7. MAINTENANCE OF TRACEABILITY AND RECALIBRATION
Page 177
© 2007 DH Instruments, a Fluke Company
7.5.6
EXTERNAL PRT TEMPERATURE ADJUSTMENT
PURPOSE
To view and adjust the output of the external (Reference Mass) PRT.
The ambient temperature sensor and External PRT temperature sensor are of the same
technology and read in the same manner. However, their recommended calibration procedures
differ reflecting the different measurement uncertainty specifications of the two measurements.
The LCM default front panel display shows the output of the External PRT in its lower left
corner on powerup (see Figure 112).
It is recommended to calibrate the external PRT with the PRT connected to the LCM, but the
PRT can be removed and calibrated using the 4 wire resistance method alone. If the PRT is
removed for calibration by conventional PRT methods then it is necessary to verify the LCM’s
Ohmic Measurement System and to measure and/or adjust the onboard Standard Resistors
per the steps in Section 7.5.8 and 7.5.9.
A one point verification against a temperature reference is all that is needed.
If a full adjustment is desired, at least two points should be selected within the range of the
sensor and encompass the range of measurements the sensor may encounter during normal
operation.
To adjust the value of Rzero use GFS Tools and select the
[Setup]
,
[Calibration]
,
<External
PRT>
tab (see Figure 114) to access a screen in which the Rzero value can be edited.
Normally, only Rzero is adjusted. Calculate a new Rzero using the following formula:
Rzero(new) = Rzero(current) + ((Tdisplay – Tref)*0.3896)
Where:
Rzero(new) is the new Resistance at 0°C in ohms.
Rzero(current) is the current resistance at 0°C in ohms from the
<External PRT>
tab
(see
Figure 114).
Tdisplay is the observed temperature in °C from the LCM display.
Tref is the reference temperature reading in °C.
From here, press
OK
and wait while the coefficients are uploaded to the LCM. When the
upload is complete, cycle the power on the LCM, and the front display reading should reflect
the newly applied calibration coefficients.
See Section 1.1 for External PRT specifications.