Vaisala GMW90 - Manual (Page 66)

User's Guide _______________________________________________________________________
64 ___________________________________________________________________ M211659EN-B
Notes for CO
2
Adjustment
1-point CO
2
adjustment affects either offset or gain, depending on the gas
concentration. 1-point adjustment below 700 ppm affects the
measurement offset, above 700 ppm it affects the gain.
2-point adjustment will adjust both offset and gain. Choose the
adjustment points as follows:
- For first point, use as low concentration as possible. Preferably 0 ppm,
must be below 700 ppm.
- For second point, use as high concentration as is relevant for your
application. Must be within 700 ... 5000 ppm.
Using Calibration Gas vs. Ambient Gas
You can adjust using a calibration gas with a known concentration
(recommended), or adjust using the ambient gas:
- When using a calibration gas, feed the gas to the adjustment inlet at a
flow rate of 0.3 ... 0.7 L/min. For location of the inlet, see Figure 2 on
page 15. After connecting the gas supply to the field adjustment inlet,
wait for at least three minutes for the reading to stabilize.
- When using ambient gas for adjustment, avoid breathing near the
transmitter during the adjustment.
Regardless of the CO
2
reference used, it is important to let the CO
2
measurement stabilize before performing the adjustment. Three minutes
is typically enough. Also allow for the stabilization period after
adjustment, before verifying the result.
Effect of Temperature on CO
2
Measurement
The device is fully temperature compensated using internal temperature
measurement.