For many industrial trace moisture measurements, dew-point transmitters are the ideal choice. They are low cost, easy to install and robust enough to withstand the tough environments. This includes measuring moisture at line pressure, operating at high ambient temperatures or in hazardous areas.
They are mostly used in applications where measuring the water dew point in a gas is crucial to protect equipment or ensure the quality of products. To get the best accuracy from your dew-point transmitter you need to understand how the different types of transmitter work, and how each is suited to specific applications.
Dew point temperature is an important measurement for many industrial processes because it tells operators at what temperature liquid water will condense. In applications such as compressed air used for medical gases, as an example, it’s important the gas is kept dry to avoid liquid water condensing in the pipework to avoid blockages or the growth of mold.
Dew-point transmitters work through impedance. The sensor consists of layers – two electrodes either side of an absorbent layer. As the gas passes over the sensor, the technology absorbs water vapor onto a porous hygroscopic dialectric area between two conductive layers built on top of a base ceramic substrate. Water molecules changes dielectric constant (Ke) of the dialectric which causes a change in impedance of the sensor
Using different materials for the sensors gives dew-point transmitters that provide different characteristics. The most traditional type of sensor uses aluminum oxide for the central absorbent layer. The Michell Ceramic Metal Oxide moisture sensors are based on new developments in this technology. These sensors are able to measure moisture at pressure and across a wide dew point range from -100 to +20 °C dew point.
polymer-based sensors – such as the Michell SF82 - are a later development. They provide a faster wet-to-dry response time, but work in a more limited measurement range, typically from -60 to +60 °C dew point.
From the differences described above, you can see why it’s important to pick a dew-point transmitter that can measure the range needed for your process. If you need to measure very low dew points or a wide range of dew points that include dew points below -60 °C you need a metal oxide dew-point transmitter. Whereas for industrial dryer applications, measuring dew points between -60 and +60 °C, a polymer-based transmitter is a fast-response alternative.
The datasheet for the dew-point transmitter will give you an accuracy specification. This will usually be given as a range – for example ±1 °C. This means the transmitter can be expected to be accurate to within plus or minus 1 °C of the actual dew point.
The calibration certificate is also an important indicator of how accurate the transmitter will be over a certain range.
Dew-point transmitters are calibrated to measure dew point against a chilled mirror reference hygrometer at various dew-point temperatures across their range. The more calibration points provided across the range provides a guarantee of accuracy across that range. For example, the Michell Easidew range of dew-point transmitters are provided with a 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.
Choosing the right dew-point transmitter for your application is only the first step to ensuring accuracy. Making sure your sampling system is based on best practice will ensure that moisture measurements are as accurate as possible. Avoiding common pitfalls such as dead volume, trapped moisture and using the wrong materials will ensure the accuracy and reliability of your measurements.
Once installed and operating correctly with the right sampling conditioning system, dew-point transmitter will operate reliably. However, like all precision instrumentation, they are not maintenance-free and we recommend that they are checked yearly to ensure that they are still providing reliable, accurate measurements.
For re-calibration, Process Sensing Technologies offers 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 12 months.
PST also offers a sensor exchange program for our Easidew and SF82 ranges of dew-point transmitters which ensures your process is always kept running with a calibrated dew-point transmitter. Full details of the sensor exchange and calibration services are given in our PST Services section on the website.
We also recommend that the sensors are not stored for long periods before being installed since ambient moisture and temperature can have a detrimental effect on the sensitive sensor tiles. Our manufacturing systems are optimized to allow fast supply of even large volumes of moisture sensors so there is no need to keep a large stock of spares.
Our final recommendation is to use a portable hygrometer to make spot checks on your process. While dew-point transmitters are installed online at a fixed location, portable hygrometers can take readings at different points in the system. This helps to not only confirm the online measurements, but also to detect leaks or other issues elsewhere in the process.
Various types of portable hygrometer are available with models based on the Michel ceramic metal oxide moisture sensor and the fast responding polymer sensors found in the SF82 dew-point transmitter. Small transportable chilled-mirror hygrometers, such as the Michell Optidew 401 , are available to provide a fundamental reference against the installed dew-point transmitters.
Ensuring the accuracy of entire systems is also possible with the Rotronic Monitoring System – RMS – which is a cloud-based environmental monitoring system that allows you to combine moisture, oxygen and many other parameters into a single system.
We hope this overview has been helpful to explain how you can optimize the accuracy of your dew-point measurements by choosing the right type of sensor for your application, installing it correctly and making sure calibration is kept up to date. If you have any other questions about your specific application, our application support team would love to hear from you.
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