Specifications

molbox™ RFM™ OPERATION AND MAINTENANCE MANUAL
© 1998-2007 DH Instruments, a Fluke Company Page 142
The value of PA must be entered in the pressure unit Pascal [Pa]. If your calibration data
is in a unit other than Pascal, the calculated value of PA will be in the other unit and must
be converted to Pascal before it is entered.
If you are not familiar or comfortable with the mathematics for PA and PM manipulation,
it is recommended you verify the as left readings as suggested in Step w above. This will
provide practical confirmation of the integrity of the calculations. Note also that the
CalTool for RPTs software delivered with the molbox RFM performs the necessary
calculations automatically.
5.3 OHMIC MEASUREMENT SYSTEM VERIFICATION
PURPOSE
To verify the measurements made by molbloc RFM’s ohmic measurement system.
PRINCIPLE
molbox RFM includes an ohmic measurement system used to measure the resistance of the two
Platinum Resistance Thermometers (PRTs) mounted in molblocs. The temperature of the PRTs is
calculated from the resistance. The molbloc PRTs have nominal resistance at 0 °C of 100 Ω and a slope
of 0.3896 following DIN Norm 43760. The measured resistance at 0 °C for each PRT is stored on the
molbloc EEPROM and read by the molbox RFM.
The ohmic measurement system self-calibrates using on-board 100 and 110 Ω reference resistors. On
molbox RFM power up, an automated routine adjusts the ohmic measurement system relative to on-
board, reference resistor readings. During this routine, “System autocal” is displayed on the molbox RFM
display. The on-board reference resistors used to calibrate the molbox RFM measurement system have
accuracy of ± 0.01 % and stability of ± 0.0025 % for one year, ± 0.005 % for three years.
The self-calibration feature and the very high accuracy and stability of the reference resistors relative to
the accuracy tolerance on the molbox RFM temperature measurements make it unnecessary to calibrate
the ohmic measurement system independently. It is a good practice, however, to verify the ohmic
measurement system. This is most easily accomplished globally by connecting a known resistance value
to the measurement circuit where the molbloc PRTs are normally connected and verifying that the molbox
RFM reads the correct corresponding temperature when that resistance is connected. Reference
resistors mounted in a molbloc simulator for easy connection to molbox RFM using the normal molbloc
connection cable can be purchased from your DHI Sales Representative (molbloc simulator, 107 Ω, P/N
401232). These are nominally 107 Ω resistors (roughly equivalent to a 20 °C measurement) whose
values are reported and can be measured independently if desired. The measured resistance of the
reference resistors and a value for resistance at 0 °C of 100 Ω are programmed on the
simulator EEPROM. Using these values, if the molbox RFM ohmic measurement system and associated
connections are working properly, when the simulator is connected the molbloc temperature indicated by
molbox RFM should equal the reference temperature corresponding to the molbloc simulator reference
resistor following:
Reference temperature [
°
C] =
Reference resistor value [Ω] - 100
0.3896
The Reference temperature [°C] calculated from the actual reference resistor value is written on the
molbloc simulator. The temperature value read by the molbox RFM when the molbloc simulator is
connected can be observed by pressing [P&T] (see Section 3.4.5). The disagreement between the
reference temperature and the molbloc indication should not exceed 0.05 °C.