of the AC voltage. If the Model 1395 is now transferred to a source of DC voltage
and this source is adjusted until the DC couple of the thermocouple is exactly the
same value as it was when the AC voltage was applied, the resulting DC heater
current is equal to the RMS AC heater current. Assuming a constant transadmittance
of heater current to input voltage and no frequency errors in the thermocouple
response, this DC input voltage is now exactly equal to the unknown AC voltage.
Any measurement errors resulting from nonconstant transadmittance or thermocouple
frequency response are expressed as the AC/DC transfer difference. Such errors can
be corrected to obtain the best measurement accuracy.
5. To measure the RMS value of an unknown AC voltage, refer to the hookup diagram
of Figure 2-14 and proceed as follows:
a. Select a Model 1395 whose voltage range includes the voltage to be
measured.
If the approximate value of the voltage is unknown, either obtain an
approximate value from an auxiliary voltmeter or start the measurement process with
the highest range Model 1395 and subsequently select lower range Model 1395s until
the correct range is found.
b. Connect an appropriate DC monitoring instrument to the DC couple output of
the Model 1395. This connection is made by a twin-contact shielded connector.
c. Reduce the level of the AC voltage to be measured to its lowest value.
d. Connect the unknown AC voltage to the coaxial input of the Model 1395.
e. Adjust the AC voltage to its desired level.
f. Note the indication of the DC monitor connected in step 1. For greatest
accuracy allow 1 minute for the thermocouple to stabilize to a final output.
g. Reduce the level of the AC voltage.
h. Disconnect the input of the Model 1395 from the AC source.
i. Reduce the level of an appropriate DC source to its lowest level.
j. Connect the DC source to the coaxial input of the Model 1395.
k. Adjust the level of the DC source until the DC monitor indicates the same
reading as obtained in step 6.
For greatest accuracy, allow one minute for the
thermocouple to stabilize to a final output.
l. Measure the value of the DC source. This value, corrected for any AC/DC
transfer error, is the RMS value of the unknown AC voltage.
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