RESISTANCE TERMOMETERS RTD

  1. Working principle
  2. Platinum (Pt) resistence thermometers
  3. Nickel resistence thermometers
  4. Measuring methods for resistances thermometers
  5. Construction of resistance thermometers
  6. Traditional insulation thermoresistances
  7. Mineral Mgo insulation thermoresistances
  8. Main causes of errors in measurement with thermoresistances
  9. Reference tables
  10. Tolerances

The working principle for metal resistance thermometers, normally called thermoresistances, is bases on the variation of the electrical resistance of a metal with variations in the surrounding temperature.
In the industrial field the materials most frequently used are platinum and nickel which, due to their high resistivity and stability, permit the production of thermoelements which are highly reproducible, small and with excellent dynamic characteristics.
The temperature measurements carried out with thermoresistances are far more precise and reliable than those carried out with other types of sensor such as thermocouples or thermomistors.
Normally resistance thermometers are identified with the code of the material used to construct them (platinum = Pt, nickel = Ni etc.) followed by their nominal resistance at a temperature of 0°C .

 

TERMOTECH manufactures platinum resistance thermometers which comply with the international standard EN 60751; sensitive elements which conform to other standards, for example JIS C 1604 etc., may be supplied on request.
According to standard EN 60751 the platinum used for the manufacture of resistance thermometers should have a temperature coefficient of
alpha
= 3,851E-03
Standard EN 60751 allows for thermoresistances with a nominal value at 0 °C (Ro) of between 5 and 1000 ohm; however, the values most commonly used are 100 ohm, 500 ohm and 1000 ohm.
The equation linking resistance at temperature t° (Rt) and resistance at 0° (Ro) is a follows:


in the range -200°C / 0 °C
Rt = Ro [ 1+At+Bt²+C ( t-100 ) t³ ]

in the range 0 °C / 850 °C
Rt = Ro ( 1+At+Bt² )

Where the coefficients A, B and C have the following values:
A = 3,9083E-03
B = -5,775E-07
C = -4,183E-12

 

The classes of precision for platinum resistance thermometers refer to temperature and are standardized as follows:


Class AA = 0,1+0,0017* | t | ( °C )
Class A = 0,15+0,002* | t | ( °C )
Class B = 0,3+0,005 | t | ( °C )

Class C = 0,6+0,01 | t | ( °C )

 

The temperature ranges of validity of the tolerance classes above mentioned are reported in the table below.
All the resistance thermometers of tolerance class better then class B shall have three or four wire configuration.

 

Nickel resistance thermometers are standardized by the German Standard DIN 43760.
As opposed to platinum, nickel has a higher temperature coefficient (alpha= 6,17E-03) which, compensating for its lower electrical resistivity, makes its sensitivity comparable to that of platinum thermoresistances.
Their poor resistance to oxidation limits the range of use of nickel resistance thermometers to temperatures between -100°C e +200°C.
The equation linking resistance at temperature t° (Rt) and the resistance at 0° (Ro) is as follows:

 

There is only one class of precision for nickel resistance thermometers in the standard which refers to temperature:


In the range -60°C / 0°C: 0,4 + 0,028 | t | (°C)
In the range 0°C / 180°C: 0,4 + 0,007 | t | (°C)

 

There are different methods for connecting the resistance thermometers to the measuring devices, the choice of one method rather than another basically depends on the precision required in the measurement.

 

The two-wire technique is the least precise and is used only in cases where the connection of the thermoresistance is carried out with short and low resistivity wires; indeed testing the equivalent electrical circuit, it can be noted that the electrical resistance measured is the sum of that of the sensitive element (and, therefore, dependent on the temperature being measured) and the resistance of the conductors used for the connection.
The error introduced with this type of measurement is not constant: it depends on temperature.

2 wires connection

 

Due to the good degree of precision obtainable in measurements, the three-wire technique is the most used in the industrial field.
With this measurement technique the errors caused by the resistance of the conductors used for the connection of the thermoresistance are eliminated; at the output of the measuring bridge the voltage present depends entirely on the variation of the resistance of the resistance thermometer and consequently only on the temperature.

3 wires connection

 

The volt-ammeter four-wire technique offers the greatest precision possible; little used in the industrial field, it is almost exclusively used in laboratory applications.
On an equivalent electrical circuit it can be seen that the voltage measured depends solely on the resistance of the thermoelement; the precision of the measurement depends exclusively on the stability of the measuring current and the precision of the voltage reading across the thermoelement.

 

There are two construction types of thermoresistances: with traditional insulation or mineral MgO insulation.
The following table shows the main characteristics of the two types:

 

Response speed Electrical Insulation Vibration resistance Pressure resistance
Traditional insulation Good Excellent Good Good
Mineral (MgO) Insulation Excellent Good Excellent Excellent

Traditional insulation thermoresistances comprise:

1- Sensitive element
The sensitive element is the most important part of the thermoresistance, a poor quality sensitive element would jeopardize the correct functioning of the entire sensor. Once connected with the connection wires, it is placed inside the protective sheath. Sensitive elements with different degrees of precision and with double winding are available.

2- Connection wires
The connection of the sensitive element can be carried out using 2, 3 or 4 wires; the wire material depends on the conditions of use of the probe.

3- Ceramic insulators
Ceramic insulators prevent short circuits and insulate the connection wires from the protective sheath.

4- Filler
The filler is composed of extremely fine alumina powder, dried and vibrated, which fills any gaps so as to protect the sensor from vibrations.

5- Protective sheath
The protective sheath protects the sensitive element and the connection wires.
Since it is in direct contact with the process it is important that it is made of the right material and has the right dimensions.
In certain conditions it is advisable to cover the sheath with additional casing (thermowell).

6- Connection head
The connection head contains the terminal board made of insulating material (normally ceramic) which permits the electrical connection of the thermoresistance. Depending on the conditions of use explosionproof casing may be used. A 4-20mA converter can be installed instead of the terminal board.

This particular construction type permits the manufacture of high performance thermoresistances with excellent mechanical characteristics.
The main characteristics which differentiate this type of construction from the traditional type, in addition to those already described, are: the possibility of bending the sheath with a sharp bending radius, the possibility of soldering the sheath upon installation and the possibility of creating very long probes.

1- Sensitive element
With the use of particular techniques, the sensitive element is connected to the conductors of the cable insulated in mineral oxide.
To meet different requirements, it is possible to use double sensitive elements or elements with different degrees of precision.

2- Connection wires
The sensitive element can be connected using 2, 3 or 4 wires.

3- Mineral insulation sheath
This comprises an external metal sheath with the conductors insulated inside from one another and the sheath using extremely pure and highly compressed metal oxides; the standard insulator is magnesium oxide (MgO).

4- Connection head
The connection head contains the terminal board made of insulating material (normally ceramic) which permits the electrical connection of the thermoresistance.
Depending on the conditions of use explosionproof casing may be used. A 4-20mA converter can be installed instead of the terminal board.

 

Measuring temperature with thermoresistances is quite simple to carry out compared to that using other types of sensors, however certain steps should be taken to remedy any measurement errors. There are three main causes of errors introduced into temperature measurements with thermoresistances:
- Error due to the selfheating of the sensing element
- Error due to the poor electrical insulation of the sensitive element
- Error due to the sensitive element not being immersed at sufficient depth


The sensitive element heats up by itself during measuring when it is crossed by a current which is too high which, due to the Joule effect, increases the temperature of the element.
The increase in temperature depends both on the type of sensitive element used and the measuring conditions.
At the same temperature, the same thermoresistance will heat up by itself less if placed in water rather than air; this is due to the fact that water has a higher dispersion coefficient than air.
Normally all measuring devices which use thermoresistances as sensors have an extremely low measuring current, however it is advisable never to exceed a measuring current of 1 mA (EN 60751).

For the correct measurement with thermoresistances it is very important that the electrical insulation between the conductors and the external sheath is sufficiently great, particularly at high temperatures.
The insulation resistance may be seen as an electrical resistance positioned parallel to those of the sensitive element.
It is, therefore, clear how, at a constant temperature, should the electrical insulation diminish, the voltage measured across the sensitive element will also diminish thus introducing an error into the measurement.
Insulation resistance can fall when the probe is used at temperatures which are too high, when there are strong vibrations or because of the influence of physical or chemical agents.


The immersion depth of the sensitive element is also extremely important for correct measurements; unlike in thermocouples where measurements can be considered punctiform, if the depth is not sufficient it can cause errors in the measurement of as much as several degrees °C.
This is due to the fact that the sheath, usually metallic, with which the sensitive element is protected disperses heat in proportion to the difference in temperature between the hot and cold areas; we, therefore, have a thermal gradient along part of the sheath's length.
The immersion depth must, therefore, be sufficient so that the sensitive element inside the sheath is not subjected to this thermal gradient.
The minimum depth will depend on the physical measuring conditions and the dimensions of the thermoresistance (length of the element etc.).

°C 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 °C
Ohm
0 100,0 94,6 89,3 84,2 79,1 74,3 69,5 0
°C 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 °C
°C 0 10 20 30 40 50 60 70 80 90 °C
Ohm - Ohm
0 100,0 105,6 111,2 117,1 123,0 129,1 135,3 141,7 148,3 154,9 0
100 161,8 168,8 176,0 183,3 190,9 198,7 206,6 214,8 223,2 100
°C 0 10 20 30 40 50 60 70 80 90 °C
°C 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 °C
Ohm - Ohm
-200 18,49 14,45 10,49 6,99 4,26 2,51 -200
-100 60,26 56,19 52,11 48,00 43,88 39,72 35,54 31,34 27,10 22,83 -100
0 100,00 96,09 92,16 88,22 84,27 80,31 76,33 72,33 68,33 64,30 0
°C 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 °C
°C 0 10 20 30 40 50 60 70 80 90 °C
Ohm - Ohm
0 100,00 103,90 107,79 111,67 115,54 119,40 123,24 127,08 130,90 134,71 0
100 138,51 142,29 146,07 149,83 153,58 157,33 161,05 164,77 168,48 172,17 100
200 175,86 179,53 183,19 186,84 190,47 194,10 197,71 201,31 204,90 208,48 200
300 212,05 215,61 219,15 222,68 226,21 229,72 233,21 236,70 240,18 243,64 300
400 247,09 250,53 253,96 257,38 260,78 264,18 267,56 270,93 274,29 277,64 400
500 280,98 284,30 287,62 290,92 294,21 297,49 300,75 304,01 307,25 310,49 500
600 313,71 316,92 320,12 323,30 326,48 329,64 332,79 335,93 339,06 342,18 600
700 345,28 348,38 351,46 354,53 357,59 360,64 363,67 366,70 369,71 372,71 700
800 375,70 378,68 381,65 384,60 387,55 390,48 800
°C 0 10 20 30 40 50 60 70 80 90 °C
Tolerence class
Temperature range of validity
°C
Wire wound resistors
Film resistors
AA
-50 ÷ +250
-0 ÷ +150
±(0,1 + 0.0017 *| t |)
A
-100 ÷ +450
-30 ÷ +300
±(0,15 + 0.002 *| t |)
B
-196 ÷ +600
-50 ÷ +500
±(0,3 + 0.005 *| t |)
C
-196 ÷ +600
-50 ÷ +600
±(0,6 + 0.001 *| t |)
Temp. °C
0,4+0,07*| t | (°C) 0 °C<t<180 °C
0,4+0,028*| t | (°C) -60 °C<t<0 °C
Ohm
°C
-60
±1,00
±2,10
0
±0,20
±0,40
100
±0,80
±1,10
180
±1,30
±1,70