Calibration of Relative Humidity Transducers for use in the Texas LoanSTAR Program

Abstract

In support of the Texas LoanSTAR Energy Conservation Program, the Texas A&M University Monitoring and Analysis Program (MAP) calibration facility has been performing relative humidity calibrations for transducers being used in field installations as well as for new units before installation. The sensors are all of the capacitive type with the element mounted on a polymer chip at the sensor probe tip. Saturated aqueous inorganic salt solutions are used as the calibration standard due to their wide use and acceptibility as a relative humidity standard. The tests are conducted through a temperature range of 0 to 44°C (32 to 111°F) and five nominal relative humidity levels of 11 %, 33%, 55%, 75%, and 83%. Early results from these tests indicate that for a given temperature, the relative humidity indicated may be significantly different than the true value. Many manufacturers claim an accuracy of ±2 to 3% over the range 0 to 100% relative humidity. Differences found during the tests for some of the transducers have been typically 10% relative humidity high at low temperatures and 10 to 15% relative humidity low at high temperatures. Accurate relative humidity measurement is important in the analysis of building energy consumption, especially in hot and humid regions where the latent cooling load is a large percentage of the total load. Clean room facilities and health care facilities also have the requirement of accurate relative humidity measurement and control.