7.9.12.5.1 FieldCal() Zero or Tare (Opt 0) Example
Most CRBasic measurement instructions have a multiplier and offset parameter.
FieldCal() Option 0 adjusts the offset argument such that the output of the sensor
being calibrated is set to the value of the FieldCal() KnownVar parameter, which
is set to 0. Subsequent measurements have the same offset subtracted. Option 0
does not affect the multiplier argument.
Example Case: A sensor measures the relative humidity (RH) of air. Multiplier is
known to be stable, but sensor offset drifts and requires regular zeroing in a
desiccated chamber. The following procedure zeros the RH sensor to obtain the
calibration report shown. To step through the example, use the CR1000KD
Keyboard Display or software numeric monitor
(p. 521) to change variable values as
directed.
Table 29. Calibration Report for Relative Humidity Sensor
CRBasic Variable At Deployment At 30-Day Service
SimulatedRHSignal output 100 mV 105 mV
KnownRH (desiccated
chamber)
0 % 0 %
RHMultiplier
0.05 % / mV 0.05 % / mV
RHOffset
-5 % -5.25 %
RH
0 % 0 %
1. Send CRBasic example FieldCal() Zero (p. 214) to the CR1000. A terminal
configured for excitation has been programmed to simulate a sensor output.
2. To place the simulated RH sensor in a simulated-calibration condition (in the
field it would be placed in a desiccated chamber), place a jumper wire between
terminals VX1 and SE1. The following variables are preset by the program:
SimulatedRHSignal = 100, KnownRH = 0.
3. To start the 'calibration', set variable CalMode = 1. When CalMode increments
to 6, zero calibration is complete. Calibrated RHOffset will equal -5% at this
stage of this example.
4. To continue this example and simulate a zero-drift condition, set variable
SimulatedRHSignal = 105.
5. To simulate conditions for a 30-day-service calibration, again with desiccated
chamber conditions, keep variable KnownRH = 0.0. Set variable CalMode =
1 to start calibration. When CalMode increments to 6, simulated 30-day-
service zero calibration is complete. Calibrated RHOffset will equal -5.2 %.
214
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