Fluke 732a - Minimizing Error Sources; Thermal Errors

732A
2-31.
A potential
difference may exist between
the
power
line
grounds
of the 732A and an
instrument
to
which
it is connected.
This
potential
difference
can
cause
circulating
ground
currents which cause
errors
in the
output voltage.
2-32.
To
prevent
these
errors
the 732A
GUARD
terminal
should be
connected to
the load in
such
a
manner
as to provide
a
separate
path for the
circulating
currents.
Connect
the
GUARD terminal
to
the
grounded
side
of
the
load, at
the
load. Figure
2-3
illustrates
the
correct
GUARD
terminal
connection
and
the
rerouted ground
currents.
The
circulating
current
path may
also
be broken
by operating the
732A on battery power as
described
later
in
this
section.
2-33.
Oven
Temperature
Thermistor
2-34.
A
Thermistor, mounted
inside
the Oven
Assembly
senses changes
in the internal oven
temperature.
Use
the
Oven
Temperature
Thermistor in
conjunction
woth an
external
ohmmeter
to monitor the temperature
stability
of the oven vs
time. The thermistor
terminals
are on to the
front
pane! of the 732A.
Both
leads of the thermistor
are
isolated
from all
parts
of the 732A circuitry. A
maximum
potential difference
of
60V
is allowed
between
either of
the
thermistor
terminals
and any
other
front panel
terminal
(Guard,
Ground,
Reference
Standard Outputs).
2-35.
The
nominal
value
of
the
Oven
Temperature
Thermistor
is
between
3 kQ
and
4
kQ
at
the normal
oven
operating
temperature.
The thermistor
has
a
temperature
coefficient
of
3.8%/°C. The actual
operating value
is
shipped
with
the
instrument.
2-36.
Minimizing
Error
Sources
2-37.
The inherent
accuracy
and stability
of the
732A
may
be easily
degraded
if the effects
of
thermal
emf, lead
resistance
and
other
factors are
not
considered
and
minimized.
2-38.
THERMAL
ERRORS
2-39.
When
parts of
a circuit
operate
at different
temperatures,
thermal
voltages
will
normally
be
present
at
the
equipment
connections.
These
thermal
voltages
can
exceed
10
uV.
Use the
following
techniques
to minimize
thermal
errors:
i. Use
the
Fluke 5440-7002
Low
Thermal
EMF
Interconnecting
Cable Assembly
. See
Section
6,
Accessories.
CASE{CHASSIS)
WHERE:
R1
Cl
=
Leakage
Resistance
&
Capacitance
From
Guard to
Chassis
Ec
=
Potential
Difference Between
Power
Line
Grounds
Rq
=
732A Output
Resistance
Rl
=
Load
Resistance
Figure
2-3.
Guard
Connection
2-6