Three-wire RTDFigure 8-3:
Consult the table for resistance-temperature data. Lead resistance is about 0.05 ohm/ft at 25 °C.
Therefore, 15 feet of cable increases the resistance by about 1.5 ohm. The resistance between the
RTD return and RTD sense leads should be less than 2 ohms.
RTD resistance valuesTable 8-1:
Temperature Pt-100 resistance Pt-1000 resistance
0 °C (32 °F) 100.0 ohms 1000 ohms
10 °C (50 °F) 103.9 ohms 1039 ohms
20 °C (68 °F) 107.8 ohms 1078 ohms
25 °C (77 °F) 109.6 ohms 1096 ohms
30 °C (86 °F) 111.7 ohms 1117 ohms
40 °C (104 °F) 115.5 ohms 1155 ohms
50 °C (122 °F) 119.4 ohms 1194 ohms
60 °C (140 °F) 123.2 ohms 1232 ohms
70 °C (158 °F) 127.1 ohms 1271 ohms
80 °C (176 °F) 130.9 ohms 1309 ohms
90 °C (194 °F) 134.7 ohms 1347 ohms
100 °C (212 °F) 138.5 ohms 1385 ohms
• If a connections is open or shorted and it should not be, the sensor has failed.
Replace the sensor.
• If the measured resistances are acceptable, go to Step 3.
3.
Check the transmitter. Disconnect the RTD sensor leads and wire the circuit shown
in Figure 8-4. Set the resistance to the value for 25 °C (77 °F) shown in Table 8-1. The
measured temperature should equal 25 °C (77 °F) within ±1 °C.
Diagnostics and troubleshooting
62 Rosemount 5081
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