Inexpensive temperature and relative humidity sensors, how good are they?

With a number of inexpensive temperature and relative humidity sensors on the market one can ask, how good are these devices, and can they be used in applications requiring relatively accurate and repeatable measurements? This paper describes the use of two such instruments as comparison devices in regulatory audit programs. A thermister based digital thermometer was used for water bath audits of station temperature sensors. Relative humidity was audited using a self contained data logging system that measured both temperature and relative humidity. Prior to the use of the sensors as audit devices they were evaluated both in the laboratory and in the field to determine their accuracy and stability. The results of the temperature evaluation showed excellent linearity and accuracy when compared to the standard method of measurement using a mercury in glass thermometer. Over a temperature range of 0 to 45 degrees C the digital thermometer was accurate and repeatable to within 0.1 degree C. The relative humidity measurement, which is a more difficult measurement to make, showed average differences within acceptable tolerances to be used as audit devices over a range in temperature of 15 to 40 degrees C and humidities of 10% to 90%. The results from a significant number of audits showed the use of the two inexpensive devices not only provided accurate results, but increased the efficiency of the audits and durability of the audit instrument packages.