Making sense of relative humidity

An Able Instruments and Controls product story

Edited by the Engineeringtalk editorial team Dec 12, 2005

Monitoring relative humidity is vital in maintaining efficient control of an HVAC system; here Dave Quelch discusses some criteria for selecting RH sensors.

Maintaining optimum environmental conditions, such as humidity and temperature, is essential for any workspace, but particularly so for a clean room environment.

Of these parameters, relative humidity (RH) is one of the most challenging to measure because it is a truly analytical measurement where the sensor must make direct contact with the environment.

RH is also one of the most critical to measure as it has a direct influence on the perception of comfort by staff in the area and can affect the performance of equipment; so it's important to feed the right information into the control system, so it can make the right decisions.

This necessitates choosing the right type and quality of sensor - and ensuring they are properly calibrated.

For HVAC applications these include psychrometry, displacement, resistive, capacitive and liquid sorption sensors.

Psychrometry is a well-established method that has the benefit of being simple and inexpensive, although it does have intensive maintenance requirements.

Psychrometers are very accurate at near-saturation (100%) conditions, but at lower RH the accuracy is poor and they cannot be used at temperatures below 0C.

Perhaps the oldest type of RH sensor still in common use is the displacement sensor, which is inexpensive to manufacture and highly resistant to contamination.

Unfortunately, this type of sensor has a tendency to drift over time and there is a significant lag between the change occurring and it being registered by the sensor (hysteresis).

A particularly popular choice for HVAC applications is the bulk polymer resistive sensor, which provides a direct, secondary measurement of RH.

These use a humidity-sensitive salt embedded in a protective coating that is permeable to water vapour, providing protection against contaminants in the air while ensuring an accurate measurement.

What makes these types of sensors popular for HVAC applications is their relative immunity to surface contamination.

However, it's important to bear in mind that while a surface build up will not have an adverse effect on accuracy, it will delay the response time, so surfaces will need to be cleaned as part of a general maintenance regime.

In addition, these sensors are better suited for use in RH ranges above 20% because of the extremely high resistance at RH values below this figure.

Where a high degree of sensitivity is required at low RH, the capacitive sensor is frequently chosen.

These are usually designed with parallel plates with porous electrodes, or with interdigitated fingers on a substrate.

The sensor material is very thin to achieve a large signal change with humidity.

It also permits water to enter and leave easily and allows for fast drying and easy calibration.

Capacitive sensors are ideal for high temperature environments because the temperature coefficient is low and the polymer dielectric can withstand high temperature.

At low humidity levels these sensors are very sensitive and provide a relatively fast response; but at higher RH values they have a tendency to saturate and become nonlinear.

Whatever type of sensor is specified, it is important to ensure correct calibration before delivery - ideally individually rather than batch tested.

The manufacturer should also be able to provide an audit trail for calibration, using methods approved by the National Physical Laboratory.

To that end, chilled mirror (optical condensation) hygrometers are widely accepted as the most precise method for calibrating the sensors used in day-to-day HVAC applications.

These use the actual condensation point of the ambient gas and can easily be made traceable to international calibration standards such as UKAS and NIST.

Alternative methods for calibrating RH sensors include salt baths, but these require a long equilibrium time to achieve accurate results and, with commercial pressures in play, it is all too easy to rush the calibration at the expense of accuracy.

Salt baths are also very prone to cross-contamination.

In an environment where accurate measurements are critical to effective environmental control, the accuracy of the information from sensors is vital.

In choosing the most appropriate sensor for the particular space, HVAC designers can ensure the system's response is based on meaningful information.