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Resistance thermometers

Temperature-dependent resistors measure the temperature using one or more built-in electrical resistors. We offer our customers a range of designs for the many possible applications they use. The basic resistance thermometers are distinguished into various types. Other key distinguishing features based on their physical properties are the sensor type, whether the resistor is made from metal or a semiconductor, and the connection type.

2 Types of resistor

2.1 Metal resistors

As is widely known, metals expand as they heat up. An electrical resistor does something similar. It also changes, depending on the temperature of the metal. This dependence is described by the temperature coefficient, which should be as large as possible. In other words, a temperature change causes a large change in the resistance. The pressure and temperature dependence of the coefficient should also be as low as possible, and uninfluenced by chemicals. Platinum in particular has proven itself especially useful in industrial applications for resistance thermometers, although nickel is also used.

Platinum has a positive temperature coefficient: its resistance increases as the temperature rises. Platinum can be produced with a high degree of purity and is resistant to chemical influences. The electrical properties are easily reproducible, making platinum resistance thermometers universally exchangeable. The baseline series for platinum resistors has been defined in accordance with the European standard IEC 60751 and ranges from -200 °C to 850 °C. The standard also defines the permissible limit value deviations. Platinum resistors are categorised according to their rated resistance (R0), which corresponds to the resistance value at 0 °C. A resistor with a rated value of 100 Ω at 0 °C is referred to as a Pt100 measuring resistor. Consequently, a Pt1000 measuring resistor has a rated value of 1000 Ω at 0 °C. The limit deviations for resistance thermometers are divided according to the IEC 60751:2008 standard into four classes. These apply for any R0 values and are distinguished according to wire-wound or sheet resistors. The most commonly used accuracy classes for resistance thermometers are AA, A and B. Class AA (previously known as 1/3B) defines the highest standardised accuracy for resistance thermometers. In special cases, a measuring resistor with a 1/5 or even 1/10 Class B can be installed.

These are used less frequently than platinum resistors since their range of use is only between -60 and +260 °C. Compared to platinum, nickel resistors have a higher temperature coefficient, which is why they are still popular. Like platinum resistors, they are also designated according to their rated value at 0 °C, e.g. Ni120.

2.2 Semiconductor resistors

Semiconductors are materials whose conductivity lies between electrically conducting materials and electrically non-conducting materials. They are grouped, according to their temperature coefficients, into PTC and NTC resistors.

These are also referred to as cold conductors since they have better electrical conductivity at low temperatures than they do at high ones.

These are also referred to as hot conductors since their electrical conductivity increases as they heat up. The resistance is highly non-linear and is specified as an R/R25 logarithm. A hot conductor is not chemically resistant.

3 Connecting wires

Connecting wires are needed to connect resistance thermometers to a measuring device. Unlike thermowires and compensating wires, wires made from zinc-plated, nickel-plated or silver-plated copper or nickel wire are used. Nickel wires are only used for very high temperatures and are only available with fibreglass insulation. To make resistance thermometers as delicate as possible, preference is given to using connecting wires with a small diameter (≤ AWG 24). This does however lead to an increased wire resistance which can be compensated for with a three or four-wire circuit.

4 Connection types

The resistance change in the thermometer is determined using a measuring current applied to the connecting wire and by measuring the voltage drop. The resistance is determined on the basis of this using Ohm's law:

U = R * I

To avoid self-heating of the resistor, a very low current is selected. The voltage drop must be measured ideally with no distortion. There are various circuit types available for this. We produce our resistance thermometers with all possible circuit configurations, accommodating two, three or four-wire circuits.

A wire is connected to each of the two arms of the measuring resistor and fed to the measuring device. The voltage is only measured in the measuring device, which means that the inherent resistance of the connecting wire influences the measuring result. The inherent resistance of the wire depends on its length and cross-section. It is possible to compensate the error electrically, however this process is laborious and highly inflexible.

An additional wire is connected to one of the arms of the measuring resistor and forms a second measuring circuit. This allows a measuring circuit to be used as reference and therefore the inherent resistance of the wire, as well as temperature-dependent fluctuations in the wire resistance, to be compensated.

The two arms of the measuring resistor are each connected to two wires - one measuring wire and one current-carrying wire. The voltage drop is therefore picked up directly on the measuring resistor and is therefore completely independent of wire and temperature influences. A four-wire circuit is the best connection option for error-free measurement.

Typical area of application

  • No details given

Pros

  • Very high accuracy

Cons

  • Self-heating

Images of resistance thermometers