Pt100 Sensors for Industrial Use

Pt100 Sensors for Industrial Use

 The use of a platinum resistance thermometer probe in industrial and scientific applications will normally result in good, accurate temperature sensing without the need for special cables (unlike thermocouples). To achieve the best possible accuracy and reliability, care is needed with  installation of the probe, connections, and choice of the host instrumentation.

 

Choice of wire-wound or flat film sensing resistor:

The resistance element is produced in one of two forms, either wire-wound or flat film. Metal film resistors consist of a platinum layer on a ceramic substrate; the coil of a wire wound version is fused into ceramic or glass.

 

a) Wire-wound resistors.

Various methods of detector construction are employed to meet the requirements of differing applications. The unsupported "bird cage" construction is used for temperature standards, and the partially supported construction is used where a compromise is acceptable between primary standards and use in industrial applications. Other constructional methods include the totally supported construction which can normally withstand vibration levels to 100g, and the coated wire construction where the wire is covered with an insulating medium such as varnish. The maximum operating range of the latter method is limited by the wire coating to usually around 250oC.

 

b) Metal Film Resistors

Metal film Pt resistors take the form of a thin (1 micron) film of platinum on a ceramic substrate. The film is laser trimmed to have a precise Ro value and then encapsulated in glass for protection.

A wide range of styles and dimensions are produced to allow for different applications. Such sensors have fast thermal response and their small thermal mass minimises intrusion in the media being tested. Such sensors are known variously as flat film, thin film or chip sensors.

 

 2,3 or 4 wire termination

Terminating the Resistance Thermometer

Fundamentally, every sensing resistor is a two wire device. When terminating the resistor with extension wires during probe construction, a decision must be made as to whether a 2,3 or 4 wire arrangement is required for measurement purposes.

In the sensing resistor, the electrical resistance varies with temperature. Temperature is measured indirectly by reading the voltage drop across the sensing resistor in the presence of a constant current flowing through it using Ohm's Law:  V = R.I

The connection between the thermometer assembly and the instrumentation is made with standard electrical cable with copper conductors in 2,3 or 4 core construction. The cabling introduces electrical resistance which is placed in series with the resistance thermometer. The two resistances are therefore cumulative and could be interpreted as an increased temperature if the lead resistance is not allowed for. The longer and/or the smaller the diameter of the cable, the greater the lead resistance will be and the measurement errors could be appreciable. In the case of a 2 wire connection, little can be done about this problem and some measurement error will result according to the cabling and input circuit arrangement.

For this reason, a 2 wire arrangement is not recommended.  If it is essential to use only 2 wires, ensure that the largest possible diameter of conductors is specified and that the length of cable is minimised to keep cable resistance to as low a value as possible.

The use of 3 wires, when dictated either by probe construction or by the input termination of the measuring instrument, will allow for a good level of lead resistance compensation. However the compensation technique is based on the assumption that the resistance of all three leads is identical and that they all reside at the same ambient temperature; this is not always the case.

Optimum accuracy is therefore achieved with a 4 wire configuration. The Pt100 measuring current is obtained through the supply. The voltage drop across the sensing resistor is picked off by the measurement wires. If the measurement circuit has a very high input impedance, lead resistance and connection contact resistances have negligible effect. The voltage drop thus obtained is independent of the connecting wire resistivity. In practice, the transition from the 2 wires of the Pt100 to the extension wires may not occur precisely at the element itself but may involve a short 2 wire extension for reasons of physical construction; such an arrangement can introduce some error but this is usually insignificant.

Note: The wiring configuration (2,3, or 4 wire) of the thermometer must be compatible with the input to the associated instrument.

 

 Extension cable considerations

Unlike thermocouples, resistance thermometers do not require special cable and standard electrical wires with copper conductors should be used. The heavier the gauge of the conductors, the less the impact is on errors due to lead resistance effects as described. Typically 7/0.2mm or 14/0.2mm conductors are specified with insulation chosen to suit a particular application.

 

Installation Notes:

a) Always observe colour codes and terminal designations; the wiring configuration of the thermometer must match that of the instrument input arrangement.

b) Avoid introducing "different" metals into the cabling; preferably use copper connecting blocks or colour coded (or other dedicated) connectors for the greater accuracy, reliability and convenience of installation.

c) Use screened or braided cable connected to ground in any installation where ac pick-up or relay contact interference is likely.

d) For very long cable runs, ensure that cable resistance can be tolerated by the instrumentation without resulting in measurement errors. Modern electronic instruments usually accept up to 100 Ohms or so for 3 or 4 wire inputs. Refer to the relevant instrument specifications for full details.

e) Cabling is usually available with many different types of insulation material and outer covering to suit different applications. Choose carefully in consideration of ambient temperature, the presence of moisture or water and the need for abrasion resistance.

f) If errors occur, be sure to check the sensor, the cable, interconnections and the instrument. Many such problems are due to incorrect wiring or instrument calibration error rather than the sensor.

Interchangeability is facilitated by the use of plug and socket interconnections. Special connectors are available for this purpose.

 

 High Accuracy Options (tolerance classes)

Assuming a 3 or 4 wire connection, and the use of a class B sensing resistor, a standard thermometer assembly will provide an accuracy of around 0.5oC between 0oC and 100oC. Considerable improvement on this figure can be achieved by various means including the use of closer tolerance sensors..

The principle of operation is the non-linear variation of the electrical resistance of the resistor with temperature. The commonly internationally used Pt100 has a resistance of 100Ohms at 0oC and 138.51Ohms at 100oC; alternative Pt1000 sensing resistors are less commonly used as are a few other types which meet the needs of special applications. The guidance stated in this article broadly applies to all types.

The use of virtually pure rare metal (platinum) in the sensing resistor endows this type of temperature sensor with both high accuracy and long term stability; good quality instrumentation is therefore essential to realize these virtues.

It is important to remember that the overall accuracy of any measuring system  (e.g. sensor, instrument, interconnection, application etc) is compromised by the sum of the uncertainties in that system.