(6) The Model 540B imposes two load conditions on the AC source.
The AC
source sees the 182 ohms/volt input impedance of the Model 540B when the MODE
switch is in the AC TRANSFER position. When the MODE switch is in the OFF or DC
TRANSFER positions, the AC source sees the Model 540B as an open circuit.
This
impedance change across the AC source causes a corresponding output level change
from the AC source.
When calibrating a device that is quite sensitive to level
changes, the user should determine if the AC source level changes could adversely
affect the device. The user should take appropriate precautions, such as switching
the device out of the calibration circuit before turning the MODE switch to the OFF
or DC positions, should such a situation arise.
(7) Though the internal range selection components and thermocouple in the
Model 540B are protected from overloads in the voltage mode; no protection is
provided in the current mode or for external shunts or high frequency converters.
These devices are easily damaged or destroyed by overloads and all possible
precautions should be observed.
External switching transients cause many such
overloads and should be avoided.
It is possible to damage a thermal converter
without completely destroying it.
One of the characteristics which change, and
indicate a damaged thermal element, is DC reversal error.
Should the reversal
error of any thermal element change after a suspected overload, it is reasonable to
assume that damage has occurred and the device should be recalibrated.
(8) During all transfer measurements the operator must remain alert to
changes in readings regardless of their cause. All such changes should be analyzed
and corrective measures taken in order to ensure the accuracy of the transfer
measurements.
e. Prior to operating the Model 540B, you should always check the battery
charge. When the POWER switch is positioned at either REFERENCE, GALV, or SEARCH,
the PERCENT INPUT meter must indicate above the lower limit of the BAT band on the
meter scale in all BAT CHECK positions.
f. It is recommended that a DC reversal error check be made and recorded for
future reference, in the event that suspected overload damage has occurred to the
thermocouple. This value can be used for comparison, as discussed in step (g) of
para 9.
The amount of DC reversal error at a particular input level may be
measured as follows:
(1) Apply the desired DC voltage or current to the A54 DC INPUT terminals.
Determine the deflection sensitivity in the HIGH SENSITIVITY position.
(2) Adjust the REFERENCE ADJUST control until a GALVANOMETER indication of
zero is obtained in the HIGH SENSITIVITY position.
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