Simulating Temperature; Three-Wire Rtd Configuration; Simulating Rtd Inputs - Emerson Rosemount Analytical HART 5081-A-HT Instruction Manual

MODEL 5081-A

16.12 SIMULATING TEMPERATURE

16.12.1 General
The transmitter accepts either a Pt100 RTD (used in pH,
499ADO, 499ATrDO, 499ACL-01, 499ACL-02, and 499AOZ
sensors) or a 22k NTC thermistor (used in HX438 and Gx448
DO sensors and most steam-sterilizable sensors from other
manufacturers). The Pt100 RTD has a three-wire configura-
tion. See Figure 16-4. The thermistor has a two-wire config-
uration.
16.12.2 Simulating temperature
To simulate the temperature input, wire a decade box to the
analyzer or junction box as shown in Figure 16-5.
To check the accuracy of the temperature measurement, set
the resistor simulating the RTD to the values indicated in the
table and note the temperature readings. The measured tem-
perature might not agree with the value in the table. During
sensor calibration an offset might have been applied to make
the measured temperature agree with a standard thermome-
ter. The offset is also applied to the simulated resistance. The
controller is measuring temperature correctly if the difference
between measured temperatures equals the difference
between the values in the table to within ±0.1°C.
For example, start with a simulated resistance of 103.9 W,
which corresponds to 10.0°C. Assume the offset from the
sensor calibration was -0.3 W. Because of the offset, the ana-
lyzer calculates temperature using 103.6 W. The result is
9.2°C. Now change the resistance to 107.8 W, which corre-
sponds to 20.0°C. The analyzer uses 107.5 W to calculate
the temperature, so the display reads 19.2°C. Because the
difference between the displayed temperatures (10.0°C) is
the same as the difference between the simulated tempera-
tures, the analyzer is working correctly.
FIGURE 16-4. Three-Wire RTD Configuration.
Although only two wires are required to connect
the RTD to the analyzer, using a third wire allows
the analyzer to correct for the resistance of the
lead wires and for changes in the lead wire resist-
ance with temperature.
FIGURE 16-5. Simulating RTD Inputs.
The figure shows wiring connections for sensors
containing a Pt 100 RTD. For sensors using a 22k
NTC thermistor (Hx438 and Gx448 sensors), wire
the decade box to terminals 1 and 3 on TB6.
Temp. (°C) Pt 100 (W)
0 100.0
10 103.9
20 107.8
25 109.7
30 111.7
40 115.5
50 119.4
60 123.2
70 127.1
80 130.9
85 132.8
90 134.7
100 138.5
SECTION 16.0
TROUBLESHOOTING
22k NTC (kW)
64.88
41.33
26.99
22.00
18.03
12.31
8.565
6.072
4.378
3.208
2.761
2.385
1.798
93