[ftir_tga] pressure sensor zero drift

  • From: Dan Smale <Dan.Smale@xxxxxxxxxx>
  • To: "ftir_tga@xxxxxxxxxxxxx" <ftir_tga@xxxxxxxxxxxxx>
  • Date: Mon, 12 Dec 2011 19:39:09 +0000

Hi Nick, Dave, Johannes

Yes, we saw a slow drift in the MKS, this affected the N2O retrievals the most.

In an ideal world (unlimited $$ + hrs) I agree with Dave-a nice robust hi-vac 
pump pulling <<1mb would be ideal, then also the cell having 2 pressure 'ports' 
to allow dual pressure gauges , so the cell pressure mks gauge could be checked 
against another short term (or long term) pressure gauge.

I attached an extra pressure gauge to the cell output line to calibrate/confirm 
the cell MKS pressure sensor.
Recalibration of the mks sensor has been done twice in 6 years. After the 
initial drift (within 3 months of IFTS purchase) the mks sensor has been 
working fine.

I keep a close eye on the evacuated cell pressure...

Cheers
Dan



> -----Original Message-----
> From: ftir_tga-bounce@xxxxxxxxxxxxx [mailto:ftir_tga-bounce@xxxxxxxxxxxxx]
> On Behalf Of Nicholas Deutscher
> Sent: Monday, 12 December 2011 5:44 p.m.
> To: ftir_tga@xxxxxxxxxxxxx
> Subject: [ftir_tga] Re: FW: FTIR intake numbers & memory effect
>
> Thanks Dave (and Johannes),
>
> One additional note regarding the pressure gauges at low pressure. Not only
> is there a temperature dependence, but after we installed hte instrument at
> Darwin there was a slow drift in the zero of the pressure gauge. I'm fairly
> certain that Dan saw something similar at Lauder post installation there, in
> both cases with the MKS sensors.
>
> Cheers,
> Nick
>
> On 12/12/2011 3:28 PM, David Griffith wrote:
>
>
>
>
>
>       From: David Griffith
>       Sent: Monday, 12 December 2011 14:38
>       To: 'Johannes Laubach'
>       Subject: RE: FTIR intake numbers & memory effect
>
>
>
>       Dear Johannes
>
>       I think your analysis is pretty good - while I am a bit surprised that
> Pevac comes out as high as 4 mb, and the error at 2 mb, we have never
> actually done anything to check it - we don't have any pressure gauge in that
> low pressure region.  But we DO know that the zero of the gauge is
> temperature dependent.  I will pass this on to Ecotech for the future - they
> should be able to measure this low pressure with more accuracy.  The other
> option would be to use a high vacuum pump to evacuate the cell to < 0.1 mb
> with confidence, and then zero the pressure gauge.
>
>
>
>       I'm not convinced that the 900-898 mb difference is due to the gauge
> though -  after a fill, the pressure can readjust after the valves close for a
> static measurement.  The PID that controls the pressure uses the indicated
> pressure as it's control, so if the gauge reads 900mb, that is what it will 
> adjust
> to, whatever the true value may be.
>
>
>
>       Cheers
>
>       David
>
>
>
>
>
>       From: Johannes Laubach [mailto:LaubachJ@xxxxxxxxxxxxxxxxxxxxxx]
>       Sent: Friday, 9 December 2011 11:14
>       To: David Griffith
>       Cc: Graham Kettlewell; John Hunt
>       Subject: FTIR intake numbers & memory effect
>
>
>
>       Hi Dave,
>
>       thank you, it is switched off now.
>
>       Yes the Aux Inlet numbering was correct in our test data.
>
>
>
>       I've been thinking more about the "residual-fill" error due to
> inaccurate pressure reading at low pressure. I think the best way to figure it
> out would be to use two different, reasonably constant, air sources (2
> cylinders, or 1 cylinder and the air in a closed lab) and define a measurement
> sequence of 3 successive fills of the cell with Air 1, then 3 fills with Air 
> 2, then
> Air 1 again etc. The first of the three fills should be subject to measurable
> memory effect, the 2nd and 3rd fill should not be (and thus give
> indistinguishable results). A few repetitions of this should suffice to work 
> out
> if the effect is consistent between gas species or not.
>
>
>
>       Not having done that yet, the closest I've come to such a test is to
> use the 4 to 14 Nov data for N2O, which had very small gradients between
> the two ambient-air intakes (<1 ppb) and a relatively large difference, of 13
> %, between tank and ambient. So every time I measured Intake 1 after tank
> air, I had (on average) 0.21 ppb higher N2O than when I measured Intake 1
> after Intake 2. This works out as follows:
>
>       X_prev = 370.5 ppb (tank)
>
>       X_true = 327.7 ppb (ambient)
>
>       X_meas-X_true = 0.21 (+/-0.02) ppb, where the given uncertainty
> combines the standard error of the mean of 317 measurements of [N2O] in
> Intake 1 after tank air with that of 1066 measurements of [N2O] in Intake 1
> after Intake 2.
>
>       Hence:
>
>       f=(X_meas-X_true)/ (X_prev-X_true) = 0.0049 (+/-0.0005)
>
>       in excellent agreement with the estimates from linear regressions in
> my earlier notes, which range from 0.003 to 0.007 for the different species.
>
>       With p_cell=898 hPa, this gives p_evac = f p_cell = 4.4 hPa (compared
> to 2.36 hPa usually recorded). I presume this is an offset, not a scale factor
> error. An offset of 2 hPa is even consistent with regularly finding p_cell
> recorded as 898 hPa when the target pressure was actually 900 hPa(!).
>
>
>
>       I tried the same procedure for CO, where we know X_prev = 0, but
> ambient [CO] was too variable, giving
>
>       (X_meas-X_true) = -0.1 (+/-1.3) ppb which has too large relative
> error to be useful. And for the other gases, the concentration difference
> between tank and ambient was too small as well as ambient too variable.
>
>
>
>       Cheers
>
>       Johannes.
>
>
>
>       ****************************************************
> ****
>
>       Johannes Laubach
>
>       Ph.  +64 - 3 - 321 9865 <tel:%2B64%20-%203%20-%20321%209865>
> (direct dial)
>
>       email: LaubachJ@xxxxxxxxxxxxxxxxxxxxxx
>
>       Fax  +64 - 3 - 321 9998 <tel:%2B64%20-%203%20-%20321%209998>
>
>
>
>       Postal Address:
>
>       Landcare Research, P.O.Box 40, Lincoln 7640, New Zealand
>
>
>
>       Street Delivery Address:
>
>       Landcare Research, Gerald St., Lincoln 7608, New Zealand
>
>       ****************************************************
> ****
>
>
>
>       From: David Griffith [mailto:griffith@xxxxxxxxxx]
>       Sent: Thursday, 8 December 2011 4:01 p.m.
>       To: Johannes Laubach
>       Cc: Graham Kettlewell
>       Subject: RE: FTIR intake numbers
>
>
>
>       Johannes,
>
>       Thnaks for the help - Graham has found and fixed the bug with the
> numbering.
>
>       Can you confirm that it was only in the main inlet numbering, not the
> aux inlet?
>
>       In any case you can turn it off, we replicated the problem on our test
> bench.
>
>       Cheers
>
>       David
>
>
>
>
>
>       From: Johannes Laubach [mailto:LaubachJ@xxxxxxxxxxxxxxxxxxxxxx]
>       Sent: Thursday, 8 December 2011 13:03
>       To: David Griffith
>       Cc: Graham Kettlewell
>       Subject: RE: FTIR intake numbers
>
>
>
>       Hi Dave, Graham,
>
>       do you still need to access our FTIR for testing? Please let me know
> when I can switch off.
>
>       Cheers
>
>       Johannes.
>
>
>
>       ****************************************************
> ****
>
>       Johannes Laubach
>
>       Ph.  +64 - 3 - 321 9865 <tel:%2B64%20-%203%20-%20321%209865>
> (direct dial)
>
>       email: LaubachJ@xxxxxxxxxxxxxxxxxxxxxx
>
>       Fax  +64 - 3 - 321 9998 <tel:%2B64%20-%203%20-%20321%209998>
>
>
>
>       Postal Address:
>
>       Landcare Research, P.O.Box 40, Lincoln 7640, New Zealand
>
>
>
>       Street Delivery Address:
>
>       Landcare Research, Gerald St., Lincoln 7608, New Zealand
>
>       ****************************************************
> ****
>
>
>
>       From: David Griffith [mailto:griffith@xxxxxxxxxx]
>       Sent: Tuesday, 6 December 2011 12:40 p.m.
>       To: Johannes Laubach
>       Cc: John Hunt; Samuel Hammer; Benoit Wastine
> (benoit.wastine@xxxxxxxxxxxx); Dan Smale; Vanessa Sherlock; Graham
> Kettlewell
>       Subject: RE: FTIR temperature
>
>
>
>       Hi Johannes,
>
>
>
>       Meanwhile Graham has replicated the error with the mis-named
> channel numbers, and is working on it.
>
>
>
>
>
>       Dave
>
>
>
>
>
>
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>
> --
> ------------------------------------
> (Dr.) Nicholas Deutscher
> Postdoctoral Fellow
> Institute of Environmental Physics
> University of Bremen
> D-28334 Bremen
>
> Ph: +49 (0) 421-218-62177 <tel:%2B49%20%280%29%20421-218-62177>
>
> n_deutscher@xxxxxxxxxxxxxxxxxxxxxxxx
>
> nicholas.deutscher@xxxxxxxxx
>
> -and-
>
> Honorary Fellow
> Centre for Atmospheric Chemistry
> University of Wollongong
> Wollongong, NSW, 2522
> Australia
>
> ndeutsch@xxxxxxxxxx
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