Connection of Voltage Measuring Instrument
About the connection of voltage measuring
instrument
No account so far has been taken of the
actual connection between the thermocouple extension leads and the
voltage measuring instrument. This introduces two further junctions
between dissimilar metals and hence two extra voltage sources E4
and E5, and the total number of voltage sources in the
equivalent circuit is increased to five. Provided that these extra
junctions are maintained at the same temperature, this is unimportant
as E4 and E5 cancel out (E4 = -E5)
and the net effect on the temperature indicated by the thermocouple
tables is zero. However, in various circumstances, the junctions can
be at different temperatures and correction then has to be made for E4
and E5. The junction between a copper connection lead and,
say, a chromel extension lead does not involve a standard pair of
thermocouple materials and is therefore not covered by thermocouple
tables. Use of a thermoelectric law known as the law of intermediate
metals has to be used in these circumstances. This states that the
e.m.f. generated at the junction between two metals or alloys A and C
is equal to the sum of the e.m.f. generated at the junction between
metals or alloys A and B and the e.m.f. generated at the junction
between metals or alloys B and C, where all junctions are at the same
temperature. This can be expressed more simply as:
eAC = eAB + eBC
Example
Suppose that the junction between a
copper connection lead and a chromel extension lead is maintained at a
temperature of 80°C. Equation (above) can be applied by introducing
the intermediate alloy, constantan. Then:
| ecopper-chromel |
= ecopper-constantan |
+ econstantan-chromel |
| |
= ecopper-constantan |
- echromel-constantan |
Therefore, from thermocouple tables:
ecopper-chromel = 3.357 –
4.983 = -1.626 mV
More
Temperature Measurements
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