Campbell Scientific TGA100A Overview User Manual

Page 11

TGA100A Trace Gas Analyzer Overview

OV6.4 Absolute Concentration / Isotope Ratio Measurements

The TGA100A can be configured for highly accurate measurements of trace
gas concentrations by performing frequent calibration. The TGA100A has a
small offset error caused by optical interference. This offset error changes
slowly over time, with a standard deviation roughly equal to the short-term
noise. Offset errors have little effect on flux measurements by either the
gradient or eddy covariance technique, but may be important in other
applications. For measurements of absolute trace gas concentration, the offset
error can be removed by switching between a nonabsorbing gas (e.g. zero air)
and the sample, using the gradient mode of operation.

Applications such as isotopic ratio measurements require the highest possible
accuracy. This is achieved using a frequent two-point calibration to correct for
drift in the instrument gain and offset. High accuracy requires the flow rate for
the calibration gases to be the same as for the sample air. Even though the
sampling system can be designed so that calibration gases flow only when they
are used, frequent calibration (every few minutes) consumes a large amount of
calibration gas if high flow rates are used. The site means sampling mode is
normally used because it works well at low flow rates.

Figure OV6.4-1 illustrates a typical CO

isotope application. It is similar to the

site means example above, but it includes two intakes connected to calibration
tanks. A tank of nitrogen or CO

-free air is also shown connected to the

analyzer to purge the air gap between the laser dewar and sample cell. This
purge is required for CO

isotope measurements because of the high ambient

concentration of CO

and the need for high accuracy.

Sample
Intakes

TGA100A

Analyzer

Digital Control
Cable

Purge
Gas

Site

Selection

Sampling

System

Bypass

Sample Pump

Sample

Purge Dryer

Sample Dryer

Reference Gas

Sample

Dryer Purge

Calibration Gases

FIGURE OV6.4-1. Example CO

Isotope Application

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