Thermocouples - West Control Solutions - #1

Depending on version, industrial thermocouples can be used between -200 and +2315 °C, whereby the sensor as also the protection tube is exposed directly to the measured temperature. The type of sensor and the construction of the protection tube are essential factors for the useful life of the sensor and for the stability of the measurement.

Depending from application, industrial thermocouples can be mounted in various ways, by means of an adjustable flange or an adjustable sheeth mounting fitting; with a welded bush or by welding (the thermowell). For the last mentioned type, the measuring insert is screwed into the thermowell. To reduce temperature effects to the head and possibly to a built-in transmitter, a distance between mounting fitting and head is necessary. For fixed bushes this is between 120 and 140 mm.

As the junctions (4) represent the limits of the + and - conductors generating the e.m.f., the remaining conductors linking the measuring device being copper wire, the output of the thermocouple will effectively become a function of all the junction temperatures. Thus the extension lead must have similar thermoelectric characteristic as the main conductor. Usually the temperature of the cold junction Q_c is used as reference by the the transmitter and compensated for (internal temperature compensation).

Protective fittings for industrial thermometers always consists of a protection tube and the termination head. Fitted inside the protection tube are one or two thermocouples. The termination head contains the connector block for connecting the extension (thermo or compensating) leads. The connector block can be replaced by a two-wire head transmitter. More suitable and easier for mounting and service is the use of a head with high doomed cover which will take the transmitter instead. This transmitter converts the thermovoltage signal into a standardized temperature linear 4...20 mA signal.

If a temperature gradient is present in an electrical conductor, the heat flow will create a movement of electrons and an electromotive force (e.m.f.) will be generated in that region. Magnitude and direction depend of magnitude and direction of the temperature gradient and the material forming the conductor. The voltage existing across the ends will represent the sums of the e.m.fs. generated along it. In practice two materials having different electrothermal characteristics are combined to produce a thermocouple.

EMK_h

12

1 = measuring junction 5 = copper lead

2 = thermocouple S_h = temperature of

measuring junction

3 = compensation/ thermo S_c = temperature of cold

lead

4 = reference junction

junction (reference)

-— = direction e.m.f.

E.m.f. = electromotoric force