TGA200 Trace Gas Analyzer Overview
6. Specifications
6.1 Measurement Noise
The typical 10 Hz-concentration measurement noise, given in Table 6-1, is
calculated as the square root of the Allan variance with 100 ms averaging (i.e.,
the two-sample standard deviation). This is comparable to the standard
deviation of the 10 Hz samples calculated over a relatively short time (10 s).
The typical 30-minute average gradient resolution is given as the standard
deviation of the difference between two intakes, averaged over 30 minutes,
assuming typical valve switching parameters.
TABLE 6-1. Typical Concentration Measurement Noise
Gas Wave
number
(cm
-1
)
10 Hz Noise
(ppbv)
30-min Gradient
Resolution (pptv)
Nitrous Oxide
N
2
O 2208.575
1.5
30
Methane CH
4
3017.711
7
140
Ammonia NH
3
1065.56
6
200
The TGA200 multiple-scan mode can be used to measure suitable pairs of
gases. Typical performance for some examples is given in Table 6-2.
TABLE 6-2. Typical Concentration Measurement Noise
for Multiple Scan Lasers
Gas Wave
number
(cm
-1
)
10 Hz Noise
(ppbv)
30-min Gradient
Resolution (pptv)
N
2
O 1271.077
7
140
Nitrous Oxide
and Methane
CH
4
1270.785
18
360
N
2
O 2243.110
1.8
35
Nitrous Oxide
and Carbon
Dioxide
CO
2
2243.585
300
6000
Typical performance for isotope ratio measurements is given in delta notation.
For example, the
δ
13
C for CO
2
is given by
1000
1
13
×
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
=
VPDB
S
R
R
C
δ
where
R
s
is the ratio of the isotopolog concentrations measured by the TGA200
(
13
CO
2
/
12
CO
2
) and
R
VPDB
is the standard isotope ratio (
13
C/
12
C).
δ
13
C is
reported in parts per thousand (per mil or ‰). The 10 Hz noise is the square
root of the Allan variance with no averaging. The calibrated noise assumes a
typical sampling scenario: two air sample intakes and two calibration samples
measured in a 1-minute cycle. It is given as the standard deviation of the
calibrated air sample measurements.
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