Measurement Procedure; Measuring Example - Keithley 7014 Manual

which are usually copper. The measured voltage is pro-
portional to the difference between temperatures Tand
T7.
Fi@~re 2- 1
Thermocouple measurement
To determine the difference, the thermoelectric proper-
ties of the thermocouple are needed. Data is available
to determine the voltage Y~~-SLIS temperature relation-
ship based on a reference temperature (T1) of 0°C.
Thus, if the thermocouple-to-copper junctions were
maintained at O"C, it would be possible to determine T
by referring to the Thermocouple Reference Tables.
(See NIST Monograph 125). The tables list temperature
as a function of the meter reading E,. Since these junc-
tions are not OT, a voltage E, is inhoduced, where:
E, = E - E,
2.2.3 Measurement procedure
The temperature of a thermocouple junction is deter-
mined by the following summarized procedure:
l
Measure the reference voltage (E,).
. Calculate the reference temperature (TI).
. Determine the reference correction voltage (E,).
l
Measure the thermocouple voltage (E,).
l
Calculate the thermocouple correction voltage (E).
l
Determine the thermocouple temperature (T).
The complete step-by-step procedure follows:
1. Read the voltage (E,) developed by the Model 7014
reference junction. Assuming a temperature coeffi-
cient of +2OOpV/"C, and an offset voltage of
2.
3.
4.
5.
+54.63mV at O"C, convert the voltage reading to
temperature (TI) with the formula:
T, = (El - 54.63mV) / 0.2mV per "C
T, represents the temperature of the Model 7014
isothermal connections.
Using the thermocouple look-up tables or the fol-
lowing formula, convert the temperature (T,) from
step 1 to a voltage. Use tables matching the type of
thermocouple connected to the Model 7014. E, COT-
responds to the reference voltage that would result
if an actual thermocouple were used as a reference
junction.
E, = a0 + a,T + a,F + a,T3 + a,T4
Make a measurement of the voltage (E,) developed
by the thermocouple connected to the Model 7014.
Add the reference voltage derived in step 2 to the
thermocouple voltage measured in step 3.
E = E, + E,
Convert the voltage sum (E) from step 4 to a tem-
perature (T) using either thermocouple look-up ta-
bles or the formula:
T = a, + a,E + a,E2 + a,E" + a4E4
The values for a, through a4 for the supported thermo-
couples are listed in tables located in Appendix A.
2.2.4 Measuring example
A measurement setup uses a Type J thermocouple. The
voltage developed by the reference junction (channel 1)
is 61.83mV. The voltage read from the thermocouple is
14476pV.
1. Find the temperature of the isothermal connec-
tions. The voltage from the reference sensor is
61.83mV
(61.83mV - 54.63mV) / 0.2mV per "C = 36°C
Using the appropriate formula or thermocouple
look-up tables (see Table 6.32 of NIST Monograph
125), find the equivalent voltage developed by a
Type J thermocouple at 36°C. This voltage is found
to be 1849.11iv. The formula shown in Step 2 above
would yield 1849.085cLv.
2-2