Types of Temperature Transducers
When two wires with dissimilar electrical properties are joined at both ends and one junction is made hot and the other cold, a small electric current is produced proportional to the difference in the temperature. Seebeck discovered this effect. It is true no matter how the ends are joined so the cold end may be joined at a sensitive millivolt meter. The hot junction forms the sensor end.
The picture shows a typical industrial probe with a flexible extension and standard plug.
Thermocouples come in several forms. They may be wires insulated from each other with plastic or glass fibre materials. For high temperature work, the wire pairs are put inside a tube with mineral insulation. For industrial uses the sensor comes in a metal enclosure such as stainless steel.
Below are the types of thermocouples and their corresponding temperature ranges.
Type J 0 to 800oC
Type K 0 to 1200oC
Type T -199 to 250oC
Type E 0 to 600oC
Type R/S 0 to 1600oC
Type B 500 to 1800oC
Type N 0 to 1200oC
Type L 0 to 800oC
Resistance type sensors (RTD)
These work on the principle that the electrical resistance of a conductor change with temperature. If a constant voltage is applied to the conductor then the current flowing through it will change with temperature. The resistivity of the conductor change with temperature. This usually means the resistance gets bigger as the conductor gets hotter.
A special type of resistance sensor is called a THERMISTOR. They are made from a small piece of semiconductor material. The material is special because the resistance changes a lot for a small change in temperature and so can be made into a small sensor and it costs less than platinum wire. The temperature range is limited. They are only used for a typical range of -20 to 120oC and are commonly used in small hand held thermometers for every day use.
Liquid expansion and vapour pressure sensors
These are thermometers filled with either a liquid such as mercury or an evaporating fluid such as used in refrigerators. In both cases the inside of the sensor head and the connecting tube are completely full. Any rise in temperature produces expansion or evaporation of the liquid so the sensor becomes pressurised. The pressure is related to the temperature and it may be indicated on a simple pressure gauge.
Ways and means exist to convert the pressure into an electrical signal. The movement may also directly operate a thermostat. These instruments are robust and used over a wide range. They can be fitted with electric switches to set off alarms.
It is a well-known principle that if two metals are rigidly joined together as a two-layer strip and heated, the difference in the expansion rate causes the strip to bend.
In the industrial type, the strip is twisted into a long thin coil inside a tube. One end is fixed at the bottom of the tube and the other turns and moves a pointer on a dial. The outward appearance is very similar to the pressure type. They can be made to operate limit switches and set off alarms or act as a thermostat.
The ordinary glass thermometer is also a complete system. Again the bulb is the sensor but the column of liquid and the scale on the glass is the processor and indicator. Mercury is used for hot temperatures and coloured alcohol for cold temperatures.
The problems with glass thermometers are that they are
· Mercury solidifies at -40oC.
· Alcohol boils at around 120 oC.
· Accurate manufacture is needed and this makes accurate ones expensive.
· It is easy for people to make mistakes reading them.
Glass thermometers are not used much now in industry but if they are, they are usually protected by a shield from
accidental breakage. In order to measure the temperature of something inside a pipe they are placed in