Highlighted - Pt100

The Pt100 is a common temperature sensor used in measurement and control technologies and it is the sensor element of a RTD temperature probe (RTD stands for Resistive Temperature Device). Of all industrial temperature measurements 70 percent is performed by RTD temperature probes with a Pt100 sensor element. Mainly because of the large measuring range, linear behaviour, long lifecycle, simple connections and the accuracy of the sensor.

Although the term Pt100 refers to the temperature sensor element, it is often used in combination with sensor or probe to refer to the complete RTD temperature probe. Two other common names for a RTD temperature probe are resistance temperature sensor and resistance thermometer.

A temperature dependent resistor measures the change in the electrical resistance in metal when temperature fluctuates. The reason is that electrical conduction in metals is caused by the electrons moving freely between the atoms. If the movement of the atoms becomes more active and heavier, the electrons will experience more resistance passing the atoms, which results in an increase of electrical resistance in the metal.

The resistance value according to the European standard:RPt100 = 100 + 0,385 055 × T. That means that the positive temperature coefficient of a Pt100 is 0.385 055 ohms per kelvin. Another accurate approximation is: RPt100 = (100 + 0,390 802 × T - 0,000 058 0195 × T2).

Pt100 temperature probe with connection head type B

The production of JUMO vibrotemp sensors

Construction forms

Traditional, the platinum wire is wounded on an insulating ceramic core, which is slid into a tube of the same material. In another form the platinum wire is wounded on a glass rod and is heated to a temperature which melts the glass with the platinum wire. This is called the sensor element. Depending on the application, the element is placed in a metal or sometimes in a glass casing. This housing must be able to withstand the mechanical stress to which the sensor is subjected in the industrial applications. There are now also other construction forms such as vapour-deposited, platinum on a plate of ceramic or glass and SMD versions.

Connections

Because it concerns a resistance measurement in the order of 100 ohms, the variable temperature has an influence on the connecting wires. Three systems are used to eliminate the influence of temperature on the wiring:

  • The two-wire connection: the wires are short-circuited at the end during adjustment. The resistance of the cables is adjusted by means of a potentiometer. Then the Pt100 is placed back in the pipe. There is therefore no compensation for temperature influence on the pipe.
  • The three-wire connection: the resistance of the Pt100 and the line resistance are measured between the first and third connecting wire of the Pt100. Only the line resistance is measured between the first and second connecting wire. The lead resistance between the first and third wire of the Pt100 will have approximately the same value as that between the first and second connecting wire. This second measurement is subtracted from the first measurement. The result is the resistance of the bare Pt100, compensated for temperature influences on the pipe.
  • The four-wire connection: two leads lead to one side of the Pt100 and two leads to the other side. The measurement is made according to the Kelvin resistance measurement method. Two leads carry a current through the sensor - in the order of magnitude of less than 1 mA to prevent heating - via the other two leads the measuring voltage from the Pt100 is fed to the measuring instrument. Due to the high impedance of measuring equipment, cable losses can therefore be disregarded. This method also compensates for the contact resistance at the terminals.

diagram 2 to 4 wire connections

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