How Accurate is your Dew Point Meter or Transmitter?

Why product choice, sampling technique and calibration are key to dew point measurement

If you make trace moisture measurements in an industrial process, you’ll be familiar with the use of dew point transmitters and portable dew point meters. These instruments are normally easy to use and built to withstand repeated use in demanding environments, including high line pressures, extremes of temperature, and hazardous or Ex-classified areas. They are also capable of measuring extremely low levels of moisture in a wide range of process gases, and of producing results to high degrees of accuracy.

But, how do you know these results are accurate or consistent?

To be certain – and in many applications this is critical for process performance, quality and safety – you need to understand the basics of trace moisture measurement.

Dew point is an important measurement for many industrial processes because it indicates the temperature at which moisture suspended in a gas will condense. For example, in applications such as compressed air used for medical gases, it’s important the air is dry to avoid liquid water condensing and corroding pipework or leading to the growth of mold.

Not all dew point sensors are created equal, which means that selecting the correct technology can have a direct impact on the quality of your trace moisture measurement. So, let’s start by taking a look at how a dew point sensor or transmitter works.

How does a dew point sensor work?

The most common industrial process dew point sensors work by impedance. Each sensor consists of three layers: two conductive electrodes placed either side of a porous hygroscopic dielectric material. As the process gas passes over the sensor assembly, the porous layer absorbs water vapor. This alters the dielectric constant (Ke), causing a change in impedance that is measured across the electrodes, which can then be detected, conditioned and output using the onboard electronics.

Using different sensor materials creates different sensor characteristics. The traditional type of sensor has aluminum oxide for the central dielectric layer. Our Ceramic Metal Oxide moisture sensors are based on recent developments in this technology and are capable of measuring moisture at high pressure and across a wide range, from -100 to +20^°C dew point. 

How a ceramic metal oxide moisture sensor is constructed

By comparison, polymer-based sensors, such as our SF82, are a more recent development. These devices provide a faster atmospheric-to-dry response time, but work across a more limited measurement range, typically from -60 to +60^°C dew point.

We should also mention two other trace moisture detection technologies: cooled mirror and tunable diode laser absorption spectroscopy (TDLAS). These are each capable of producing exceptionally accurate results, down to a few parts per billion of water molecules in gas samples. They are, however, more complex and expensive than impedance-based devices, making them more suitable to laboratory, or especially demanding, applications.

Selecting the correct moisture sensor for your measurement range is the first step to accuracy

From the differences described above, you can see why it’s important to choose a dew point transmitter that can measure the range needed for your particular application. If you need to measure extremely low dew points valued below -60^°C, a metal oxide dew point transmitter is the best option. Conversely, for applications where the requirement is to measure dew points between -60 and +60^°C, a polymer-based transmitter with a fast response is the way to go.

We have developed a range of dew point sensors and transmitters, based on both types of impedance technology, and each is suitable for many different industrial applications.

One of the fastest growing areas of application is compressed air, especially where systems include industrial dryers and gas generators. These systems are all susceptible to the presence of moisture. This can cause corrosion, microbial growth and affect the properties of lubricating oils. In dryers and gas generators, trace moisture levels can increase energy consumption, reduce the operating life of desiccant materials and adversely affect the purity of generated nitrogen and oxygen gases.

Typically, these systems are engineered to comply with the ISO 8573 standard, which governs the quality of compressed air and specifies acceptable levels of contamination – moisture, oil aerosols and particulates – in different types of application. For example, Class 1 dryers used in the electronics or semiconductor sectors, have to produce extremely dry air with a pressure dew point of -70^°C or lower, while Class 2 dryers, such as those used in food production or general industrial processes, require a pressure dew point of -40^°C or better.

This is where our Easidew EA2 dew point transmitter comes in. It is ideal for use with all classes of dryer and gas generator. A single sensor can cover all applications. So, OEMs and end users can easily standardize on a trace moisture measurement solution, which is proven, robust and compact, making it simple to integrate into a dryer or generator. Where trace moisture sensors are not integrated into a compressed air system, dryer or gas generator, we also offer our latest Easidew PDP Dryer Portable, which provides a lightweight, accurate and easy-to-use solution for taking spot measurements.

What a calibration certificate tells you about moisture sensor accuracy

Although the datasheet for a dew point transmitter will tell you the expected accuracy of the device, normally given as a range such as ±1^°C, you should also refer to the calibration certificate, which should be supplied with every new or reconditioned dew point transmitter.

Dew Point transmitters are calibrated against a chilled mirror reference hygrometer across a range of calibration points. The greater the number of calibration points, the better the guarantee of accuracy for that particular device. For example, our range of dew point transmitters is provided with 13-point calibration. This means that each transmitter is calibrated against a chilled mirror reference hygrometer at 13 separate points across its entire measurement range of -100 to +20^°C dew point, providing a detailed indication of how it will perform in use.

Correct sampling and installation are vital for accurate moisture measurements

Choosing the right dew point transmitter for your application is, of course, only the first step in ensuring accuracy and repeatability.

It is also critical to ensure that your sampling system is based on best practice. There are various common problems that affect sensor performance, such as dead volume, trapped moisture, condensation and leaks. We have produced a separate article covering common problems with trace moisture measurements, which you can read here.

Keeping your moisture sensor calibration up to date

Once installed and operating correctly with the correct sampling conditioning system, a dew point transmitter should operate reliably, with minimal drift over time. However, like all precision instrumentation, moisture sensors and transmitters require regular maintenance. We recommend that they are checked annually to ensure that they continue to provide reliable, accurate measurements.

For re-calibration, we offer two services. To maintain full traceability, you can send your dew point transmitter to one of our calibration and service centers. Your sensor will be returned to you with a fresh calibration certificate and the assurance that you will benefit from accurate trace moisture measurements for another twelve months.

We also offer a sensor exchange program for our Easidew and SF82 ranges of dew point transmitters. This minimizes downtime and ensures you always have a fully calibrated device ready to use.

Learn more about our sensor exchange and calibration services.

Our manufacturing systems are optimized to allow fast supply of even large volumes of moisture sensors, so you do not need to keep a large stock of products.

Make spot checks of your process for accuracy with dew point meters

Dew point transmitters are normally installed in-line at fixed process locations. For complete process integrity, we recommend the use of a dew point meter or portable hygrometer, for making spot checks across your system. Dew point meters typically use the proven ceramic metal oxide or polymer sensor technologies described above, enabling you to confirm your in-line measurements in real time, ensuring that there are no issues, such as leaks, at other areas in the process.

Portable dew point meters for fast and precise spot checks

As a final point, small transportable chilled mirror hygrometers, such as the  Optidew, can also be used to provide a fundamental reference against installed dew point transmitters. Additionally, integrated cloud-based environmental monitoring systems are available, which allow you to combine moisture, oxygen and many other parameters into a single system for complete peace of mind.

With 50 years’ experience in the development of moisture and dew point technology, we are the application experts for all trace moisture measurement in industrial applications. If you would like to discuss your requirements, please contact our team today. 

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