Z-23
Lets replace the ice bath with another isothermalblock
The new block is at Reference Temperature TREF, andbecause J3
and J4 are still at the same temperature, wecan again show that
V = (T1-TREF)This is still a rather inconvenient circuit because
we
have to connect two thermocouples. Lets eliminate theextra Fe
wire in the negative (LO) lead by combiningthe Cu-Fe junction (J4)
and the Fe-C junction (JREF).
We can do this by first joining the two isothermalblocks (Figure
9b).
We havent changed the output voltage V. It is stillV = (TJ1 -
TJREF )
Now we call upon the law of intermediate metals (seeAppendix A)
to eliminate the extra junction. Thisempirical law states that a
third metal (in this case,iron) inserted between the two dissimilar
metals of athermocouple junction will have no effect upon theoutput
voltage as long as the two junctions formed bythe additional metal
are at the same temperature:
This is a useful conclusion, as it completely eliminatesthe need
for the iron (Fe) wire in the LO lead:
Again, V = (TJ1 - TREF), where is the Seebeckcoefficient for an
Fe-C thermocouple.
Junctions J3 and J4, take the place of the ice bath.These two
junctions now become the ReferenceJunction.
Now we can proceed to the next logical step: Directlymeasure the
temperature of the isothermal block (theReference Junction) and use
that information tocompute the unknown temperature, TJ1.
A thermistor, whose resistance RT is a function oftemperature,
provides us with a way to measure theabsolute temperature of the
reference junction.Junctions J3 and J4 and the thermistor are all
assumedto be at the same temperature, due to the design of
theisothermal block. Using a digital multimeter undercomputer
control, we simply:
1) Measure RT to find TREF and convert TREFto its equivalent
reference junction voltage, VREF , then
2) Measure V and add VREF to find V1,and convert V1 to
temperature TJ1.
This procedure is known as Software Compensationbecause it
relies upon the software of a computer tocompensate for the effect
of the reference junction. Theisothermal terminal block temperature
sensor can beany device which has a characteristic proportional
toabsolute temperature: an RTD, a thermistor, or anintegrated
circuit sensor.
It seems logical to ask: If we already have a devicethat will
measure absolute temperature (like an RTD orthermistor), why do we
even bother with athermocouple that requires reference junction
Reference Circuit
J3
J4
Cu
Cu J1C
T REF
Fe
+
vJ3
J4
Cu
CuVoltmeter
HI
LO
Isothermal Block
J1
C
T REF Isothermal Block
J REF
Fe
Fe
J3
J4
Cu
Cu
J1
C
Isothermal Block @ T REF
J REF
Fe
Fe
HI
LO +
J1+
V1v
VoltmeterC
FeJ3
RT
Cu
Cu
Block Temperature = TREF
J4
Isothermal Connection
T REF
T REF
Thus the low lead in Fig. 9b: Becomes:
Metal A Metal CMetal B=
Metal CMetal A
Cu CFe=
CCu
ELIMINATING THE ICE BATHFigure 9a
JOINING THE ISOTHERMAL BLOCKSFigure 9b
LAW OF INTERMEDIATE METALSFigure 10
EQUIVALENT CIRCUITFigure 11
EXTERNAL REFERENCE JUNCTION-NO ICE BATHFigure 12
