732B/734A
Instruction Manual
3-18
Minimizing Error Sources 3-20.
You can inadvertently defeat the low uncertainty of the 732B output if you do not pay
proper care to avoidable error sources. Errors can occur from the effects of thermal emfs,
lead and connection resistance, and other sources.
Note
Most importantly, always repeat measurements until you are satisfied that
the results are repeatable relative to the specification being measured.
Mechanically Induced Errors 3-21.
To avoid changing connector contact resistance and degrading the measurement, do not
change the stresses on the connectors or cables. Try not to move, jostle, or vibrate the
standard or any attached equipment during measurements.
Thermal EMFs 3-22.
When dissimilar metals at different temperatures come into contact, a thermal voltage is
induced. Thermal voltages can exceed 10 µV. All it takes to induce a thermal emf error
and adversely affect a measurement is to briefly touch and thereby warm a connector or
binding post. Typically, it takes five minutes to thermally stabilize a connection after it
has been touched. In addition, use the following techniques to avoid thermal emf errors:
1. Use the Fluke 5440A-7002 Low Thermal EMF Cable Set.
2. Use #24 AWG or larger, bare copper, Teflon\R-insulated connecting wires. It is
preferable to use shielded, twisted pair cable. Avoid splices.
3. Avoid the use of ordinary, nickel-plated, banana plugs for equipment
interconnections. Use metals that have small thermal emf’s with respect to copper.
4. Use low thermal emf copper spade lugs. Crimp the lug onto the wire and solder the
connection. Loosen the top of the binding post, insert the lug, and tighten the
binding post on the lug with fingers only.
Output Cable Loading 3-23.
The assigned value of the 10V output is guaranteed at the binding posts. A voltage drop
in test leads can cause a loading error. Refer to Figure 3-11. In this example, the 1 kΩ
load sees a 2 mV degradation of the 10V output. To reduce loading errors in applications
where low-impedance loads are unavoidable, use lower resistance test leads (shorter and
heavier gauge).
Digital multimeters usually have much higher impedance than 1 kΩ. Most DMMSs have
a 10 MΩ input impedance. On the 20V dc range, Fluke 8840A 8505A, and 8506A
DMMs present a load impedance higher than 10,000 MΩ. For a DMM with a 10 MΩ
input impedance, the loading error would be 0.2 µV. For a DMM with a 10,000 MΩ
input impedance, there is no significant loading error.
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