Temperature switches

How does a resistance thermometer work?

The electrical resistance of a resistance thermometer's sensor changes with the temperature. As the resistance of measuring resistors to EN 60751 (2009-05) increases with rising temperature, we refer to it as PTC (Positive Temperature Coefficient). Pt100 or Pt1000 measuring resistors are normally used for industrial applications. The thermometers based around EN 60751 are defined in DIN 43735.

What are 2-, 3- and 4-wire circuits?

They describe the number of wires with which the measuring resistor (e.g. a Pt100) is connected. While with the simplest 2-wire connection, the lead resistance can falsify the measuring result, this negative influence can be compensated within the 3- or 4-wire connection, and thus the accuracy of the measurement improved.

What does „negative temperature coefficient thermistor“ mean?

Negative temperature coefficient thermistors conduct electricity better at higher temperatures than at lower temperatures. They are also known as NTC resistances (Negative Temperature Coefficient). Typically, NTC is used in the plastics and food and beverage industries.

What does "positive temperature coefficient thermistor" mean?

Positive temperature coefficient thermistors conduct electricity worse at higher temperatures than at lower temperatures. They are also known as PTC resistances (Positive Temperature Coefficient). Typically PTC are used in high-value temperature measuring points, e.g. in the chemical industry.

What effect does poor insulation resistance have?

In accordance with DIN EN 60751 section 6.3.1 the insulation resistance between each measuring circuit and the sheath, at a minimum test voltage of 100 V DC, must not be less than 100 MOhm. Should the insulation resistance be too low, a measuring error occurs that causes the display of too low a temperature. In relation to a resistance thermometer (with sheathed cable) this results, with an insulation resistance of 100 kOhm, in a display error up to 0.25 Ohm and at 25 kOhm up to 1 Ohm. On all WIKA resistance thermometers, an insulation test with 500 V DC and an insulation resistance of > 1,000 MOhm is carried out, i.e. we test to a factor of 50 better than specified by the standard.

What minimum insertion lengths are recommended, as a rough guide, for protection tubes in order to minimise the heat dissipation error?

for gaseous media: 15 ... 20 x protection tube diameter
for liquid media: 5 ... 10 x protection tube diameter
for solid media: 3 ... 5 x protection tube diameter
(these standard values are only valid for static mediaThe gap between the protection tube and measuring insert should be < 0.5mm)

Why has there been, for some time, a separation between the accuracy classes for "wire-wound resistance" and "film resistance" Pt100 measuring resistors?

In the past, no distinction had been made between the two basic types of measuring resistor and their temperature limits.  Practice, however, showed that film resistors (thin-film/chipset resistors) have a (not insignificant) deviation form the characteristic. This behaviour has been accommodated in DIN EN 60751:2009-5 through the splitting of the temperature ranges within the individual accuracy classes.

Why should Pt100 measuring circuits with reduced tolerance class A or AA per DIN EN 60751 be used in at least a 3- or 4-wire connection?

The 2-wire connection is not permissible for classes A and AA per DIN EN 60751 since here the internal lead resistance of the wires is added to the measured value. This will usually exceed the specified tolerance for the temperature sensor. A measurement of the cable resistance at room temperature and adjusting this in the transmitter (for example) is possible, but the temperature-dependent resistance of the inner conductor of the cable would still be added to the reading as an error.  Conclusion: A 2-wire circuit is not suitable for accurate temperature measurement.

What mounting types are available with temperature switches?

Temperature switches can be fitted with either a direct mounting or a remote capillary.

What does one understand by the accuracy of mechanical pressure or temperature switches?

With the accuracy, one refers to the deviation between the set point and the actual value of the switch point. The “accuracy” is only used very rarely with mechanical pressure or temperature switches, because the set point is often set by the customer themselves.

What is the repeatability with mechanical pressure or temperature switches?

The repeatability is one of the most important performance characteristics for pressure or temperature switches. It describes the maximum deviation between the switch points when repeatedly approaching the same pressure or temperature value. This value indicates how reliably the pressure switch will always switch the same value.

Which switch does one need for use in safety systems?

For use in safety systems, one needs a pressure or temperature switch with an SIL approval.

What do the terms hysteresis/switch differential/dead band mean with pressure or temperature switches?

The three terms refer to the difference between the switch point and the reset point. At the switch point, the switch changes the connected circuit through its activation. At the reset point, the original status is restored. It is basically essential.