Vaisala GMP231 - Manual (Page 76)

User's Guide _______________________________________________________________________
74 ___________________________________________________________________ M211501EN-C
Using Ambient Air as Reference
You can use ambient air to do a 1-point calibration with the background
CO
2
level as your reference. You will need a calibrated reference
instrument to compare against, for example a GM70 hand-held meter
with a calibrated GMP222 probe.
With the GMP231 and the reference instrument in the same space (for
example, inside the incubator with the door open), allow the
measurement to stabilize before comparing the readings. Try to provide
as stable environment as you can during this time. Avoid working around
the incubator during this time.
Using a Calibration Gas as Reference
There are two easy ways to use a calibration gas as a reference:
- You can supply the gas to the GMP231 using the calibration adapter
accessory. Gas flow should be in the range 0.5 ... 1.1 l/min,
recommendation is 1 l/min. Allow the measurement to stabilize for
three minutes before starting the calibration.
- You can fill the entire incubator with the calibration gas.
To perform a two-point adjustment, you need two calibration gases:
one gas that is below 2 %CO
2
(low-end reference) and one that is above
2 %CO
2
(high-end reference).
When supplying the gas from a gas bottle, make sure the gas bottle has
stabilized to room temperature before starting.
Effect of Environmental Compensations
GMP231 has various environmental compensations that improve its CO
2
measurement accuracy (see section Environmental Compensation on
page 17). As the calibration and adjustment environment may differ from
the actual measurement environment, you must make sure that the
compensation settings are properly set. Here are some key points to
remember:
- Pressure and temperature compensations have a significant effect on
accuracy. If you are using setpoint values instead of the values from
built-in pressure and temperature sensors, make sure to correct the
setpoints so that they corresponds to your calibration situation.
Consider switching the compensations to use the internal sensors
when calibrating, and then switching back when calibration and
adjustment is done.