Experimental Comparison among the Psychrometer and the Two-Pressure Humidity Generator and the Dew Point Hygrometer Speaker: Jinpeng Fan Authors: Xueli Quan Yuren Cai Jinpeng Fan Brain Instruments Corporation, Hebei Vocational T. T. College, P. R. China Phone (973) , Fax:

Abstract to check the psychrometer coefficient A Three ventilated psychrometers were compared with the two-pressure humidity generator and with the precise dew point hygrometer average value of A near 6.28×10 -4 K -1 Dr. Wylie and Lalas, 6.24×10 -4 K ×10 -4 K -1 is in general used. This difference of value A creates a 1.2%rh error at 20 ℃ and 50%rh.

1. Introduction R. Wylie and T. Lalas[1-3] from CSIRO Australia developed a WMO Reference Psychrometer for WMO- CIMO. uncertainty can be as little as ±0.16% rh, coefficient A much closer to 6.2×10 -4 K -1 D. Sonntag[4,5]from German, values of A approximately 6.7×10 -4 K -1. Fan from China determinated with standard gravimetric hygrometer, 6.26×10 -4 K -1.

2.The facilities and instruments for the comparison Two-pressure humidity generator similar to the NIST (NBS) two pressure humidity Mark-2. Dew point hygrometer M4 is made by General Eastern Instruments and calibrated by NRCCRM of China and traceable to NIST Three axially ventilate psychrometers, Model 99-2, Two Assmann-like psychrometers.

Fig. 1 Experimental schematic drawing of comparison 8. Psychrometer sensor 9. Psychrometer instrument 5. expansion valve 6. Test chamber 7. Dew point Hygrometer 1. Gas cylinder 2. Pressure regulator 3. Gas flow meter 4. Saturator

A: Front panel InputCalibrationAir pressurePrinter T ＆ H 9 9—2 Digital Psychrometer SelectionReset Power ℃ % 1. Display windows 2.Function keys 3. Main switch

B: Ventilator 1. Electric fan 3. dry-bulb 5. Water cup 2.Radiation tube 4.wet-bulb

Table 1 Summarized characters of three psychrometers Model Scale Length Diameter Airspeed Wick of bulb of bulb length ℃ mm mm m/s mm HM J-SK

3.The calibration of the thermometers calibrated by standard platinum resistance thermometer 99-2 ±0.03 ℃ HM6 ±0.06 ℃ JSK. ±0.06 ℃

4. Experimental procedure The generator produces enough continuous stream of air that can march the airspeed of the psychrometer. pumped to the M4 dew point hygrometer by a diaphragm pump. During each run the wet and dry bulb temperature were red at interval 1 minute and 10 times

Table 2 Experimental points Dry- temperature ℃ Wet- temperature ℃ 4, 6, 9, 14 16, 20, 26 Relative humidity %rh

5. Experimental results The A tp and A M4 are the coefficients of the psychrometer calculated from A=[e(t w )f w -e] / p (t a -t w ) (2) e = U ×e(t a )×f w (3) t a =dry temperature t w =wet-bulb temperature P= atmosphere pressure e(t w ), e (t a )= saturation vapor pressure e=water partial pressure in the air f w =enhancement factor,Greenspan’s formula

Table 3 Summary of the results for three psychrometers A : K -1 σ: A tp A M A m 6.26 A tp HM6 A M4 6, A m 6.25 A tp J-SK A M A m 6.24

6. Discussion 6.1 The theory value of psychrometer coefficient One hundred years ago psychrometer coefficient A of Assmann and whirling psychrometer is about 6.7×10 -4 K -1. Wylie gives A w =6.24×10 -4 K -1 ±1% for transverse ventilate psychrometer and 6.08× K -1 for Assmann psychrometer Sonntag [9] insisted that the A s =6.66×10 -4 K -1

Difference come from : Sonntag’s omission of the Steffan factor (1-x w ) Sonntag’s radioactive heat transfer coefficient a r contains no allowance Sonntag ignores significance of the length of the wet bulb, although this parameter is equal in importance to the airspeed

6.2 Experimental values of A Wylie: A for WMO Reference Psychrometer 6.24×10 -4 K -1 ±0.4%. closely similar of the true Assmann, value of A about 6.25×10 -4 K -1. Sonntag: A of EAP determined German and China is 6.7×10 -4 K -1 respectively. International comparison of Assmann psychrometer is 6.62×10 -4 K -1 Fan: checked the values of A of HM6 Assmann, Psychrometer is 6.26×10 -4 K -1 in China.

key differences What is true theoretical value of the A, 6.2×10 -4 K -1 or 6.6×10 -4 K -1 ? Why the A for Assmann psychrometer HM 6 is different tested in China and German?

6.3 The relation between A and wet bulb temperature Sanntog’s A s =6.53×10 -4 K -1 ( ×10 -3 t w ) (4) Wyle’s A w =6.10×10 -4 K -1 (1-x w )( ×10 -3 t w ) (5)

Fig. 3 Curve of A tp -(t w ) corresponding to the data of 99-2 given in table 3.

7. Conclusions coefficient A in general use is higher 8% than true value of A which is much closer to 6.2×10 -4 K -1. proved through the comparison with a standard gravimetric by [Fan]. tested by two-pressure humidity generator and dew point hygrometer prove the true value of A again. well-designed psychrometer with the value of A near 6.2×10 -4 K -1 and ±0.03 ℃ accuracy of the dry-and wet-temperature can get higher 1%rh uncertainty as Brain 99-2 done.