Compensating Leads





By the use of compensating leads, formed of

the same material as the thermo-couple, the cold junction can be

removed from the head of the thermo-couple to a point 10, 20 or 50

ft. distant from the furnace, where the temperature is reasonably

constant. Where greater accuracy is desired, a common method is

to drive a 2-in. pipe, with a pointed closed end, some 10 to 20

ft. into the ground, as shown in Fig. 128. The compensating leads

are joined to the copper leads, and the junction forced down to

the bottom of the pipe. The cold junction is now in the ground,

beneath the building, at a depth at which the temperature is very

constant, about 70 deg.F., throughout the year. This method will usually

control the cold-junction temperature within 5 deg.F.



Where the greatest accuracy is desired a compensating box will

overcome cold-junction errors entirely. It consists of a case enclosing

a lamp and thermostat, which can be adjusted to maintain any desired

temperature, from 50 to 150 deg.F. The compensating leads enter the box

and copper leads run from the compensating box to the instrument,

so that the cold junction is within the box. Figure 129 shows a

Brown compensating box.






If it is desired to maintain the cold junction at 100 deg.: the thermostat

is set at this point, and the lamp, being wired to the 110- or

220-volt lighting circuit, will light and heat the box until 100 deg.

is reached, when the thermostat will open the circuit and the light

is extinguished. The box will now cool down to 98 deg., when the circuit

is again closed, the lamp lights, the box heats up, and the operation

is repeated.





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