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. Peter Kammel wrote: > Dear Dave, > > I tested the calculator and got exactly the result I expected: > ppmv @ x bar = ppmv @ 1bar /x. > Example: > frostpoint (dewpoint) = -50 C, corresponds to 38.84 ppm @ 1 bar and > to 3.88 ppm @ 10 bar. > > Thanks for the attached article, which is educational. > > I asked my colleagues at PSI about their measurements. They measured the > T dependence by slowly cooling the sensor and warming it up again. > There was no hysteresis observable. Initially the noticed the different > readings, when the door of the experimental hall was opened and the > overall temperature changed. At constant temperature the sensor is very > stable. > > We are waiting for your advice. > > Thanks > > Peter > > On Mon, 27 Jun 2005 info@kahn.com wrote: > > > Dear Peter: > > > > Attached are the following documents: > > > > * An article providing practical suggestions on improving response > > speed at low dewpoints > > * A humidity calculator > > > > I will call you tomorrow with comments regarding temperature effects on > > the Pura sensor. > > > > Sincerely yours, > > Dave Alan > > KAHN INSTRUMENTS, INC. > > > > > > Peter Kammel wrote: > > > > >Dear Dave, > > > > > >That's the first report I got from our collaborators in Switzerland. > > >I will call you soon and ask for your advice. > > > > > >Best regards > > > > > >Peter > > > > > >Peter Kammel / pkammel@uiuc.edu > > >Department of Physics, Loomis Laboratory > > >University of Illinois at Urbana-Champaign > > >1110 West Green Street, Urbana, IL 61801 > > >Tel (217) 333-5424 / Fax (217) 333-1215 > > > > > > > > >---------- Forwarded message ---------- > > >Date: Mon, 27 Jun 2005 17:11:40 +0200 > > >From: Peter Kravtsov <pkravt@gmail.com> > > >To: Peter Kammel <kammel@npl.uiuc.edu> > > >Cc: Claude Petitjean <claude.petitjean@psi.ch>, > > > Malte Hildebrandt <malte.hildebrandt@psi.ch>, > > > Francoise Mulhauser <Francoise.Mulhauser@psi.ch>, > > > Bernhard Lauss <lauss@berkeley.edu> > > >Subject: Humidity sensor > > > > > >Dear Peter and colleagues, > > > > > >Attached is our news and questions about the humidity sensor. > > > > > > > > > > >
Attachment:
-90dp+20C plots.xls
Description: application/msexcel