Emerson Rosemount5081 Instruction Manual page 72

Diagnostics and troubleshooting
3.
62
Figure 8-3: Three-wire RTD
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.
Table 8-1: RTD resistance values
Temperature
0 °C (32 °F)
10 °C (50 °F)
20 °C (68 °F)
25 °C (77 °F)
30 °C (86 °F)
40 °C (104 °F)
50 °C (122 °F)
60 °C (140 °F)
70 °C (158 °F)
80 °C (176 °F)
90 °C (194 °F)
100 °C (212 °F)
• 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.
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
measured temperature should equal 25 °C (77 °F) within ±1 °C.
Pt-100 resistance
100.0 ohms
103.9 ohms
107.8 ohms
109.6 ohms
111.7 ohms
115.5 ohms
119.4 ohms
123.2 ohms
127.1 ohms
130.9 ohms
134.7 ohms
138.5 ohms
Pt-1000 resistance
1000 ohms
1039 ohms
1078 ohms
1096 ohms
1117 ohms
1155 ohms
1194 ohms
1232 ohms
1271 ohms
1309 ohms
1347 ohms
1385 ohms
Table 8-1. The
Rosemount 5081