Humidity measurement in cleanrooms: The devil is in the details!0 pages

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