Page 286 - Instrumentation Reference Book 3E

270 Temperature measurement
Ballast

.
A

Figure 14.32 Use of ballast resistor: (a) external to instrument, (b) adjustable ballast mounted inside instrument

Cold junction compensation
Amplifier
P-PL
2
ov
motor

Potentiometer
-
I slidewire

~ ~~
Input
-- - - amplifier output
----

Cold junction
sensor
compensation

(b)
Figure 14.33 Cold junction compensation: (a) in conjunction with potentiometric indicating instrument, (b) alternative
arrangement for cold junction compensation.
frequently is, a strip chart recorder, it is almost “seen” by the thermocouple is a high impedance
certain to be a potentiometric instrument. Figure amplifier. Again. there is negligible current in the
14.33(a) shows the potentiometric arrangement thermocouple circuit and as the resistance of the
diagrammatically. thermocouple circuit is of the order of 100 ohms
while the amplifier input is likely to be a megohm
Electronic instruments In modern electronic or more, the effect of the external circuit resistance
instruments for thermocouple indication, whether is negligible. Electronic instruments allow their
they be analog or digital devices, the input circuit designer much more versatility for cold junction