Zero Calibration; Auto-Zero Calibration - GasTech D-Guard2 Operator's Manual

12.3. Zero Calibration

Zero calibration consists of applying a purge gas (i.e. a gas that does not contain any trace of the detector's intended
target gas) and adjusting the detectors response accordingly. 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.
12.3.1

Auto-Zero Calibration

Unlocked Menu System Screen
After unlocking the menu system, 'CAL' gives access to
detector calibration, 'SET' gives access to the configuration
I i
functions. The  and cons access detector information
screens. Select 'CAL' to continue.
Running The Auto Zero Process
Once the baseline gas is within the allowable range, the 'Run
Auto Cal' soft-key can be used to start the process.
A countdown timer indicates the time from pressing the key to
the process beginning. During this time the trend bar may be
observed for a stable reading as D-Guard
reading.
Unstable Reading Warning
If the sensor is old and has become electrically noisy, it is
possible that the readings will not be stable. D-Guard
the noise level and flags a warning if it is too great to perform
a reliable auto-zero calibration. The extent of the instability
can be gauged by the distance the trend marker moves from
the centre position. In some cases, if the sensor noise is
variable, the detector may revert back to the auto-zero
countdown. If instability returns, the stability warning screen
will again be shown. D-Guard
number of times before stopping.
2
checks for a stable
2 checks
2
re-tries the auto-zero process a
The 'Calibrate Zero' page is now shown
This page allows the detector's 'auto-zero' function to set the
baseline value. Operate the 'Run Auto Cal; soft-key to
continue. Purge gas should be applied to the detector at this
time.
Apply Purge Gas Warning
If the baseline gas concentration is greater than 10% of the
detector's full scale, a warning to apply purge gas is displayed.
A flashing gas cylinder icon is also displayed.
The 'Run Auto Cal' soft-key will not operate at this stage.
If not already connected, connect the purge gas supply to the
detector to continue.
Auto Zero Fail Message
2
If D-Guard has determined that an auto-zero is not possible,
the up and down soft-keys will become enabled, allowing a
manual zero adjustment to be made. It is important to note
however that a zero set manually due to a noisy sensor will
have to be based on best judgment of where the zero actually
lies amongst noisy readings.
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D-Guard