Humidity sensors: what's best?

Bill Williams bwilliam at oyster.smcm.edu
Fri Aug 13 13:12:50 EST 1993


In article <24f2t0INNfrh at zephyr.grace.cri.nz> Michael Thorpe,
srgpmrt at grv.grace.cri.nz writes:
> I would be grateful for advice (and there may  be a better newsgroup
> to ask). I am looking to buy a humidity sensor for a gas exchange
system. 
> How successfully have vaisala-type sensors been used, is there 
> anything around yet that holds its calibration better? Is it still
best to 
> go for a dew-point mirror sensor, and if so from where? 

Vaisala-type humidity sensors are excellent, as long as !2% RH is OK,
and as long as you can comfortably stay within the range of about 15%
to 75%.  Beyond this range they can show significant hysteresis, but
they can be of use in these ranges if they are allowed to equilibrate
for a long time (hours to days) at the high (or low) humidity.  Vaisala
is quite frank about these limitations, as I recall, and I think they
can provide you with details.  Also, I think more recent chips may be
better in this respect than older ones.  Because they sense relative
humidity, their temperature must be precisely known in order to use the
information in calculating anything that requires absolute humidity,
such as transpiration.  Li-Cor uses Vaisalas in their LI-6200
gas-exchange system.

I use chilled-mirror humidity sensors in my system (DEW-10's), and I am
extremely pleased with them.  Periodic calibration shows that their
long-term drift and linearity are usually better than 0.2!C (they're
dew-point sensors), and they seem to be rugged.  They do not have
hysteresis problems. About the only drawback I've found is that their
time-constant is longer than the Vaisala type (can be several seconds
for large changes in dew point).
 
The true fanatics in this business seem to use IRGAs for humidity
sensing.  I belive at least Li-Cor and Binos make water-vapor sensing
IRGAs.  The chief problem here, of course, is cost.
___________________________________
William E. Williams, bwilliam at oyster.smcm.edu
Divison of Natural Sciences and Mathematics
St. Mary's College of Maryland
St. Mary's City, MD 20686



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