Calibration of Special Relative Humidity and Temperature (RHT) ......................Measurement 115 Calibration of Special Relative Humidity and Temperature (RHT) Sensors and Evaluation and Expression of Uncertainty in the Measurement BHIKHAM SINGH, HARI KISHAN* and Y. P. SINGH National Physical Laboratory (NPLI), CSIR New Delhi - 110 012, India *e-mail : hkishan@mail.nplindia.ernet.in [Received : 06.03.2007 ; Revised : 07.01.2008 ; Accepted : 29.05.2008] Abstract This paper reports the calibration of special type of relative humidity and temperature sensor, which gives the output in terms of millivolts, measured by precision digital multimeter (DMM) to further estimation of relative humidity in the range of 25 %RH to 95 %RH. The sensor has been calibrated against the standard aspirated psychrometer having a pair of matched dry and wet bulb quartz thermometers. The temperature of wet bulb thermometer is effectively depressed when a ventilated airflow is maintained at a speed more than 3 metre/second. The relative humidity of reference standard psychrometer is computed using psychrometric equation to relate with the output of the RH indicator under comparison. The paper also describes the details of estimation and expression of expanded uncertainty in the calibration of the sensor at measured relative humidity. A sufficient number of data were undertaken to evaluate the uncertainty at a confidence-level of approximately 95% for a normal probability distribution. MAPAN - Journal of Metrology Society of India, Vol. 23, No. 2, 2008; pp. 115-121 © Metrology Society of India, All rights reserved. 1. Introduction The measurement and control of relative humidity plays a significant role in industry, including various sectors such as electrical, pharmaceutical, textile, food and aerospace. Its need is increasing day by day with the modernization of industries and awareness of quality system in regard to ISO 9000. This awareness has led to the industries for getting their RH indicators calibrated against the National Humidity Standard. The gravimetric method of water vapour measurement is considered the most precise and accurate. This method permits the measurement of the amount of water vapour in a given mass of the moist gas sample. The quantities measured in this method are fundamental; it gives an absolute measure of humidity. Hence the gravimetric method is used as a primary standard of humidity against which other methods are compared. The National Institute of Standards and Technology (NIST), USA, National Physical Laboratory (NPL), UK, Physikalisch-Technische Bundesanstalt (PTB), Germany, National Metrology Institute of Japan (NMIJ), Japan and Korea Research Institute of Standards and Science (KRISS), Republic of Korea use gravimetric method [1-6]. Various other techniques such as two-pressure, two-temperature, two pressure-two temperature, mixed flow and aspirated psychrometer [7-11] are used as the secondary humidity standards. At NPL, India an aspirated psychrometric technique has been established [12-14] as the reference humidity standard. No measurement is complete without assigning the value of uncertainty to it. The detailed study of the uncertainty associated with the measurement has been evaluated and discussed