Vaisala GMP252 - Background Gas Compensation; Pressure Compensation; Probe Startup; Filtering Factor

2.4.2 Pressure Compensation
The probe does not have on-board pressure measurement. However, a pressure reading
from an external source can be used as a setpoint value for compensation using Vaisala
Industrial Protocol or Modbus.
If pressure compensation is turned
o, the probe uses the default compensation value of
1013 hPa.
2.4.3 Background Gas Compensation
The probe does not have on-board oxygen or relative humidity measurement. However,
oxygen and relative humidity readings from an external source can be used as setpoint
values for compensation using Vaisala Industrial Protocol or Modbus. The default setpoint
values are as follows:
• Oxygen concentration: 0 %O
2
or 21 %O
2
• Relative humidity: 0 %RH or 50 %RH
If background gas compensations are turned
o, the probe uses the value 0 % for both.
In practice, when CO
2
is measured at a ppm level, O
2
and RH compensations have a very
small eect on the accuracy of the measurement.
2.5 Probe Startup
When powered on, the probe starts up within 12 seconds. Measurements from the outputs
(digital and analog) become available during this time but note that they will only reach
specified accuracy after a 2-minute warm-up period. For this reason, you should design your
system so that it does not rely on measurements from the probe during this time. When the
probe is in analog output mode, the probe remains in an error state during the start-up
phase until measurement output becomes available.
Specifically
note that the CO
2
reading will rise to the correct reading as the sensor’s
infrared emitter achieves operating temperature.
2.6 Filtering Factor
You can set a filtering factor that aects the speed at which the latest CO
2
measurement is
integrated into the output of the probe. This allows averaging the output if the measuring
environment produces occasional exceptionally high or low readings.
The filtering factor can be set either with Modbus configuration registers or with an Indigo
transmitter's wireless
configuration interface.
Chapter 2 – Product Overview
17