LESSON 2
DCLF THERMAL TRANSFER STANDARDS
TASK
Describe the function and operation of each thermal transfer standard.
CONDITIONS
Given the information and illustrations about theory and applications relating to
thermal transfer standards.
STANDARDS
Demonstrate competency of the task skills and knowledge by correctly responding to
at least 80 percent of the multiple-choice test covering theory and application of
thermal transfer standards.
REFERENCES
None
Learning Event 1:
DESCRIBE MODEL 540B THERMAL TRANSFER STANDARD
1. As illustrated by Figure 2-1, there are only three instruments involved in a
basic AC-DC thermal transfer measurement; a thermal transfer standard, a DC voltage
standard, and the AC source that is to be calibrated.
The thermal transfer
standard is the center of the measurement.
It contains three basic items; a
thermocouple, a galvanometer, and an internal DC reference supply.
a. To perform the measurement, you apply the AC source voltage to the input
of the thermal transfer standard.
This AC voltage is attenuated by the range
switch prior to being felt on the thermocouple heater.
The resulting DC output
from the thermocouple is applied to one side of the galvanometer. The internal DC
reference voltage is present on the other side of the galvanometer and it is
manually adjusted to balance against the thermocouple DC voltage, causing the
galvanometer to indicate a null or zero.
b. The next step is to remove the AC input and connect the DC voltage
standard output to the thermal transfer standard.
The reference controls on the
thermal transfer standard are not touched. The DC voltage output is now adjusted
until the galvanometer reads zero.
You can now read the RMS value of the AC
voltage directly from the DC voltage standard. This is made possible because the
heating effect of an RMS value of the AC voltage applied to a thermocouple is the
same as an equivalent value of DC voltage applied to the same thermocouple.
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