AMC-DTM-Sensor Transmitter
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6.2.2 Span
Adjustment
ALARM 1
> 50.0%LEL
F
A1 A2 A3
Figure 6-2: Alarm 1 Menu
Operator then chooses “Span” in the “calibrate” menu. Operator is then required to enter the
concentration of the gas used for calibration. Operator then applies the gas used for calibration.
After this is done, operator continues in the same fashion as with Zero concentration. After the
button “yes” is pressed again, calibration is completed.
Note: The current gas concentration displayed during the calibration procedure may be different
from expected by a small percentage of the Range because the instrument is not yet calibrated.
(e.g. when calibration concentration of 100ppm is applied, instrument can display e.g. 102ppm.).
The calibration concentration (Span) is permitted to be greater than the Range concentration as
long as the concentration can be properly measured with the given parameters (e.g. Range
concentration is 100ppm but the gas used for calibration is 106ppm).
Note: The transmitter electronics itself is
not
calibrated. It is assumed that the transmitter
outputs 20mA at Range and 4mA at Zero without actually measuring these values. It is the
responsibility of the monitoring unit to calibrate the current loop.
To guard against the possibility of an incorrect or incomplete calibration, the newly calibrated
points are compared with the previous calibration points. If new Span point is off by more than
20% of the value of the previous point, a warning is displayed requiring the operator to confirm if
the values are correct or not. If the new Zero value is different from the old one by more than
3% of Range, similar warning is displayed.
Operator is allowed to calibrate only one point. Unit detects this condition and asks the operator
to confirm this intention.
Helpful messages such as: “set Span”, “apply Span”, “apply Zero”, “calibration completed”,
“calibration cancelled” are displayed during the calibration to guide the operator.
6.2.3 Using Gas Depletion As A Measure Of Another Gas’s Concentration
In some applications depletion of one gas (usually oxygen) is used to detect the presence of
another gas. In these cases, concentration of the target gas is displayed on the LCD although it
is oxygen that is being measured. The transmitter is configured to work in such a mode by
simply calibrating it. At calibration completion, if the transmitter recognizes that the Span point
is lower than the Zero point, a warning message is displayed. The operator is required to
confirm the values. If the operator confirms the values, the transmitter applies an inverse
function to all the measured values making it appear as though the target gas is being
measured.
