Which relative humidity measurement technology fits my industrial application?

Typical requirements for rh measurements are:

• Performance: accuracy, repeatability, speed of response, recovery time
• Signal outputs
• Mechanical: size, process connections.

And constraints often include:

• Operational factors: temperature, pressure, flow, available power supply, size versus space for installation, maintenance schedules
• Budget
• Environmental factors such as hazardous areas or the need for chemical resistance.

Once you have made your list, you have a choice of a wide range of instruments. However, don’t waste your time considering all possible options. Of all the technologies capable of making RH measurements only two are commonly used in industrial settings:

Polymer based capacitive sensors: 

These sensors measure humidity using the impedance method, where adsorption of water vapour in a polymer film changes the capacitance resulting in an electric signal proportional to the humidity present.

Chilled mirror sensors, also referred to as using the condensation method: 

These sensors measure the temperature at which moisture condenses on a mirrored surface. This provides the dew point temperature, from which the humidity is determined, relative to the ambient (or process) temperature and pressure.

As a general rule, chilled mirror sensors provide the most accurate, drift-free and repeatable humidity measurements, but are also larger and more expensive to buy. Capacitive transmitters using polymer sensors are generally small and inexpensive; however they lack the accuracy and repeatability of chilled mirror instruments.

The key points to consider are:

• If you have a tight budget, you may struggle to afford a chilled mirror instrument and you should focus your search towards polymer sensors. But you will still need to factor in total cost of ownership. Polymer sensors drift and need to be recalibrated at least once every twelve months, so be prepared for both the cost and extra work in sending them off. To keep your process running continually, you would also need spare sensors in stock (which will also need to be recalibrated in turn!)
• If neither budget nor space are squeezed, your decision may be based purely on performance. If you require the best accuracy and repeatability you should think about a chilled mirror instrument. Many polymer sensors offer acceptable accuracies on paper, without mentioning the higher temperature dependence. A change in temperature of only one degree centigrade may result in an error of almost 6 %rh.
• If you need a portable instrument, a light weight, polymer-based hand-held is the best choice.
• If speed of response is critical, polymer sensors should be your choice. But again, this is dependent on the range – at 90 %rh and higher, a chilled mirror would perform better.
• If your application involves a harsh or corrosive atmosphere, a chilled mirror instrument is usually less prone to be affected by contamination.

Manufacturers are constantly working to improve and extend their product lines. This means that polymer sensing tiles are increasing their accuracy and reliability while chilled mirror products are becoming smaller and more cost-effective so it’s worth considering both options in your initial research.

We are happy to discuss all details, which could not be covered in this short article personally. Please do not hesitate to contact us or to learn more about Moisture Sensing Technologies.