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Resistive sensors mean devices that will react to varying temperature this way that electrical resistance of an in-built internal resistor will change appropriately.
Their principle consists in the physical phenomenon that resistance of a metal changes when temperature changes. With increasing temperature the mean amplitude of vibration of atomic core will increase and so is probability of collisions between free electrons and atoms of the core; impeded movement of the free electron will result as increased resistance.
Metals that are used for thermometric resistors should have the following properties:
- Temperature coefficient of resistance possibly high
- Possibly high resistance that will allow making resistors with small size
- Possibly high melting point
- Constant physical properties
- Corrosion resistance
- Easy repeatability of the metal parts with equal properties
- Continuous temperature-resistance relation with no hysteresis
- Suitable continuity and strength
Platinum (Pt) is a metal that suitably meets above mentioned criteria. Also nickel and copper are used for making thermometric resistors.
Temperature range: -200..+850°C
Temperature range: -50..+600°C
Besides of classical solutions for thermometric resistors significant miniaturization has progressed within last years. Furthermore coiled wire has often been replaced with resistors of construction made with sputtered layer techniques that was taken from production of integrated circuits.
Admissible tolerances of measuring errors for platinum resistive sensors are detailed in the PN-EN 60751:1997+A2-standard. This standard distinguishes two classes of accuracy: A and B.
Below two formulas for calculating admissible deviation are presented.
Class A: t = ( 0.15 + 0.002 x |t| )
Class B: t = ( 0.30 + 0.005 x |t| )
t = temperature in oC
It is also possible to apply platinum resistors with higher accuracy, i.e. of 1/3 DIN B – class and of 1/10 DIN B-class. Application areas of these resistors, however, are limited to narrower temperature ranges according to the table below.
Equations for calculating admissible errors for resistors from the classes of higher accuracy are presented below.
Class 1/3 B : t = ( 0.10 + 0.0017 x |t| )
Class 1/10 B : t = ( 0.07 + 0.0007 x |t| )
Electric resistance of a resistive sensor will vary with temperature. In order to determine input signal constant current it passed through the resistor and voltage drop is measured. The Ohm law states for this voltage drop that:
V = R x I
Measuring current should be as low as possible in order to avoid heating of the resistor. It may be assumed that 1mA of measuring current will not cause significant errors. Such current will give voltage drop of 0,1 V in Pt100 at 0ºC. This signal, with minimal changes, has to be transferred to a displaying or analyzing point through connecting cables. Three types of circuits are used for this purpose.
Connecting the sensor with transforming electronics is made through a 2-wire cable. This cable, like any other conductor, has electrical resistance connected in series to the sensor. Two resistances are thus added which results as systematically higher temperature indications. Resistance of connections at longer distance may be as high as several ohms and thus may produce a significant shift of measuring values. In order to avoid this shift the resistance is electrically compensated.
The measuring device is designed to that it has to give resistance of connections of – for instance – 10 Ohms. When a resistive sensor is connected to, compensating resistance is connected with one of measuring cables and the sensor is initially replaced with a 100.00 Ohm resistor. Then the compensating resistance is being changed as long as the reading of the measuring device is 0ºC.
Since applying a 2-wire circuit means relatively much work to do and temperature of the connecting cable is not taken into account such circuits are rather seldom used.
Impact of resistance of the connecting cables and their fluctuation with temperature are reduced to minimal in a 3-wire circuit. In this circuit an additional terminal is connected to the resistive sensor. This way two measuring circuits will be obtained where one of these will mean as a reference circuit.
The 3-wire circuit allows compensating both, the value of resistance of connections and its dependence on temperature. It is required, however, that all these 3 wires are identical and are at the same temperature. These requirements are mostly met with accuracy good enough thus the 3-wire circuit in now one of the most frequently used ones. Compensation of connections is not required.
A 4-wire circuit is an optimal form of connection for resistive sensors. Measuring results will not depend on resistance of connection and on temperature changes, either. Compensation of connections is not required. The resistor will get measuring current I through the supplying terminals. Voltage drop V on the resistor is taken by the measuring terminals.
When the input resistance of the electronics is several times higher than the resistance of the connections is this latest can be neglected. Voltage drop determined this way doesn’t depend on the properties of the connecting cables.
One has to note that the 3- and 4-wire circuit is not necessary like that up to the sensor element itself. Connection from the sensor to the block of terminals, i.e. an internal connection, is frequently made as a 2-wire circuit. It results in similar problems like those discussed above for a 2-wire circuit, however they are not as much significant. Total resistance that consists of the sum of internal connection and the sensor itself is defined by DIN 16160-standard as resistance of a resistor.