HP 3468A - Page 29

,
you
are
making
temperature
measurements
with
a
type
44004
thermistor.
Refer
to
Figure
2-4.
At
20°C,
40
feet
of
#24
A.W.G.
copper
wire
has
a
resistance
of
1.02
ohms.
Two
such
wires
would
have
a
total
resistance
of
2.04
ohms.
With
a
type
44004
thermistor
this
would
result
in
an
error
of
.1%.
7
a | Command
F3
Selects
2-wire
ohms
mode
4-Wire Ohms
The
use
of
4-wire
ohms
measurements
eliminates
the
errors
caused
by
the
test
lead
resistance.
Figure
2-4
illustrates
this
point.
The
current
through
the
thermistor
is
the
same
regardless
of
the
lead
resistance,
and
the
voltmeter
measures
only
the
voltage
across
the
thermistor,
not
across
the
combined
lead
resistance.
The
4-wire
resistance
measurements
are
essential
when
highest
accuracy
is
required,
or
where
long lead lengths
are present.
| Command
F4 Selects 4-wire ohms mode
Extended Ohms
The
extended
ohms
feature
of
the
3468A
is
available
only
via
the
F7
remote
pro-
gramming
command.
With
extended
ohms,
the
3468A
can
be
used
to
measure
resistances
in
excess
of
30
Mohms.
When
in
the
extended
ohms
mode,
the
3468A
goes
to
the
30
Mohm
range,
2-wire
mode.
An
internal
resistance
of
ap-
proximately
10
Mohms
is
placed
in
parallel
with
the
input
terminals
by
the
3468A.
If
this
resistance
is
measured
first
and
then
your
unknown
resistor
con-
nected
to
the
input
terminals;
the
parallel
combination
can
be
measured.
A
calcula-
tion
can
then
be
performed
to
determine
the
unknown
resistance.
The
formula
for
the calculation
is:
Rie = Ri * Rt
Ri — Rt
Rx
is
the
unknown
resistance,
Ri
is
the
measured
value
of
the
internal
10
Mohm
resistor
and
Rt
is
the
measured
value
of
the
parallel
combination.
The
test
leads
used
for
the
measurement
should
be
very
short,
preferably
a
shielded
twisted
pair,
to redure
noise
pick up.
Programs
to
make
the
necessary
measurements,
perform
the
calculations,
and
display
the
value
of
the
unknown
resistor
is
given
in
Chapters
Ill
and
IV,
23