How to Convert RTD Resistance to Temperature

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An RTD resistance can be converted into temperature using standard tables that gives values of temperatures for any given resistance value of the RTD.
The table below shows temperature versus resistance data in degree celsius with temperature coefficient of resistance of: 0.003916 ohm/ohm/°C.
               Fundamental Interval 39.16 ohms
0C
0 100 200 300 400 500 600
0C
0 100 139.16 177.14 213.95 249.59 284.04 317.33 0
10 103.97 143.01 180.88 217.57 253.09 287.43 320.59 10
20 107.93 146.85 184.60 221.17 256.57 290.79 323.84 20
30 111.87 150.68 188.31 224.77 260.05 294.15 327.08 30
40 115.81 154.49 192.01 228.35 263.51 297.50
40
50 119.73 158.30 195.70 231.92 266.96 300.83
50
60 123.64 162.09 199.37 235.47 270.40 304.15
60
70 127.54 165.87 203.03 239.02 273.83 307.47
70
80 131.42 169.64 206.69 242.55 277.25 310.76
80
90 135.30 173.40 210.33 246.08 280.65 314.05
90
100 139.16 177.14 213.95 249.59 284.04 317.33
100
Ω/0C Ave
0.390 0.380 0.368 0.356 0.345 0.333 0.325

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An example of RTD temperature/resistance determination from the standard tables:

Example 1:
Calculate the resistance of an RTD thermometer when the temperature is:
a) 0 degree Celsius
b) 100 degree Celsius
c) 50 degree Celsius
d) 75 degree Celsius

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Solution:
(a) From the Temperature vs Resistance data tables,
At 0 degree Celsius, Resistance = 100 ohms
(b) At 100 degree Celsius, Resistance = 139.16 ohms
(c) At 50 degree Celsius, Resistance = 119.73 ohms
(d) At 700C, resistance = 127.54ohms

At 800C, resistance = 131.42ohms
At 750C, let resistance = X

Using interpolation method, we have:
(70 - 80)/(75 - 80) = (127.54 - 131.42)/(X - 131.42)
-10/-5 = -3.88/(X - 131.42)
-10(X - 131.42) = 19.4
X = 131.42 - 1.94 = 129.48

Example 2:
An RTD with a temperature coefficient of resistance of 0.390 has a resistance of 100Ω at 00C and a resistance of 139.16Ω at 1000C . If the RTD is used to measure the temperature of water in a water bath heater, what is the temperature of the water bath if the resistance of the RTD is 120Ω?

Solution:
At
0 degree C , RTD resistance = 100Ω

100 degree C, RTD resistance = 139.16Ω

Let X be the temperature of the water when RTD resistance is 120Ω
Using interpolation, we have:
(X - 0)/(100 - 0) = (120 - 100)/(139.16 - 100)
X/100 = 20/39.16
X = (20 X 100)/(39.16)
X = 51.07250C

Hence temperature of the water bath is 51 degree C
For more information on RTD Sensors, check out:
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