Resistance Thermometer Information

Resistance thermometers are widely used temperature sensors in the temperature range of -200°C, +850°C (Especially in laboratory applications). At low temperature values, they have much more accuracy than the thermocouples have.

Resistance thermometers are constructed on the idea of resistance change while a small current is applied for itself. Encircled resistance is plunged into process environment and a constant current is applied. Encircled resistance is changed by the temperature change. Therefore, a voltage value is created which is bounded with applied current.

Resistance thermometers require a small current to be passed through in order to determine the resistance. This can cause self-heating, and manufacturers' limits should always be followed along with heat path considerations in design. Care should also be taken to avoid any strains on the resistance thermometer in its application. Lead wire resistance should be considered, and adopting three and four wire connections can eliminate connection lead resistance effects from measurements.

Temperature change of resistance thermometer is formulated as the one below:

a : Temperature change factor of resistance thermometer
R₀ : Resistance at 0°C
R₁₀₀ : Resistance at 100°C

a=	R₁₀₀ - R₀

	100R₀

Temperature-resistance chance values are compatible with IEC751 standards. PT100 and NI-100 have 100ohm resistance value with the tolerance of ±0,1 at 100C. Temperature-resistance change values can be calculated using the formula below:

Rt = Ro (1 + At + Bt)
Rt = Resistance value at temperature T.
Ro = Resistance at 0°C
t = Temperature
A = 0390784x10¯²C¯¹(constant)
B = 0,578408x10¯ 6 C¯²(constant)

Below, you can find the resistance thermometer inductance graph for PT100 (IEC 751)

TEMPERATURE °C	TOLERANCE IEC 751:1983 (BS EN 60751:1996)
	A CLASS		B CLASS
	± °C	± OHM	± °C	± OHM
-200	0.55	0.24	1.3	0.56
-100	0.35	0.14	0.8	0.32
0	0.15	0.06	0.3	0.12
100	0.35	0.13	0.8	0.30
200	0.55	0.20	1.3	0.48
300	0.75	0.27	1.8	0.64
400	0.95	0.33	2.3	0.79
500	1.15	0.38	2.8	0.93
600	1.35	0.43	3.3	1.06
650	1.45	0.46	3.6	1.13
700	-	-	3.8	1.17
800	-	-	4.3	1.28
850	-	-	4.6	1.34

Resistance thermometers that is mentioned in this catalog are tought to be able to connect with mounting bush and flange.

Cupper wires are used between devices and resistance thermometer head. Two wires for 10m, three wires for 10-150m and four wires are used for 150+

The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is not required as the resistance of the connecting wires is always included with that of the sensor leading to errors in the signal. Using this configuration you will be able to use 100 metres of cable. This applies equally to balanced bridge and fixed bridge systems. The values of the lead resistance can only be determined in a separate measurement without the resistance thermometer sensor and therefore a continuous correction during the temperature measurement is not possible.
In order to minimize the effects of the lead resistances a three wire configuration can be used. Using this method the two leads to the sensor are on adjoining arms, there is a lead resistance in each arm of the bridge and therefore the lead resistance is cancelled out. High quality connection cables should be used for this type of configuration because an assumption is made that the two lead resistances are the same. This configuration allows for up to 600 metres of cable.
The four wire resistance thermometer configuration even further increases the accuracy and reliability of the resistance being measured. In the diagram above a standard two terminal RTD is used with another pair of wires to form an additional loop that cancels out the lead resistance. The above Wheatstone bridge method uses a little more copper wire and is not a perfect solution. Below is a better alternative configuration that should be used in all RTD's. It provides full cancellation of spurious effects and cable resistance of up to 15 Ω can be handled.

Process flow speed is an important factor for correct measurement. Generally, resistance thermomers are put vertically to flow direction.

In order to make a correct measurement, RT should be put at least 10-15 times of its outher steath diameter.

Up to 500°C standard products can be used. Between 500-850°C special products are used. Resistance thermometers are generally used for machines, tanks, pipes, gas and liquid environments and surface measurements.

INSET

Resistance thermometers are produced with insets. Inset is a protecting tube that is put into the outher protecting tube. Resistance thermometer element is put into inset and filled with metal oxide dust. Then it is put into the outher tube. It can be changed without stopping the process.

ELEMENT

Resistance thermometer element is put in glass or mica materials. Generally ceramic isolators are used. Altoresistance thermometer element is covered with platinium wire and put into ceramics, glass or mica. Also there are some other versions that are covered with paper material which is called film element. Because the performance values are better than other elemtns, generally platinium and nicel are used to product resistance thermometers. PT100 which is produced with Nicel is used in the temperature range of -60 / +150°C. Platinium is used in a wider range. -250°C +850°C. .

For connections between PT100 element and terminal, Copper(Cu), Silver(Ag) and Nickee Chrome (NiCr) wires are used. For copper and silver, resistance values are not important becouse of their low resistance values and can be tolarated. For applications higher than 550°C, Nickel is used to make connection. Because of its high resistance value, its resistance is measured and print on the terminal or head.

PROTECTING TUBE

Protecting tube material, length and dimater is selected according to process type. Protecting tube can be selected as being 1.4301(304), 1.4571(316) stainless materials. While selecting the protecting tubes, chemical and mechanical status of the environment should be checked. In order to make a correct measurement, it should be put into the environment about 6-15 times of its protecting tube diameter.

CONNECTION HEAD

Inset is put into the protecting tube by using two screw by bowing. This decreases the vibration. Also problems created by the thermal expansions are eliminated.

Generally, B Type aliminium head is used for resistance thermometers. If neccesary C Type head can be used. Heads are produced according to DIN 43729 standards.

CABLES

Between resistance thermometers and devices, copper wires should be used. Because of wire resistance, generally wires that have 1,5 mm² dimension are used. Also in standards, 2 wires till 10m, 3 wires till 150 m and four wires should be used upper than the length of 150m. For long distances, resistance/current converter can be used.

STANDARD and SPECIAL TYPES

By using the instructions in catalog, resistance thermometers can be selected. Also special resistance thermometers can be produced according to users choices. Here are the useful information that can be used while buying resistance thermometers

If resistance thermometer is bought before, sending a sample
Using technical image number or order number
Length, diameter and shape information
If resistance thermometer is being bought first time, describing the process briefly
Continious and maximum temperatures
Process flow information, pressure values and chemical properties

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Resistance Thermometer Information