(10) The SHUNT terminals are provided to accept current shunts when the
RANGE switch is set to the SHUNT position.
(11) The PROTECTION DISABLE push button, when depressed, disconnects the
diode protection circuit which is parallel with the thermocouple.
d. High order accuracy can be obtained with this instrument only if the user
is aware of certain limitations and takes appropriate precautions as follows:
(1) Bead type thermocouples, like that used in the Model 540B, exhibit a
small exponentially decreasing drift of EMF after a steady heater current is
initially applied. Allow approximately 10 minutes for the thermocouple element to
stabilize. After the stabilization time, the drift should be less than two small
divisions per minute in the HIGH sensitivity position. When changing to different
levels of input after the stabilization period, a wait of only a few minutes is
necessary before continuing with measurements.
Any observed drift should be
evaluated quantitatively.
(2) Connection errors should be evaluated and minimized when the transfer
standard is used to calibrate other instruments.
These errors can often be
detected by changing the length and position of the leads.
Short coaxial leads
should be used when possible.
These errors can be significant at high test
frequencies.
(3) Radio frequency interference can be particularly troublesome with low-
current thermoelements. The thermocouple in the Model 540B is encased in a steel
shield to minimize stray field pick-up.
However, to ensure accuracy, the Model
540B should be used in areas of relatively low field strength.
(4) The number of transfers will vary with the degree of accuracy required.
In general, it is suggested that three separate transfers (DC to AC and back to DC
constitutes one transfer) be made for any single measurement.
This will be
adequate only if a good degree of repeatability is achieved.
Difficulty in
repeating any measurement can be caused by any number of problems, the most common
of which are poor connections, unstable sources, and interference from stray
capacitance or energy fields. Any factor which detracts from repeatability must be
eliminated before accurate transfers can be made.
(5) All of the components and elements in the Model 540B have been selected
to minimize environmental sensitivity.
It is suggested that the Model 540B be
given time enough to arrive at the environmental temperature before it is used. If
the Model 540B is used before it has arrived at the temperature of the environment
where it is to be used, excessive thermal drift will be observed, making accurate
transfers more difficult.
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