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14.0 Zero calibration additional
Information
A zero calibration menu routine is included in the Series 200, however it
is important to understand when this should be used. All Aeroqual sen-
sors are supplied pre-calibrated and zero calibration is performed in the
factory in an atmosphere containing < 0.5 ppm hydrocarbons, 50% rela-
tive humidity and 25 oC. Zero baseline reading checks should be per-
formed ideally under the same conditions.
Why is the baseline not zero when no sensor gas is pre-
sent ?
Under some circumstances the sensors may exhibit a baseline reading
higher than zero. This will be due to one of the following:
•
The sensor has not warmed up sufficiently. The sensor
should be left to run in clean air for 24 hours if it is new or
has not been used in the past week.
•
There are trace levels of gases present in the air to which the
sensor is cross-sensitive. In many cases, the non-zero base-
line reading occurs in an environment which is supposed to
be "clean" but for which no analysis has been carried out,
such as, an office or urban outdoor air. Re-check the base-
line reading in clean air from a known source such as a zero
air cylinder or clean outdoor air.
•
The humidity of the air is significantly different to 50 % RH.
Under this condition the sensor may display a small shift in
the baseline reading (<1% of fullscale).
•
There has been some drift in the baseline.
When should I perform a zero calibration ?
•
If the sensor has been warmed up in a clean air environment
and it still displays a small positive after 24 hours then a zero
calibration can be performed. This process takes only a few
minutes and can be performed repeatedly without problems.
(See section 4.5 Zero calibration.)
