Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 45)

Table 9. Suggested Reference Gas Concentration
Gas Species Concentration (ppm)
Methane (CH
4
) 10,000 – 20,000
Nitrous Oxide (N
2
O) 1500 – 2500
Ammonia (NH
3
) 4000 – 6000
Carbon Dioxide (CO
2
) isotopic ratios 50,000- 100,000
Other Contact Campbell Scientific
Generally, any value in the range given should be acceptable. A high concentration is recommended if a relatively weak
absorption line is used, or if the TGA is to be operated at very low pressure. A low concentration is recommended for
strong absorption lines and high pressure, or if the laser has significant multimode power. Concentration errors caused
by multimode lasers can be minimized by reducing the reference absorbance.
The concentration of the reference gas is used to calculate the concentration of the sample gas; therefore, it must be
entered into the TGA software. The calculated sample concentration is scaled by this value. If it is not correct, the
measured concentration will have a corresponding scale error. For many trace gas flux measurement applications, a
measurement of the reference gas accurate to 2% is adequate. For applications that require a very accurate
concentration measurement (such as isotope ratios), the TGA100 must be calibrated on-line using a pair of well-known
calibration tanks that bracket the concentration range of interest. For these applications the reference gas concentration
provides only a preliminary estimate that is superceded by the measurements of the calibration gases. This makes a
highly accurate measurement of the reference gas concentration unnecessary.
The user must provide a tank of reference gas with an appropriate regulator for low flow rates and low delivery
pressure. The delivery pressure is normally set to approximately 0 psig. A reference gas connection assembly (part
number 15837) can be obtained from Campbell Scientific to connect the reference gas to the TGA100. This assembly
includes a flow meter, needle valve, and 20 ft. (6.2 m) of tubing, with Swagelok fittings to connect to the TGA100. The
outlet of the regulator must have a ¼” Swagelok fitting to attach this assembly. Alternatively, the user may provide this
connection to the TGA100. The reference gas inlet connection on the TGA100 is a ⅜” Swagelok fitting. A 200 ft
3 (5.7
m 3 ) tank of reference gas will last approximately one year at a continuous flow of 10 ml/min.
4.1.3 Detectors
The most commonly measured trace gases, methane and nitrous oxide, (as well as many other gases) have strong
absorption lines at wavelengths below 5 µm, and can be measured with the standard thermoelectrically-cooled
detectors. Some gases, such as ammonia, must be measured at longer wavelengths requiring special liquid nitrogen-
cooled detectors. Contact Campbell Scientific for information regarding long-wavelength detectors for the TGA100.
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