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c.
Radio Frequency Interference can be particularily
troublesome with low-current thermoelements.
The thermocouple in the Model 540B is incased in
a steel shield to minimize stray field
pick-
up.
However, to insure accuracy, the Model 540B
should
be
used in areas of relatively
low
field
strength.
d.
The number of transfers will
vary
with the degree
of accuracy required. In general it is suggested
that three seperate transfers (de to ac and back to
de 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 collllections, unstable sources, and inter-
ference from stray capacitance or energy fields.
Any factor which detracts from repeatability must
be eliminated before accurate transfers can be
made.
e.
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.
f.
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.
g.
Though the internal range selection components
and
thermocouple in the Model 540B are protected from
overloads in the voltage mode; no protection is pro-
vided in the current mode or
for
external shunts or
high frequency converters. These devices are
easily damaged or destroyed by overloads and
all
STEP
CONTROL
POSITION
a
POWER
REFERENCE
GALVO
SEARCH
540B
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 indi-
cate a damaged thermal element, is de 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.
h.
During all transfer measurements the operator
must remain alert to changes in readings regard-
less of their cause. All such changes should be
analyzed and corrective measures taken, in order
to insure the accuracy of the transfer measure-
ments.
2-11.
BATTERY CHECK
2-
12.
Use the procedure of Figure 2-4 to check for
sufficient battery charge each time the instrument is
used.
2-13.
DEFLECTION SENSITIVITY
2-14. To determine the percentage represented by the
GALVANOMETER at a desired de voltage or current
level applied to the DC INPUT terminals, use the
pro-
cedure of Figure
2-
5.
2-15.
DC REVERSAL ERROR
2-16. The measurement technique used in the following
procedures does not utilize
the
de reversal error as
expressed in percent change. However, it is recom-
mended that a de reversal error check be made and
re-
corded for future reference, in event that suspected
overload damage has occured to the thermocouple.
This
value can be used for comparison, as discussed
in step g of paragraph 2-10. The amount of de
reversal
error at a particular input
level
may be measured as
follows:
a.
Apply the desired de voltage or current to the A54
DC INPUT
terminals.
Determine the
deflection
sensitivity in the HIGH SENSITIVITY position as
described in Figure
2-
5.
b.
Adjust the REFERENCE ADJUST controls until a
GALVANOMETER indication of zero
is
obtained
in
the HIGH SENSITIVITY
position.
c.
Set the GALV switch
to
OPEN. Depress
the
PO-
LARITY switch until it locks and remains in the
depressedposition.
Wait approximately 5 seconds;
then place the GALV switch in
the
LOCK position.
OPERATION
The PERCENT INPUT meter must read above the
lower limit of the BAT band on the meter scale in
all BAT CHECK positions.
Figure
2-
4
.
BATTERY CHECK
2-5
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