Sensors

In this Section the subject of temperature measurement will be covered more broadly. There are a wide variety of sensors and transducers available for measuring pressure, level, humidity, and other physical properties. The sensor is the part of the control system, which experiences the change in the controlled variable.

The sensor may be of a type where a change in temperature results in a change of voltage or perhaps a change in resistance.

The signal from the sensor may be very small, creating the need for local signal conditioning and amplification to read it effectively. A small change in resistance signalled by a sensor in response to a change in temperature, may, for example, be converted to an electrical voltage or current for onward transmission to the controller.

The transmission system itself is a potential source of error.

Wiring incurs electrical resistance (measured in ohms), as well as being subject to electrical interference (noise). In a comparable pneumatic system, there may also be minute leaks in the piping system.

The term 'thermostat' is generally used to describe a temperature sensor with on/off switching. 'Transducer' is another common term, and refers to a device that converts one physical characteristic into another; for example, temperature into voltage (millivolts).

An example of a transducer is a device that converts a change in temperature to a change in electrical resistance.

With pneumatic devices, the word 'transmitter' is frequently encountered. It is simply another description of transducer or sensor, but usually with some additional signal conditioning.

However, the actual measuring device is usually termed as the sensor, and the more common types will be outlined in the following Section.

Filled system sensors

With pneumatic controllers, filled system sensors are employed. Figure bellow illustrates the principles of such a system.

Liquid filled system sensor and gas filled or vapour pressure system

 When the temperature changes, the fluid expands or contracts, causing the Bourdon tube to tend to straighten out. Sometimes a bellows is used instead of a Bourdon tube.

In the past, the filling has often been mercury. When heated, it expands, causing the Bourdon tube to uncoil; cooling causes contraction and forces the Bourdon tube to coil more tightly. This coil movement is used to operate levers within the pneumatic controller enabling it to perform its task. A pressure sensing version will simply utilise a pressure pipe connected to the Bourdon tube. Note: for health and safety reasons, mercury is now used less often. Instead, an inert gas such as nitrogen is often employed.

Resistance temperature detectors (RTDs)

RTDs (Figure bellow) employ the fact that the electrical resistance of certain metals change as the temperature alters. They act as electrical transducers, converting temperature changes to changes in electrical resistance. Platinum, copper, and nickel are three metals that meet RTD requirements and Figure bellow shows the relationship between resistance and temperature.

A resistance temperature detector is specified in terms of its resistance at 0°C and the change in resistance from 0°C to 100°C. The most widely used RTD for the typical applications covered in these Tutorials are platinum RTDs. These are constructed with a resistance of 100 ohms at 0°C and are often referred to as Pt100 sensors. They can be used over a temperature range of -200°C to +800°C with high accuracy (± 0.5%) between 0°C and 100°C.

  Typical resistance temperature sensors

 

 RTD element typical resistance/temperature graphs

As can be seen from Figure above, the increase of resistance with temperature is virtually linear. RTDs have a relatively small change in resistance, which requires careful measurement. Resistance in the connecting cables needs to be properly compensated for.

Thermistors

Thermistors use semi-conductor materials, which have a large change in resistance with increasing temperature, but are non-linear. The resistance decreases in response to rising temperatures (negative coefficient thermistor), as shown in Figure bellow.

 Negative coefficient thermistor

Positive coefficient thermistors can be manufactured where the resistance increases with rising temperature (Figure bellow) but their response curve makes them generally unsuitable for temperature sensing.

Thermistors are less complex and less expensive than RTDs but do not have the same high accuracy and repeatability. Their high resistance means that the resistance of the connecting cable is less important.

Positive coefficient thermistor

Thermocouples

If two dissimilar metals are joined at two points and heat is applied to one junction (as shown in Figure bellow), an electric current will flow around the circuit. Thermocouples produce a voltage corresponding to the temperature difference between the measuring junction (hot) and the reference junction (cold).

 Thermocouple connection

The cold reference junction temperature must be accurately known if the thermocouple itself is to provide accurate sensing.

Traditionally, the cold junction was immersed in melting ice (0°C), but the temperature of the cold junction is now measured by a thermistor or an RTD and, from this, the indicated temperature, generally at the measuring junction, is corrected. This is known as cold junction compensation.

Any pair of dissimilar metals could be used to make a thermocouple. But over the years, a number of standard types have evolved which have a documented voltage and temperature relationship. The standard types are referred to by the use of letters, that is, Type J, K, T and others.

                                 Standard range of thermocouples and their range (°C)

The most widely used general-purpose thermocouple is Type K.

The dissimilar metals used in this type are Chrome (90% nickel, 10% chromium) and Alumel (94% nickel, 3% manganese, 2% aluminium and 1% silicon) and can be used between the range 0°C to 1 260°C. Figure bellow illustrates the sensitivity of Type K thermocouples, and it can be seen that the output voltage is linear across the complete range.

 Sensitivity of Type K thermocouple

 Extension tail wires are used to connect the measuring junction to the reference junction in the instrument case. These extension tails may be of the same material as the wires in the thermocouple itself, or may be a compensating cable made of copper and copper-nickel alloy. Both extension tails must be of the same material.

Thermocouples are available in a wide variety of sizes and shapes. They are inexpensive and rugged and reasonably accurate, with wide temperature ranges. However, the reference junction temperature must be held at a constant value otherwise deviations must be compensated for. The low junction voltages mean that special screened cable and careful installation must be used to prevent electrical interference or 'noise' from distorting signals.