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Humidity measurement in cleanrooms: The devil is in the details!

Humidity measurement in cleanrooms: The devil is in the details!
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Humidity measurement in cleanrooms: The devil is in the details! - 1

Thoughts on selecting and using appropriate humidity measurement equipment for cleanrooms. Every cleanroom has specifications for environmental control that define upper and lower limits for temperature and relative humidity (RH). Creating and maintaining these conditions in the cleanroom is no small feat, and this must be achieved while also controlling pressure, flow, and contamination. Devices used for measuring RH (variously referred to as sensors, transmitters, or instruments) play a small role in the overall picture, but problems with these devices can cause significant headaches for cleanroom operators. What follows are some observations that may be relevant to anyone in the cleanroom community who has struggled with humidity issues. Identify the correct installation location Relative humidity is a temperature dependent parameter. This has implications for the placement and installation details of RH instruments. For example, if an instrument with a 200 mm stainless steel probe is installed with only 50 mm of the probe in the process air, the other 150 mm of the probe may serve as a heat sink or source, depending on the environment surrounding it. Even in carefully executed probe designs this can change the temperature of the actual humidity sensing element and introduce an RH measurement error that will vary with the difference between the process air temperature and the environment surrounding the exposed portion of the probe. Does this sound implausible? It is easily measured in a laboratory environment. Local heating can also cause problems for wall mounted RH instruments, so care must be taken to locate the devices away from any equipment or apparatus that generates heat. Good airflow in the Jim Tennermann / Regional Market Manager / Vaisala / Boston, MA, USA Humidity measurement in cleanrooms: The devil is in the details! Careful placement of humidity sensors results in accurate measurements and reliable long-term performance. 16 181/2009

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Humidity measurement in cleanrooms: The devil is in the details! - 2

cleanroom usually minimizes convective heat problems, but warm or hot equipment will radiate heat, creating the possibility of a temperature discrepancy and corresponding measurement error. Also,when specifying RH instruments for wall installations, consider that any loop powered device with a 4 – 20 ma output will necessarily dissipate some power as heat. Incredibly, some wall mounted RH devices housed in attractive enclosures self-heat by several degrees, causing significant measurement error. Avoid this possibility altogether by specifying RH devices with short probes that isolate the...

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