In the constant quest to help buildings perform more efficiently without compromising on comfort levels, we are seeing increasing use of passive cooling systems such as chilled beams and chilled ceilings. Very often, these need to be combined with lower Relative Humidity (RH) to achieve a better perception of comfort.

It is well accepted that our perception of thermal comfort is based on a combination of temperature and RH. High RH can make spaces with even relatively low temperatures feel clammy, whereas higher temperatures with a lower RH can feel perfectly comfortable.

At the same time, it’s important to ensure that RH levels aren’t allowed to fall too far. If moisture levels in the air are too low, the eyes and the mucous membranes in the nose will dry out leading to itchy eyes and increased vulnerability to airborne germs. Low RH will also allow a build up of static electricity on office equipment so that users are likely to receive small shocks.

All of which means that it’s essential to exercise accurate control of RH levels and this can only be achieved with accurate RH sensors. Sophisticated control systems may have the intelligence to carry out the necessary fine tuning but the decisions they make will only be as good as the information they receive from the conditioned space.

Unfortunately, of all the environmental parameters that building services systems have to monitor, RH is one of the most challenging to measure accurately. Unlike parameters such as temperature and pressure, RH is a truly analytical measurement where the sensor must make direct contact with the environment. In contrast, temperature and pressure sensors are invariably insulated from the environment by a thermowell or a diaphragm respectively.

Consequently, choosing the right type and quality of RH sensor is critical if the design conditions are to be met and maintained. Not only do they have to employ a measurement mechanism to suit the environment they are in, they also need to be properly calibrated before installation and regularly re-calibrated when in service.

There is a wide range of RH sensors to choose from for HVAC applications - including psychrometry, displacement, resistive, capacitive and liquid sorption sensors – so it pays to understand how these differ before making a final specification.

Psychrometry has been a popular way of measuring RH for many years, principally because of its simplicity and low cost. It uses a pair of matched electrical thermometers, one of which is in a wetted condition. In operation, evaporation cools the wetted thermometer, resulting in a measurable difference between it and the dry bulb, or ambient, measurement. When the wet bulb reaches its maximum temperature depression, the humidity is determined by comparing the wet bulb/dry bulb temperatures on a psychrometric chart.

Psychrometers provide high accuracy at near saturation (100 percent) RH conditions and they are simple to use and repair. However, their accuracy at lower RH (<20 percent) is poor and they have very intense maintenance requirements – which has a bearing on life cycle costing. Also, they can’t be used at temperatures below 0°C.

Perhaps the oldest type of RH sensor still in common use is the displacement sensor. These sensors use a strain gauge or other mechanism to measure expansion or contraction of a material such as hair, nylon or cellulose in proportion to changes in RH. Displacement sensors are inexpensive to manufacture and highly resistant to contamination. Unfortunately, they have a tendency to drift over time and the hysteresis effects are significant.

A particularly popular choice of RH sensor for HVAC applications is the bulk polymer resistive sensor, which provides a direct, secondary measurement of RH. These sensors comprise a grid of interdigitated electrodes, coated with a humidity-sensitive salt embedded in a polymer resin – laid on a ceramic substrate.

The resin is covered by a protective coating that is permeable to water vapour. As water permeates the coating, the polymer is ionised and the ions become mobile within the resin. When the electrodes are excited with an alternating current, the impedance of the sensor is measured and used to derive the RH.

What makes these types of sensors popular for HVAC applications is their relative immunity to surface contamination, making them suitable for a wide range of demanding conditions, including aggressive environments such as swimming pools. 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 to avoid significant hysteresis effects over time.

In addition, these sensors are better suited for use in RH ranges above 20 percent, 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 a popular choice. 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.

The dielectric material absorbs or desorbs water vapour from the environment with changes in humidity, causing a capacitance variation. This, in turn, provides an impedance that varies in relation to humidity. A dielectric constant change of approximately 30 percent corresponds to a 0-100 percent variation in RH.

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. At higher RH values, however, they have a tendency to saturate and become non-linear.

Whichever type of sensor is specified, it’s important to ensure they have been properly calibrated 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 have a long equilibrium time to achieve accurate results and, when commercial pressures are brought to bear, it is all too easy to rush the calibration at the expense of accuracy. Salt baths are also very prone to cross-contamination.

It is now clear that the pressure to optimise a building’s energy performance is going to increase over the next few years, particularly with the implementation of the Energy Performance of Buildings Directive in 2006. With that pressure will come stronger drivers to minimise the use of active air conditioning systems. At the same time, workers are expecting higher levels of comfort from their workplace and are more willing to complain if conditions do not meet their expectations.

Passive cooling systems offer an ideal solution for many projects but have often been rejected by end users or developers because of the poor performance of early systems. This is particularly true for speculative developments where the landlord needs a guaranteed, tried and tested system that will appeal to tenants.

As passive cooling systems have been refined and fine-tuned, their performance has become more reliable and predictable. However, they still rely on effective environmental control to maintain comfort conditions on particularly hot days or in spaces with high heat gains. Realising that potential depends heavily on the ability to obtain meaningful, real-time information from the space so that the control system can act on it in the most appropriate fashion.

Choosing the most appropriate type of sensor, with the right sensitivity and build quality, and ensuring it has been calibrated to the required standards, is the most effective way of achieving this. And the only way to ensure you are specifying the best sensor for the job is to ask the right questions of the supplier.

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