Why are some dew-point transmitters supplied with sensor guards as standard and others not?

Benefits of an HDPE sensor guard for trace moisture measurements

Moisture sensing tiles, such as the Michell Ceramic Metal-Oxide Moisture Sensor, are delicate. Although the particulates in typical sample gases are very small, they can damage the sensor in two ways.

Firstly, particulates that are traveling at speed can cause severe and irreversible damage to the sensor if it is unprotected.

Secondly, particulates traveling at low velocity can cling to the sensor which reduces its effective surface area and leads to a slower response speed.

A sensor guard can also be used as baffle when dew-point transmitters are place directly in the flow. This type of installation is not recommended, however.

What are the drawbacks of using a sensor guard for trace moisture measurements?

The only drawback of using a sensor guard is a slightly slower response speed and recovery time from wetter measurements, since both the guard and the sensor need time to dry. However, in most applications this is outweighed by the benefits. There are some exceptions, though, such as the Pura.

Why is the Pura Trace Moisture Transmitter supplied with no sensor guard?

The Michell Pura Trace Moisture Transmitter is supplied with an electro-polished sample block as standard instead of a sensor guard. This is because the Pura is designed for use in applications with ultra-high purity gas – such as in the semiconductor industry.

In these applications, the entire process must be protected so particulates are filtered out before the sample gas reaches the Pura. Since moisture levels in these applications are also very low – down to parts per billion – speed of response is paramount.

The electro-polished sample block is optimized to have minimal moisture absorption to its surfaces, which also helps ensure a fast response to changes in moisture from wet to dry.

What are different types of sensor guards, and when should they be used?

We offer a choice of two sensor guards:

• HDPE (supplied as standard with SF82, Easidew Transmitter and Easidew I.S.)
• Sintered Stainless Steel (supplied as standard with Easidew PRO I.S and Easidew PRO XP)

Both guards protect the sensing tile from particulates in the sample gas, however only the stainless steel sintered guard is available cleaned for oxygen service.

Please contact us if you have any questions about using sensor guards or how to install a dew-point transmitter to ensure optimal accuracy and repeatability.