99.5% of m a x i m u m at a ventilation rate of 3 m sec -1 which was exceeded in this design. Slatyer and Bierhuizen (1964) suggest that lower ventilation rates may be adequate, particularly if the instrument design is such that sampled air is adequately mixed in the vicinity of the bulbs. The p s y c h r o m e t e r reference t h e r m o c o u p l e has a time constant of 20--25 sec. Dr y - - wet t h e r m o c o u p l e temperature differences are sensed with the same response time, as are dr y t h e r m o c o u p l e and wet t h e r m o c o u p l e t e m p e r a t u r e gradients between adjacent sensors. According to Tanner (1971) a sampling period a b o u t twice the time constant is adequate to construct an appropriate mean temperature. Thus for these t her m ocoupl es sampling every minute would be proper. If such a program is impractical time constant m a y be altered through the use of larger teflon blocks or blocks of a material with greater heat capacity. PSYCHROMETER SYSTEM ASSEMBLY An assembly of 4 psychrometers is shown in Fig.3. The psychrom et ers are suspended by T's or L's from the main line of the aspiration manifold. Aspiration is accomplished by means of a vacuum cleaner attached to the flexible hose protruding from the box at the right and through the transparent"

* Fassar Products, 250 Chester St., Painesville, Ohio 44077. Stock No. 710.

219

lucite support line which extends horizontally from the box to main line of the manifold. In the configuration shown the psychrometers are spaced at 25, 25 and 50 cm. Other spacings are possible. All vciring junctions are made in the white box shown at the right. The wiring diagram in Fig.2 applies to the configuration of psychrometers shown. A reference thermocouple (D1), electrically insulated from the remainder of

Fig.3. A s s e m b l y of 4 t h e r m o c o u p l e p s y c h r o m e t e r s w i t h t h e m a i n line of t h e a s p i r a t i o n m a n i f o l d in t h e vertical position. S u p p o r t line of t h e a s p i r a t i o n s y s t e m e x t e n d s h o r i z o n t a l l y to the c o n t r o l b o x at right. T h e a s s e m b l y is m o v e d periodically to a h o r i z o n t a l p o s i t i o n b y m e a n s o f a m o t o r c o u p l e d to t h e s u p p o r t line. T h e p s y c h r o m e t e r s swing freely at t h e i r j u n c t i o n s w i t h t h e m a i n a s p i r a t i o n line.

the system*, is e m b e d d e d in the lowest dry thermometer. At the beginning of any sampling sequence it is read first. The wet--dry temperature difference * Provision can be m a d e , as well, for using this t h e r m o c o u p l e as a r e f e r e n c e for a n y n u m b e r o f n o n - p s y c h r o m e t e r t h e r m o c o u p l e s , e.g., leaf or soil t e m p e r a t u r e sensing thermocouple networks.

220

AT (D1 -- W, ) is read next. The subscripts represent levels above the ground. Thereafter the differences AT (D, --D2), AT (W, -- W2 ), AT (D2 --D3) , AT (W2 -- W3) • • • etc., are read until a cycle is completed. With the data logging system we use the entire sampling sequence can be completed in less than 8 sec. At times the sequence is slowed but never takes longer than the time constant of a single sensor, i.e. 25 sec. No matter how carefully thermocouples are constructed, differences in response slope and zero offset will occur. In psychrometry, ventilation and wicking differences further increase the difficulty in achieving iden~;~al responses from a set of individual sensors. In field use differences in thermal insulation, radiation shielding and wick cleanliness introduce new sources of error or aggravate existing errors. Paired sensors, wired differentially, are as likely to increase as to cancel these errors. Rider et al. (1963) made periodic field tests of a set of psychrometers by manually moving all instruments to the same horizontal plane and observing the matching of sensors in this configuration. To compensate for unmatched sensors Sargeant and Tanner (1967) periodically reversed the two psychrometers in an assembly used to measure Bowen-ratio. Fritschen (1965), in a similar application, periodically switched airstreams passing his humidity sensors. Reversing sensors or airstreams for two levels is relatively simple. It is difficult, however, to devise a reversing method when more than 2 levels are sampled. The psychrometer assembly we describe here is designed to automatically tip to the horizontal position periodically during operation and to return to the vertical position after all sensors have been read. SPRINGRETAINER~

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221

The reversing mechanism (housed in the white box, Fig.3) consists of a synchronous instantaneous reversing 1 R.P.M. motor* with 600 in./oz, torque. The support aspiration tube is rotated by means of a chain and sprocket. The psychrometers are suspended by means of pressure fittings from elbows which couple to t h e T's or L's of the main line of the aspiration manifold. The m e t h o d of coupling is shown in Fig.4. Joints are machined so as to minimize loss of vacuum and y e t be free enough to allow the weight of the psychrometer to swing so that the sensor unit is vertically positioned at all times. Flow rate through the four psychrometers in the assembly is regulated by means of the butterfly valves located in the elbow coupling to the psychrometer (Fig.4). Rotameters are substituted for the baffles on all sensors and the assembly is put in the horizontal position for flow-rate adjustments. In field operation the assemblies are serviced twice daily. Water is added to the reservoir and the wicks are observed for cleanliness and wetness**. CORRECTION PROCEDURES

In the horizontal position the psychrometer unit sensors should each be sampling air of the same temperature and humidity content. After sufficient time has elapsed for equilibration of the instruments to the step change in ambient conditions the differences in electrical o u t p u t of the differential thermopiles AT (Di - - D 2 ) , AT (W1 -- W2 ) • • • etc. are interpreted as measures of instrumental imbalance. These differences are added algebraically to the o u t p u t measured for the same differential thermopiles during the following sampling periods when the psychrometer assembly is in its normal vertical position. No correction is made to the o u t p u t of the reference thermocouple D 1 or the differential AT (D1 -- W1 ). Experience has shown that the correction procedure need be used no more than one or two times hourly. Dry wicks, impeded ventilation or electrically open circuits are easily identified when the assembly is in the horizontal position. Differences greater than 0.25°C are always cause for suspicion of instrumental failure. FIELD TESTS

During summer of 1969 a series of experiments were conducted at Mead, Nebraska (41 ° 09'N 98 ° 30'W) in which the evapotranspiration rates of irrigated soybeans and micrometeorological conditions above the crop were continuously monitored. Performance of the psychrometer assembly * H u r s t Mfg. C o m p a n y , M o d e l GA, available f r o m H e r b a c h & R a d e m a n , Inc. 401 E. Erie Ave., P h i l a d e l p h i a , P e n n s y l v a n i a 1 9 1 3 4 . ** Detailed plans for c o n s t r u c t i o n of t h e p s y c h r o m e t e r a s s e m b l y are available in m i m e o g r a p h e d f o r m o n r e q u e s t to t h e s e n i o r a u t h o r .

222

was evaluated during these studies. Table I describes the range o f t e m p e r a t u r e c o n d i t i o n s during the first three studies o f the season and p e r f o r m a n c e r e c o r d o f t w o p s y c h r o m e t e r assemblies used. Failure m e a n s t h a t one or m o r e o f the 8 d r y or w e t t h e r m o p i l e s in an assembly failed t o f u n c t i o n p r o p e r l y . TABLE I Performance of thermocouple psychrometer assemblies in the field Test run

Duration dates during 1969

Assembly

Correction cycles

Failures

1

0619120006300930

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381 378

2

0702143007060730

1 2

3

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1 2

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9 4

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3 5

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28.0-12.0 27.7-11.5

Failures were identified r o u t i n e l y in the c o r r e c t i o n p r o c e d u r e . ~ y s t u d y 3 a failure rate o f less t h a n 1% was achieved. This p e r f o r m a n c e is n o t difficult to maintain. T h e mean, variance, and s t a n d a r d deviatl~a o~ d r y and w e t t h e r m o c o u p l e t e m p e r a t u r e differences, d e t e c t e d w h e n the p s y c h r o m e t e r assemblies were in the c o r r e c t i o n position, are given in Table II f o r the t w o earliest studies. T h e low variance with standard deviation a p p r o a c h i n g the value o f the m e a n is indicative o f the g a m m a d i s t r i b u t i o n -- very heavily skewed t o the lowest absolute value o f the differences. Means o f the absolute value o f t e m p e r a t u r e differences b e t w e e n sensors range f r o m 0.05 ° to 0.14°C. T h e longest s t u d y o f the season was c o n d u c t e d during t h e period August 9 - - 2 2 . 6 6 0 c o r r e c t i o n cycles o c c u r r e d during this study. A d i s t r i b u t i o n o f c o r r e c t i o n factors (absolute value) f o r adjacent d r y and w e t bulb sensors in t h e t w o p s y c h r o m e t e r assemblies is given in Table III. At least 60% o f all c o r r e c t i o n f a c t o r s were less than r0.05°Cf. E x c e p t in the case o f A T D (2 -- 3) and D(3 -- 4) in Assembly 2 m o r e t h a n 90% o f all c o r r e c t i o n factors were <10.15°CJ and the m e a n o f all c o r r e c t i o n factors was < 0 . 1 0 . T h e m e d i a n value for all differential t h e r m o p i l e s was 0.05 and s t a n d a r d deviations were close to the m e a n values. T h e d a t a in Table III suggest t h a t t h e r m o p i l e D 3 in p s y c h r o m e t e r 2 was p o o r l y adjusted d u r i n g a large p o r t i o n o f the study. Nonetheless, w h e n the c o r r e c t i o n factors are applied the t e m p e r a t u r e profiles m e a s u r e d b y the t w o assemblies are c o n s i s t e n t l y f o u n d to be nearly identical in shape. Fig.5 shows profiles o f c o r r e c t e d d r y t h e r m o c o u p l e and w e t t h e r m o c o u p l e t e m p e r a t u r e s and v a p o r pressure over s o y b e a n s at l l h 0 0 and 1 9 h 0 0 on 5 July

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1969. The u n c o r r e c t e d data points are shown as well. Maximum t e m p e r a t u r e correction necessary on this day was 0.10°C. As vapor pressure is a function of Td and ( T d - - T w ) the corrections necessary may be larger relative to the corrections in Td or T w alone. The corrections in vapor pressure at the 100-cm level are between 0.1 and 0.2 mbar. The vapor pressure profile at 19h00 would be particularly distorted w i t h o u t correction. CORRECTEDDATA • TEMPERATURE ......... VAPORPRESSURE • UNCORRECTEDDATA

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During the growing season of 1973 at Mead, the p s y c h r o m e t e r was compared with a m o t o r i z e d Assmann p s y c h r o m e t e r in a field trial. Both psychrometers were placed in the field at a level 130 cm above ground over 90--110 cm soybeans. The instruments were separated by about 10 m. The Assmann p s y c h r o m e t e r was read once shortly after the hour. The o u t p u t of t he thermo. couple p s y c h r o m e t e r was logged on the hou r and seven minutes later and the results averaged. Observations were made periodically during daylight on July 16--18. A total of 23 sets of observations were made. A summary of the comparison between Assmann and t h e r m o c o u p l e psychrometers is given in Table IV. Means differed by 0.7 ° and 0.4°C for the dry and wet bulb temperatures, respectively. Mean of the calculated vapor pressure differed by 0.6 mbar. Differences between instrumental measurements o f dry bulb temperatures were greatest at the highest temperatures e n c o u n t e r e d when the t h e r m o c o u p l e p s y c h r o m e t e r indicated 1.5°--2.0°C lower than the Assmann psychrometer. All other measurements in the range 15°--30°C were considerably closer. This difference may possibly be due to differences in radiation shielding and for wick-water temperatures likely ,to develop in the two instrumental systems. The design of the t h e r m o c o u p l e system is more likely to minimize error due to these factors. Despite the a f o r e m e n t i o n e d difference, results of the Assmann and thermocouple p s y c h r o m e t e r measurements were very closely correlated. The

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226 s t a n d a r d deviations for m e a s u r e d d r y bulb and w e t bulb a n d f o r calculated v a p o r pressure were very similar. Using all available data the t h e r m o c o u p l e s y s t e m u n d e r e s t i m a t e s t h e A s s m a n n p s y c h r o m e t e r results b y 6--8%, depending o n p a r a m e t e r measured. These observations, it s h o u l d be e m p h a s i z e d , refer t o a b s o l u t e d e t e r m i n a t i o n s . Errors in gradient m e a s u r e m e n t s are likely t o be c o n s i d e r a b l y smaller. T h e A s s m a n n p s y c h r o m e t e r lacks the r e s o l u t i o n necessary for a field c o m p a r i s o n o f gradient m e a s u r e m e n t s . ACKNOWLEDGEMENTS S u p p o r t for this p r o j e c t was received f r o m the Office o f Water R e s o u r c e s Research, U.S. Dept. o f t h e Interior, u n d e r Public Law 8 8 - - 3 7 9 p r o g r a m (Nebraska Water R e s o u r c e s Research I n s t i t u t e P r o j e c t A-017). Mr Dale Sandin, Research T e c h n i c i a n , c o n t r i b u t e d to t h e design, c o n s t r u c t i o n and testing o f the assembly. Precision Machine Co. o f Lincoln, N e b r a s k a f a b r i c a t e d m a n y o f the c o m p o n e n t parts. REFERENCES Brown, K. W. and Covey, W., 1966. The energy budget evaluation of the micrometeorological transfer processes within a corn field. Agric. Meteorol., 3: 73--96. Fritschen, L. J., 1965. Accuracy of evapotranspiration determined by the Bowen ratio method. Bull. Int. Assoc. Sci. Hydrol., 10: 38--48. Fuchs, M. and Tanner, C. B., 1965. Radiation shield for air temperature thermometers. J. Appl. Meteorol., 4: 544--547. Lourence, F. J. and Pruitt, W. O., 1969. A psychrometer system for micrometeorology profile determination. J. Appl. Meteorol., 8: 492--498. McIlroy, I. C., 1955. A sensitive temperature and humidity probe. Aust. J. Agric. Res., 6: 196--199. Pasquill, F., 1949. A portable indicating apparatus for the study of temperature and humidity profiles near the ground. Q. J. R. Meteorol. Soc., 75: 239--248. Rider, N. E., Philip, J. R. and Bradley, E. F., 1963. The horizontal transport of heat and moisture -- a micrometeorological study. Q. J. R. Meteorol. Soc., 89: 507--530; and discussions 90: 236--240, 1964. Sargeant, D. H. and Tanner, C. B., 1967. A simple psychrometric apparatus for Bowen ratio determinations. J. Appl. Meteorol., 6: 414--418. Slatyer, R. O. and Bierhuizen, J. R., 1964. A differential psychrometer for continuous measurement of transpiration. Plant Physiol., 39: 1051--1056. Tanner, C. B., 1971. Psychrometers in micrometeorology. Paper presented at the Symposium on Thermocouple Psychrometers, Logan, Utah, March 19, 1971. Proceedings in press.