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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.

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

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:

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

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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**

^{0}C | 0 | 100 | 200 | 300 | 400 | 500 | 600 | ^{0}C |

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/^{0}C Ave | 0.390 | 0.380 | 0.368 | 0.356 | 0.345 | 0.333 | 0.325 |

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

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: