Performance evaluation of duct-mounted relative humidity sensors used in building HVAC applications

Relative humidity sensors are common components in building heating, ventilating, and air-conditioning (HVAC) systems, and their performance can significantly impact energy use in these systems. A study was undertaken to test and evaluate the most commonly used relative humidity sensors in building HVAC systems, namely the capacitive and resistive types. Sensor models from six different manufacturers were used for testing. Three sensors of each model for a total of 18 sensors were tested, nine of them were capacitive-type sensors and nine were resistive-type sensors. The performance of these sensors was determined by subjecting the sensors to a series of tests that included accuracy, linearity, repeatability, hysteresis, ageing, response time and stress. For each test, a detailed method of test was developed and peer reviewed. The accuracy test results showed that two of the six humidity sensor models were within manufacturer specified accuracy, while a third sensor model did not met the manufactured specified accuracy at any humidity level tested, and finally, the remaining three sensor models, met the manufacturer specified accuracy for only part of the humidity range. Further, the repeatability of all sensors models at 50% RH and 15, 25, and 35°C was within 1.5% RH. The maximum hysteresis for all sensors was less than 3.2% for all humidities and temperatures. And finally, at 25°C, Model-B sensor had the largest nonlinearity of -3.8% while Model-C sensor had the least nonlinearity of 0.0%.