RTD Sensors Operation & Maintenance Instructions

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A RTD Sensing element consists of a wire coil or deposited film of pure metal. The element’s resistance increases with temperature in a known and repeatable manner. RTD’s exhibit excellent accuracy over a wide temperature range.

  • Temperature range: -200 to 700ºC
  • Sensitivity: the voltage drop across an RTD provides a much larger output than a thermocouple.
  • Linearity: Platinum and copper RTD’s produce a more linear response than thermocouples or thermistors. RTD non-linearities can be corrected through proper design of resistive bridge networks.

The most commonly used element material is platinum with a resistance of 100 ohms @ 0ºC and a temperature coefficient (Alpha) of 0.00385 ohms/ohm/ºC.

Other element materials also used are copper, nickel and nickel-iron. Platinum elements predominate because of their wider range, and because platinum is the most repeatable and stable of all metals.

Tolerance of PT100 Ω (Alpha = 0.003850 @ 0ºC)

CLASS ‘B’ ± 0.12 Ω OR ± 0.30ºC
CLASS ‘A’ ± 0.06 Ω OR ± 0.15ºC
1/3 DIN ± 0.04 Ω OR ± 0.10ºC
1/10 DIN ± 0.012 Ω OR ± 0.03ºC

Connection / Wiring details

Different connection Types. Standard Colour code; A is white, B is red.

2 wire Basic connection where the lead is short. No lead wire compensation, introducing an error into the reading.
3 wire Most common connection 3 wire, the instrument measures the lead wire resistance in the B legs and allows for this in its reading.
4 wire 4 wire connection is the most accurate measurement. The instrument measures the lead resistance of all four lead wires removing these values for its reading
Duplex RTD Duplex 3 wire RTD connection as per single RTD but two individual element windings.

Table for Platinum resistance elements.

Resistance values in Ohms from 0°C to + 400°C.
PT100 ohm RTD to DIN 43760 - IEC 751 – EN60 751
R(0) = 100 ohm °C 0

°C 0 1 2 3 4 5 6 7 8 9
0 100.00 100.39 100.78 101.17 101.56 101.95 102.34 102.73 103.12 103.51
10 103.90 104.29 104.68 105.07 105.46 105.85 106.24 106.63 107.02 107.40
20 107.79 108.18 108.57 108.96 109.35 109.73 110.12 110.51 110.90 111.28
30 111.67 112.06 112.45 112.83 113.22 113.61 113.99 114.38 114.77 115.15
40 115.54 115.93 116.31 116.70 117.08 117.47 117.85 118.24 118.62 119.01
50 119.40 119.78 120.16 120.55 120.93 121.32 121.70 122.09 122.47 122.86
60 123.24 123.62 124.01 124.39 124.77 125.16 125.54 125.92 126.31 126.69
70 127.07 127.45 127.84 128.22 128.60 128.98 129.37 129.75 130.13 130.51
80 130.89 131.27 131.66 132.04 132.42 132.80 133.18 133.56 133.94 134.32
90 134.70 135.08 135.46 135.84 136.22 136.60 136.98 137.36 137.74 138.12
100 138.50 138.88 139.26 139.64 140.02 140.39 140.77 141.15 141.53 141.91
110 142.29 142.66 143.04 143.42 143.80 144.17 144.55 144.93 145.31 145.68
120 146.06 146.44 146.81 147.19 147.57 147.94 148.32 148.70 149.07 149.45
130 149.82 150.20 150.57 150.95 151.33 151.70 152.08 152.45 152.83 153.20
140 153.58 153.95 154.32 154.70 155.07 155.45 155.82 156.19 156.57 156.94
150 157.31 157.69 158.06 158.43 158.81 159.18 159.55 159.93 160.30 160.67
160 161.04 161.42 161.79 162.16 162.53 162.90 163.27 163.65 164.02 164.39
170 164.76 165.13 165.50 165.87 166.24 166.61 166.98 167.35 167.72 168.09
180 168.46 168.83 169.20 169.57 169.94 170.31 170.68 171.05 171.42 171.19
190 172.16 172.53 172.90 173.26 173.63 174.00 174.37 174.74 175.10 175.47
200 175.84 176.21 176.57 176.94 177.31 177.68 178.04 178.41 178.78 179.14
210 179.51 179.88 180.24 180.61 180.97 181.34 181.71 182.07 182.44 182.80
220 183.17 183.53 183.90 184.26 184.63 184.99 185.36 185.72 186.09 186.45
230 186.82 187.18 187.54 187.91 188.27 188.63 189.00 189.36 189.72 190.09
240 190.45 190.81 191.18 191.54 191.90 192.26 192.63 192.99 193.35 193.71
250 194.07 194.44 194.80 195.16 195.52 195.88 196.24 196.60 196.96 197.33
260 197.69 198.05 198.41 198.77 199.13 199.49 199.85 200.21 200.57 200.93
270 201.29 201.65 202.01 202.36 202.72 203.08 203.44 203.80 204.16 204.52
280 204.88 205.23 205.59 205.95 206.31 206.67 207.02 207.38 207.74 208.10
290 208.45 208.81 209.17 209.52 209.88 210.24 210.59 210.95 211.31 211.66
300 212.02 212.37 212.73 213.09 213.44 213.8 214.15 214.51 214.86 215.22
310 215.57 215.93 216.28 216.64 216.99 217.35 217.70 218.05 218.41 218.76
320 219.12 219.47 219.82 220.18 220.53 220.88 221.24 221.59 221.94 222.29
330 222.65 223.00 223.35 223.70 224.06 224.41 224.76 225.11 225.46 225.81
340 226.17 226.52 226.87 227.22 227.57 227.92 228.27 228.62 228.97 229.32
350 229.67 230.02 230.37 230.72 231.07 231.42 231.77 232.12 232.47 232.82
360 233.17 233.52 233.87 234.22 234.56 234.91 235.26 235.61 235.96 236.31
370 236.65 237.00 237.35 237.70 238.04 238.39 238.74 239.09 239.43 239.78
380 240.13 240.47 240.82 241.17 241.51 241.86 242.20 242.55 242.90 243.24
390 243.59 243.93 244.28 244.62 244.97 245.31 245.66 246.00 246.35 246.69
400 247.04                  

There is no maintenance required for RTD sensors however, scheduled calibration checks at ice point (0ºC) are recommended.

RESISTANCE THERMOMETERS AND THEIR ELEMENTS

According to Australian Standard 2091 – 1981 - PT100 Ω DIN 43760 – EN 60 751 ALPHA 0.00385 BS EN 60751 1996, IEC 751 1995, DIN 43760 1980

Method for Determining Ice-Point (0C) Resistance

a. Insulated container. An insulated container at least 300 mm deep and with an internal diameter of 100 mm.

PROCEDURE. The procedure shall be as follows:

(a) Fill the insulated container with finely divided ice made from distilled water.
Note: If ice water from distilled water is not available, the clear part of a block of commercial ice will suffice, provided that all surfaces are first washed with distilled water.

b) Mix the ice with distilled water which has been pre-cooled, using the stirrer, and then drain off the excess water. The ice should be glassy-looking but there should be no free water remaining.

c) Connect the thermometer to an appropriate resistance measuring device and adjust so that the electrical power dissipated in the element does not exceed 1 m W.

d) Immerse the thermometer in the ice so that the element is at a depth of at least 150 mm. Ensure that the lower tip of the thermometer is at least 30 mm clear of the bottom of the container. Note: Thermometers with stem lengths less than 150 mm should be immersed to their maximum possible depth.

e) When the element reaches equilibrium with the ice, measurements may be taken. Measurements carried out with direct current shall be made with the current in both the forward and reverse directions. Note: The time taken for the element to reach equilibrium is normally about 3 minutes.

f) Decrease the depth of immersion of the element by 50 mm or 20 percent of the stem length, whichever is the smaller.

g) Repeat step (e). If the change in reading is more than on-third the appropriate tolerance given in Clause 2.2, the whole procedure shall be repeated with fresh ice.

Workshop certificate. The workshop certificate shall state the following information:

a. Identify instrument ie PT100 Ω DIN 43760 – EN60 751 ALPHA 0.00385
b. The measured resistance at 00C.
c. The difference between the nominal resistance and the measured resistance at 00C.
d. Uncertainty of Bath

Accuracy: RTD Sensors PT100 Ω at 0 ºC = Class B +/- 0.3 ºC, Class A +/- 0.15 ºC, 1/10 DIN = +/- 0.03 ºC