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3-7
Calibration
UsinganIceBath
Calibration Equipment Required
Millivolt Source. Precision voltage source providing outputs from –10 to 100
mV. Reflect accuracy of four times better that the 444 transmitter is
recommended (0.05% of calibrated span or 0.005 mV which ever is greater).
Voltmeter. Such as a 5-digit DVM. Accuracy is ±0.01%; resolution is 1 mV.
dc Power Supply. Power capability is 24 V dc at
35 mA.
Thermocouple Wire.Use the same type as that used in the construction of the
thermocouple.
Readout Resistor. The transmitter test terminals give a 40–200 mV signal. If
this is not suitable for the test equipment available, a ±0.1% tolerance, 0.5 W
precision wirewound resistor is needed. Suggested values include a 100-ohm
resistor to give a 0.4 to 2 volt output; 250 ohms for 1 to 5 volts; or 500 ohms for
2 to 10 volts.
Ice Bath. For highest accuracy, a stirred ice bath (such as a Rosemount 911A)
should be used, as well as ice made from deionized or distilled water.
Input Monitor Voltmeter. Use to monitor source when required. Resolution of
±0.001 mV for ranges up to 100 mV. This can be the same as voltmeter used to
measure transmitter output if rangeability and resolution are sufficient for
both levels.
Calibration Procedure 1. Remove the cover from the terminal side of the transmitter housing.
2. Install the equipment as shown in Figure 3-5 and allow the thermocouple
junctions to stabilize at the ice point. Make terminal connections with
miniature banana plugs.
3. If trimming the transmitter to a new range, you may have to reposition the
Coarse Zero Jumper on the Range Board. If so, see the disassembly
procedure on page 4-4. Position the jumper in the location shown in Table .
(A transmitter with a base temperature outside the regions shown in Table
is a special design and does not contain a Coarse Zero Jumper. Also, Model
444 TR and TS transmitters do not have Coarse Zero Jumpers.)
4. Determine the thermocouple millivolt levels at the desired base and full
scale temperatures. See NIST Monograph 125 or
Table B-1.
5. Turn the power on.
6. Set the millivolt source until the monitoring voltmeter reads the emf
corresponding to the desired base temperature. Adjust the zero pot until
the output is 4 mA.
7. Set the millivolt source until the monitoring voltmeter reads the emf
corresponding to the desired full scale temperature. Adjust the span pot
until the output is 20 mA.
8. Repeat steps 6 and 7 until you obtain the 4 and 20 mA readings without
readjusting the pots. Use the “overshoot” technique described in step 9 of
the RTD calibration procedure, if desired.
9. Mark the correct range in the “calibration” space on the nameplate.