Dear Peter:
These sensors are already calibrated and then tested to show the
temperature effects on their readings.
We'll look forward to your further results.
Sincerely yours,
Dave Alan
KAHN INSTRUMENTS, INC.
Peter Kammel wrote:
Dear Dave,
Thanks for your very interesting information. We will try to
reproduce the long term recovery after a temperature change
in our system.
Let's wait for the results and talk to Mr. Stokes after that.
Do the different curves in your excel spreadsheet correspond
to different, not yet calibrated humidity sensors?
Best regards
Peter
On Wed, 29 Jun 2005 info@kahn.com wrote:
Dear Peter:
I apologize for the day delay in responding.
We have observed on many occasions that when a sensor and sampling system is
exposed to
a temperature transient there is a two-phase reaction. The first is an
impulse spike in
moisture resulting from the disturbance to the system equilibrium. This can
be a positive
excursion on heating (outgassing) but also, interestingly, a negative
excursion (moisture
retention) on cooling. If the temperature is than allowed to either return
to �normal� or to
stabilize and hold at the new value, this impulse reaction will subside as
the moisture level
corrects itself dynamically (time frame depending on the dewpoint temperature
being
measured but at low levels typically 16-24 hours). You will then see a final
steady state offset
between the original steady temperature moisture sensor output and the final
steady state
output at T�
It is the difference between these two signals that is a measure of the
sensors true
temperature dependence. Typically this will be in the region of 0.1 deg C /
deg C as specified
in the product literature.
The attached chart shows the full data acquisition from a 10 deg C to 20 deg
C ambient
temperature variation at a steady moisture level of �90 deg C dp. As you can
see, the impulse
part of the reaction is as much as 14 deg C or thereabouts. However the
steady state values
are only a few deg C higher than the original starting conditions.
If you would like to discuss this matter further, I would suggest a
conference call with Mr. Andrew Stokes, the Technical Director at Michell
Instruments, who is most knowledgeable. We could potentially have this
discussion on Thursday morning at 9:00 am CDT (although I would have to check
on Mr. Stokes' availability) or next week.
I look forward to hearing from you.
Sincerely yours,
Dave Alan
KAHN INSTRUMENTS, INC.
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