[ftir_tga] FW: FTIR intake numbers & memory effect
- From: David Griffith <griffith@xxxxxxxxxx>
- To: "ftir_tga@xxxxxxxxxxxxx" <ftir_tga@xxxxxxxxxxxxx>
- Date: Mon, 12 Dec 2011 15:27:51 +1100
Further from Johannes,
Note the side-issue of channel numbers - as I first noticed at LSCE, the
correct inlet and aux inlet channel numbers are sometimes not reported
correctly in the log file. Graham has found and fixed the problem, which will
come to you in the next software release.
Also in the next release, each analogue input channel is measured once per
second during spectrum collection, and the mean and stdev included in the log.
Dave
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,
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]
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 (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]
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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