Range Calibration of RTD Transmitters

Use the step-by-step instructions below to field calibrate RTD Transmitters to the desired temperature range. For information about accuracy, see Special Notes on Field Calibration below.

Step 1

Assemble required equipment: temperature transmitter, 24 VDC power supply, decade box [Model RSU-280 or equal], digital VOM [Fluke Model 87 or equal], trim screwdriver, RTD Resistance vs.Temperature Chart.

Step 2

Using the RTD Resistance vs. Temperature Chart for 1000 ohm Platinum, select and record the resistance values for the high and low temperatures in your desired range. Designate these values as Low Temp Ohms and High Temp Ohms.

Step 3

Calculate the calibration factor using the ohms recorded in Step 2:

• Cal Factor = (High Temp Ohms – Low Temp Ohms) / 16

Step 4

Using the resistance decade box, select a resistance value within one ohm of the low temperature ohms in Step 2. Do not use a lower value. Measure this resistance with the VOM and record the actual value accurate to hundredths of an ohm. This value will be referred to as MIN REF OHMS. Select a resistance value within one ohm of the high temperature ohms in Step 3. Do not use a higher value. Measure this resistance with the VOM and record the actual value accurate to hundredths of an ohm. This value will be referred to as Max Ref Ohms.

Calculate the low mA reference:

• Low Ma Ref = (Min Ref Ohms – Low Temp Ohms /Cal Factor) + 4

Calculate the high mA reference:

• High Ma Ref = (Max Ref Ohms – Low Temp Ohms / Cal Factor) + 4

Step 5

Connect the transmitter as shown:

Step 6

Set the Zero and Span potentiometers:

1. Set the Min Ref Ohms on the decade box and adjust the Zero potentiometer on the transmitter for the Low mA REF calculated in Step 4
2. Set the Max Ref Ohms on the decade box and adjust the Span potentiometer on the transmitter for the High mA REF calculated in Step 4.
3. Repeat A and B as necessary.

Special Notes On Field Calibration

The accuracy of a field-calibrated RTD transmitter is highly dependent on the accuracy of the ohmmeter used to measure the sensor substitution resistances (MIN and MAX REF OHMS). The percent accuracy of the calibrated RTD transmitter is not the same as the percent accuracy of the ohmmeter.

OHMMETER ACCURACY TRANSMITTER ACCURACY (% of reading)