Making sense of relative humidity in the cleanroom environment

1st April 2013

Maintaining optimum environmental conditions, such as humidity and temperature, is essential for any workspace, but particularly so for a cleanroom 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 heating, ventilation and air-conditioning (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 (100percent) conditions, but at lower RH the accuracy is poor and they cannot be used at temperatures below 0°C.

Which sensor?

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. This is known as 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 20percent 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. However, at higher RH values they have a tendency to saturate and become non-linear.

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 an appropriate national body (Fig.1).

Precise calibration

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.

Meanwhile, ABLE has announced the approval and conformity of the Computrac Vapor Pro moisture analyzer to ASTM method D7191-05. A fast, easy, cost-effective alternative to titration methods, D7191-05 Standard Test Method for Determination of Moisture in Plastics by Relative Humidity Sensor was adopted by ASTM after its most recent round of balloting through Autumn 2005.

Relative Humidity Sensor technology has many advantages over traditional titration methods, making it an ideal choice for companies who want to make quality products as efficiently as possible, without the recurring costs of chemicals, glassware or disposal. Relative Humidity Sensor instruments, such as the Computrac Vapor Pro are durable enough for the production floor yet versatile enough for the laboratory. They require very little training to use accurately; even an inexperienced operator can produce reliable results

Dave Quelch is Senior Product Manager with ABLE Instruments & Controls Limited





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