Correction For Cold-junction Errors

: The Working Of Steel

The voltage generated by a thermo-couple of an electric pyrometer is

dependent on the difference in temperature between its hot junction,

inside the furnace, and the cold junction, or opposite end of the

thermo-couple to which the copper wires are connected. If the

temperature or this cold junction rises and falls, the indications

of the instrument will vary, although the hot junction in the furnace

may be at a constan

A cold-junction temperature of 75 deg.F., or 25 deg.C., is usually adopted

in commercial pyrometers, and the pointer on the pyrometer should

stand at this point on the scale when the hot junction is not heated.

If the cold-junction temperature rises about 75 deg.F., where base metal

thermo-couples are used, the pyrometer will read approximately 1 deg.

low for every 1 deg. rise in temperature above 75 deg.F. For example, if the

instrument is adjusted for a cold-junction temperature of 75 deg., and

the actual cold-junction temperature is 90 deg.F., the pyrometer will

read 15 deg. low. If, however, the cold-junction temperature falls below

75 deg.F., the pyrometer will read high instead of low, approximately

1 deg. for every 1 deg. drop in temperature below 75 deg.F.

With platinum thermo-couples, the error is approximately 1/2 deg. for

1 deg. change in temperature.