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RS-925D
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Operation
4.6 Environmental Conditions
For optimal accuracy, the decade box should be used
in an environment of 23ºC. It should be allowed to
stabilize at that temperature for at least four hours
after any significant temperature variation.
Humidity should be maintained at laboratory condi-
tions of 30% to 70% RH.
4.7 Switch Conditioning
The switch wipers employed in this unit are self
cleaning. They have solid silver alloy contacts.
After being left idle, the wipers and contacts must
be conditioned or “broken in” again to remove the
film of silver oxide that develops over time. This is
standard metrology practice when high accuracy is
required. This effect is of the order of less than 1 m
Ω
,
So it may be ignored whenever measurements of that
magnitude are not important.
To perform this
“breaking in,” simply rotate each
switch seven to ten times in each direction.
4.8 Meter Shunt applications
To measure the current in a 4-terminal resistor:
1. Measure the voltage drop between the two
terminals of the resistor not connected to the
current source.
2. Determine the current by the ratio of the mea-
sured voltage to the known resistance.
Using the 4-terminal technique helps avoid errors
caused by the voltage drops in the current-carrying
leads and contacts. Errors caused by lead and contact
resistances in the voltage measuring circuit are negli-
gible if the current in this circuit is small.
4.9 Kelvin Bridge applications
Use 4-terminal resistance standard for all Kelvin
bridge measurements. When connected as shown in
Figure 4-3, errors caused by lead and contact resis-
tances can be made negligible because they appear as
part of the generator or yoke resistance, or in series
with high resistance bridge arms.
Figure 4-3: Kelvin Bridge Connections
For maximum protection and accuracy, limit power
input to RS-925D to 1Watt. This may be accom-
plished by placing a resistor in series with the bridge
generator or battery.
The value of this resistance can be calculated from
the following formula:
where:
R = value of the power-limiting resistor
E = open circuit voltage of the generator
The protective resistor should have a power rating of
4 W or more. Input power should be limited to 1/10
W or less for most accurate measurements.
RS-925D
R =
E
2
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