Campbell Scientific TGA200 Trace Gas Analyzer Overview - Manual (Page 14)

TGA200 Trace Gas Analyzer Overview
TABLE 6-3. Typical Isotope Ratio Measurement Noise
Gas Isotope
Ratio
Wavenumber
(cm
-1 )
10 Hz Noise Calibrated
Noise (‰)
CO
2
2293.881 0.15 ppm 0.03 ppm Carbon Dioxide
δ
13 C only
δ 13 C 2294.481 0.5 ‰ 0.1 ‰
CO
2
2310.686 0.5 ppm 0.1 ppm
δ 13 C 2310.347 1.5 ‰ 0.3 ‰
Carbon Dioxide
δ
13 C and δ 18 O
δ 18 O 2310.206 1.5 ‰ 0.3 ‰
6.2 TGA200 Frequency Response
There are several issues related to the frequency response of the TGA200,
including measurement rate, sample rate, digital filtering, synchronicity, and
flow and pressure in the sample cell.
Measurement rate: 500 Hz
Onboard digital filter: user-configurable
Sample cell volume: 420 ml
Frequency response (half-power bandwidth at 10 slpm, 20 mb): 7.5 Hz
The TGA200’s frequency response was measured by quickly injecting a small
amount of CO
2
into the sample flow, at the TGA200’s sample inlet. The data
were sampled at 50 Hz, and the onboard digital filter was disabled. An
example of the concentration time series is shown in Figure 6-1. The flow was
10 slpm and the pressure was 20 mb, for a calculated residence time of 50 ms.
The width of the pulse at half-peak height is approximately 50 ms, as expected.
The measured frequency response (Fourier transform of the measured pulse) is
shown in Figure 6-2. The blue curve is the measured frequency response. The
purple curve is a model of the frequency response. This model includes two
components, shown individually in green and red. The green curve is a sync
function, which is the ideal frequency response assuming no mixing of the air
sample as it travels to, or through, the sample cell. The blue (measured) curve
is below the green (ideal modeled) response, indicating some mixing does
occur. The red curve is the frequency response of an ideal mixing volume,
with a residence time (exponential time constant) of 14 ms. This residence
time is chosen empirically such that the combined effect (purple curve)
matches the measured response. These results and data at other flows and
pressures confirm that the TGA200’s frequency response is well represented by
the combination of an ideal sync function and an exponential function with
time constant equal to 1/5 of the sample cell residence time.
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