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12.3.2
Saving the Zero Calibration Value
Once an auto-zero has successfully completed, the 'save' icon
will show in the top right soft-key location. Operating the save
key will store the zero calibration values and immediately start
to use the new values as the baseline reading.
Once saved, the save icon is replaced by the 'home' icon and
the soft-key will return the detector back to the main screen if
operated.
12.3.4
Notes On Zero Calibration
The purge gas source to the detector should be flowing at about 1 litre/min for the most reliable calibration. Flow rates
of 0.5 litre/min are acceptable but may take longer to bring the sensor to a stable zero point. Flow rates lower than 0.5
litre/min are not generally recommended for calibration purposes.
the purge gas must be left flowing for as long as necessary to bring the readings to a stable baseline point. Generally
this will be 60 - 120 seconds but could be as long as 5 minutes depending on the type of gas sensor and what was applied
to it prior to the purge gas source.
The trend bar on the calibration screens is designed to allow more visibility regarding the signal trend. It is often difficult
to gauge whether a reading is rising, falling or has stabilised by just looking at the displayed numbers.
The trend indicator will settle at the middle position once the raw sensor signal has stabilised and sat within a narrow
band of values for around 5 seconds. Once the trend indication is in the centre position, the reading can be deemed
stable enough to calibrate within the detectors specified accuracy limits.
It is important to note that over time, all gas sensors will show a small amount of cyclic readings. If the trend bar has
been used to gauge a stable reading and a calibration is carried out based on it indicating the centre position, the
numeric gas readings may still move from the point the calibration was set. This is normal and if observed long enough
will show up as slowly increasing and decreasing readings that sit well within the detectors specified accuracy limits.
During all calibration functions, any zero-suppression settings are over-ridden and set to the 'off' state.
Once calibration is completed and the main screen is returned to, zero-suppression will be set back to the state it was
before entering calibration modes.
Following a zero calibration, the current loop signal should be checked for a correct reading of 4.00mA ± 0.03mA.
12.4. Span calibration
Span calibration consists of applying target gas of known concentration and adjusting the detectors response
accordingly. The target gas is a mixture of gas that the detector is intended to detect and a balance gas such as nitrogen
or instrument air). The output response of the detector is both the displayed value and the value of the current output
on the current loop system. For detectors with HART communication enabled, measured values will be sent over the
HART protocol also.
12.3.3
Manual Zero Calibration
If a manual adjustment of the baseline is required after an
auto-zero completes, the up and down soft-keys will increase
or decrease the baseline reading. When a change in the
reading has been made, the save icon will be indicated again.
Saving the new manual set zero will store the reading and
revert the soft-key icon back to the 'home' function.
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