Catalogs >
PCI INSTRUMENTS LTD >
GENERAL INFORMATION ABOUT THERMOCOUPLES

GENERAL INFORMATION ABOUT THERMOCOUPLES
5Pages

Catalog excerpts

GENERAL INFORMATION ABOUT THERMOCOUPLES - 1

Data sheet INFO THERM Date 01/2012 WORKING PRINCIPLE A thermocouple comprises an electrical circuit formed by two metal conductors of different metals soldered together at the ends. When there is a difference in temperature between the two joints, due to the Seebeck effect, a voltage is generated and then, if one of the two joints is opened, there is an electromotive force (emf). The polarization and intensity of the electromotive force depends solely on the type of metals used and the temperature to which the joints are subjected. The joint exposed to the temperature to be measured is called the hot junction or the measuring junction while the joint between the thermocouple conductors and the measuring circuit is called the cold junction or the reference junction. To measure a temperature with a thermocouple the reference junction must be at a given temperature (normally 0°) so that the emf generated depends solely on the temperature of the measuring junction. The type of thermocouple depends on the materials comprising the conductors which can be summarized as follows: PCI (Process System Solutions), Unit 7 Slaidburn Crescent, Southport, Merseyside, PR9 9YF, UK, Tel: +44 (0) 1704 509090, Fax: +44 (0) 1704 509091, email: sales@pci-instruments.com, Website: www.pci-instruments.com

Open the catalog to page 1

GENERAL INFORMATION ABOUT THERMOCOUPLES - 2

Data sheet Date Page (1) Thermocouple with 0.32 mm diameter wires (2) Thermocouple with 1.63 mm diameter wires MEASURING METHODS WITH THERMOCOUPLES The methods for carrying out measurements with thermocouples can generally be divided into two types. The first, as shown in figure No. 1, is generally used in industrial fields where extreme precision is not necessary In this case the thermocouple is connected directly (fig. 1a) to the measuring device using compensated or extension cables (fig. 1b). In this case the compensation of the reference junction is carried out directly by the...

Open the catalog to page 2

GENERAL INFORMATION ABOUT THERMOCOUPLES - 3

Data sheet INFO THERM Date 01/2012 As in the case of resistance thermometers, there are also basically two construction types of thermocouples: with traditional insulation and with mineral insulation. The following table shows the main characteristics of the two construction types: Traditional insulation thermocouples comprise: 1. Measuring junction The measuring junction or hot junction is the area in which the two conductors of the thermocouple are joined together; since its dimensions are small, we can consider the measurement carried out with the thermocouples to be punctiform. This...

Open the catalog to page 3

GENERAL INFORMATION ABOUT THERMOCOUPLES - 4

Data sheet INFO THERM Date 01/2012 In traditional insulation thermocouples the limits to the use of the different thermocouples is determined not only by the type of sheath but also by the dimensions of the thermocouple wires as indicated in the table below: A. Exposed hot junction Characterized by very a short response time as it is in direct contact with the environment in which the temperature must be measured; however it is not recommended for use in corrosive environments. B. Grounded hot junction The measuring junction is an integral part of the protective sheath and consequently...

Open the catalog to page 4

GENERAL INFORMATION ABOUT THERMOCOUPLES - 5

Data sheet Date Page MINERAL MgO INSULATION THERMOCOUPLES This particular construction type enables the production of high performance thermocouples with excellent mechanical characteristics. The made construction characteristics can be summarized as follows: the possibility of producing extremely small thermocouples (from 0.5mm diameter) the possibility of bending the sheath with a very sharp bending radius the considerable increase in the average life of the thermocouple the possibility of producing very long thermocouples 1. Measuring junction Special techniques are used to create the...

Open the catalog to page 5