Like many sensors and measurement instrumentation, traditional PIDs can be affected by the environmental conditions such as those found in process environments, in particular dirt and humidity.
Humidity is a naturally occurring phenomenon and is present in the atmosphere and gasses. The presence of humidity can disrupt PID measurements in two ways leading to false low or conversely high readings.
Conventional PIDs may use humidity suppression/compensation techniques but each of them has disadvantages:
Humidity sensor – these typically have a slower response than the PID sensor itself which causes a drifting compensation
Desiccant tube – these both slow the PID response and also reduce it by adsorption plus they need replacing from time to time which adds cost
Humidify the calibration gas – this only works at one level of humidity and is no longer accurate when the humidity changes
Importantly none of these solutions solves a false positive at high humidity.