KROHNE OPTITEMP TT 51 EN - Manual (Page 39)

TECHNICAL DATA
7
39
TT 51 SERIES
www.krohne.com 09/2010 - 4000754201 - MA TT 51 R01 en
7.1.2 Thermocouples
The thermocouple features two electric conductors made from different metals, connected at
one end. Each free end is connected to a compensation cable which is then connected to a
millivolt meter. This circuitry forms a "thermal circuit". The point at which the two electric
conductors connect is called the measuring point (hot junction) and the point at which the
compensation cables connect to the conductors of the millivolt meter is called the reference
junction (cold junction).
If the measuring point of this thermal circuit is heated up, a small electrical voltage (thermal
voltage) can be measured. If, however, the measuring point and the reference junction are at the
same temperature, no thermoelectric voltage is generated. The degree of thermoelectric
voltage, also known as electromotive force (EMF), depends on the thermocouple material and
the extent of the temperature difference between the measuring point and the reference
junction. It can be measured using the millivolt meter with no auxiliary power.
Simply put, the thermocouple behaves like a battery, the voltage of which also increases as the
temperature rises.
INFORMATION!
The characteristic curves and tolerances of commercially available thermocouples are
standardised in IEC 60584.
Figure 7-2: Thermocouple measuring circuit, schematic.
1 Measuring point t
1
(hot junction)
2 Thermocouple
3 Transition junction t
2
4 Compensation cable / extension cable
5 Reference junction t
3
(cold junction)
6 Copper conductor
7 Voltage meter U
th