Campbell Scientific TGA200 Trace Gas Analyzer Overview - Manual (Page 13)

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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