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Notes on Resistance
Verification
Calibrated
Values (lOQ
-
100MQ)
For each
resistance
(RANGE)
selection, three
checks
are
required:
a.
4-wire
at
Full
Range
(Resistor Value).
b.
2-wire
at
Full
Range
(Resistor plus
internal
wiring)
c.
2-wire
at
Zero
(Internal
wiring only)
Interconnections
For
4-wire connections
in
Remote
Sense,
only
the
value
of
the
internal
Standard Resistor
is
measured.
In
Local
Sense,
a 4-wire
method
is
used
to
exclude
the
resistance
of
the external leads
from
the
measured
value.
Calibration
Memory
In
Q
function,
each
RANGE
key
selects
a nominal
value
standard
resistor.
Routine adjustment of
the
resistor
is
not
necessary.
During
calibration
the actual value
is
measured and
stored
in
the calibration
memory
to
be
displayed
whenever
that
range
is
selected.
Separate
memory
stores exist for
Remote
Sense
(4-wire),
Local Sense
(2-wire)
and
Local Sense
zero.
The
displayed values
were
obtained
at
the
most
recent
cali-
bration,
except
for the
lOMQ,
and
lOOMfi
Zero and
Full
Range
values
in
2-wire,
which
because of
their
minute
fraction
merely
repeat the
4-wire
figures.
Use
of HI
and Lo
Terminals
In
2-Wlre Q.
For
the
function, the
Remote
Sense key does
not
change
any
internal
analog
wiring,
so with
Remote
Sense
deselected,
the
1+
and
I-
terminals
remain connected
to the internal
resistors.
The
key
is
thus
used
only
to select
the appropriate
calibration
memory,
for
data
to
display
in
'Spec'
Mode.
To
obtain the value entered
when
a
resistor
was
calibrated
in
4-
wire,
it
is
necessary
for the
key
LED
to
be
lit;
but
for
2-wire
it
must be
unlit.
When
values
are
entered during
calibration, the
2-
wire
figures
are the
results
of
4-Wire measurements
at
the
Hi and
Lo
terminals,
NOT
the
1+
and
I-
terminals. Verification
check
figures are therefore
only
valid
if
they
are
obtained
from
measurements under
identical
conditons;
that
is
from
4-wire
connections
to
the
Hi and
Lo
terminals,
with
the
Remote
Sense
LED
unlit,
as described
in
the procedures.
4-Wlre
Limits
The
values
measured
in
4-wire
Remote
Sense
do
not include
the resistance
of
internal
or
external wiring.
The 4708
accepts
any
value within
±200
ppm
of nominal
as
a
valid
calibration.
For
verification,
the
measured
values are not
compared
against the
nominal
value,
but
against the previously
cali-
brated value presented
on
the
OUTPUT
Display.
2-
Wire
Limits
The
values
measured
in
2-
wire Local Sense
are greater
than
for
4-wire
Remote
Sense, as they include the
resistance
of
internal
wiring
and
relay contacts.
The 4708
will
not accept
any
2-
wire value
less
than the stored
value
for
4-wire,
so
the
4-wire
Remote
Sense
calibration
or
verification
is
always
carried
out before attempting 2-wire
Local
Sense.
The
extra
internal
resistance varies
between Ranges,
so the
4708
accepts values
up
to
the limits as
listed in
RSI
Table
3
(b)
as valid
2-wire
calibrations.
When
verifying, the
measured
2-wire values
are
not
com-
pared
against the displayed value, but against
limits
above
the
previously-measured 4-wire
value.
7-A3-3
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