1. THeRmOCOUpleS THeORy
9
Summary of the three laws
The three fundamental laws may be combined and stated as
follows:
- the algebraic sum of the thermo electric EMF’s generated in
any given circuit containing any number of dissimilar homo-
geneous metals, is a function only of the temperature of the
junction;
- if all but one of the junctions in such a circuit are maintained
at some reference temperature, the EMF generated depends
only on the temperature of that one junction and can be used
as a measure of its temperature.
There are many types of thermocouples available on today’s
market. Each has its own particular advantages and disadvan-
tages. In many cases, thermocouples and their accessories are
designed for a specifc temperature measurement problem.
In other cases, thermocouples are manufactured with a wide
variety of possible applications. It is not the intention to com-
pare one type of thermocouple with another, or to compare the
thermocouple of one manufacturer with another. No thermocou-
ple is suitable for all needs. Thermocouples must be selected to
meet the needs of a particular installation. The next basic types
can be of use to guide you in your selection process.
Three basic types of construction:
1. Wire
2. Mineral insulated
3. Thermowell
THERMOCOUPLE BODY CONSTRUCTION
Fig. 6 Law of Successive or Intermediate Temperatures
emf =
E 1
T 1
T 2
emf =
E 2
T 2
T 3
emf’s are Additive for Temperatures Intervals
emf =
E 3
=
E 1
+
E 2
T 1
T 3