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A
T1
B
THERDMDCOUPLE
JUNCTION
ISOTHERMAL BLOCK
' 1
-TERMINAL - 7
I COPPER
"'-'
E2
: t
T2
I
I COPPER
I
TERMINAL
:
L - - - - - - -
C
D
Figure 6. Typical Thermocouple Measurement
The voltage measured by the voltmeter is proportional to the difference be-
tween
temperature
T
1
and T
2 •
To determine T
1 ,
T
2
the thermoelectric proper-
ties
of
the thermocouple are needed. Data is available to determine the voltage
versus temperature relationship based on a reference temperature
(T
2 )
of 0°C.
Thus, if the thermocouple-to-copper junctions were maintained at
oo-c
it
would be possible
to
determine T
1
directly by referring to thermocouple
reference tables*, which give temperature as a function
of
the meter reading
V
1 •
If these junctions are different from 0°C a voltage E
2
will be introduced,
where
V, -
E
1
-E,.
2.6 MEASUREMENT PROCEDURE
In the Model 7057A the reference temperature Tis measured by a dual ther-
mistor in a bridge circuit. The temperature of a thermocouple connected to
Channels 2 through 10 is detennined by the following procedure:
1. Measure the voltage
for
the reference junction V and calculate T
2 •
T
2
=
30- IV, x 103).
2. Determine the reference junction voltage, E
2 ,
either from thermocouple
reference tables* or
by
the calculation**:
"2=
"o+ A
1
T + 11ir2+ a
3
T
3
+a 4 r
4
where T - T 2 as detennined in step 1
above.
~
is expressed in microvolts. The constant ap through a 4 are a
function
of
the thermocouple type connected to the channel. The coeffi-
cients, a
0 ,
a
1 ,
a
2 ,
8s
and
a
4
are
dependent on the thermocouple type and
temperature range under consideration.
3. Measure the thermocouple voltage V, at the channel output. Convert to
microvolts.
4. Add
~
to
V,
to determine
E~. E
1
=V
1
+~
{where V
1
1s expressed in
microvolts).
5. Determfne the thermocouple temperature, T
1 ,
either from thermocouple
reference tables* or by calculation**:
T,
=
8o
+
a1E1,
+
a2E12t
+
~E,3
+
a4E, 4 1
{where E
1
is expressed in microvolts).
6. For example,
consider a
type J
iron-constantan
thermocouple
at 300°C
(T
1 ).
The reference junction temperature will be 8 to 10°C above ambient,
say 3s
0
c !T,1.
6
A. The bridge output (channel 11 will be -.OOOV. Since the bridge output is
zero when T, = 30°C this indicates T
2
= 3B°C.
B. Using NBS Monograph 125 to determine the reference voltage for a
type
J
thermocouple gives
E,
=
1849.0µV. Using the most accurate quartic
approximation formula (Table 2) for this range gives 1849.085,,V.
C. The voltage at Channel 1, which
is
V
1 ,
will be 14476.0µV. (V
1 -
E
1
-E,).
D. Add
E,
to V
1
and get E
1
=
16325.0µV.
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