[ftir_tga] Re: FW: FTIR intake numbers & memory effect

  • From: Nicholas Deutscher <nicholas.deutscher@xxxxxxxxx>
  • To: ftir_tga@xxxxxxxxxxxxx
  • Date: Mon, 12 Dec 2011 15:43:36 +1100

 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<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 (direct dial)****

email: LaubachJ@xxxxxxxxxxxxxxxxxxxxxx****

Fax  +64 - 3 - 321 9998****

** **

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 <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<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 (direct dial)****

email: LaubachJ@xxxxxxxxxxxxxxxxxxxxxx****

Fax  +64 - 3 - 321 9998****

** **

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 <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
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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