8508A
Users Manual
Making Connections to the Multimeter -- General Guidelines
The 8508A Reference Multimeter is capable of providing highly accurate traceable
measurements. To attain this accuracy, it is necessary to make connections to any
external circuitry or load correctly. A few guidelines are given in Table 3-1.
Table 3-1. Identifying and Avoiding Errors
Sources of Inaccuracy Avoiding or Minimizing Inaccuracy
Thermal EMFs
These can give rise to series (Normal) mode • Screen thermal junctions from draughts.
interference, particularly where large currents
• Allow time for thermal equilibrium to be reached
have a heating effect at junctions.
before taking readings.
In otherwise thermoelectrically-balanced
• Use conductors, joints and terminals with a good
measuring circuits, cooling caused by drafts can
margin of current-carrying capacity.
upset the balance.
• Avoid thermoelectric junctions where possible:
∗ Use untinned single-strand copper wire of high
purity.
∗ Avoid making connections through Nickel, Tin,
Brass and Aluminum. If oxidation is a problem,
use gold-plated copper terminals, and replace
the terminals before the plating wears off.
∗ If joints must be soldered, low-thermal solders
are available, but crimped joints are preferred.
∗ Use low-thermal switches and relays where
they form part of the measuring circuit.
∗ Balance one thermal EMF against another in
opposition, where possible. (Switch and relay
contacts, terminals etc.)
E-M Interference
Noisy or intense electric, magnetic and
• Choose as “quiet” a site as possible (a screened
electromagnetic effects in the vicinity can disturb
cage may be necessary if interference is heavy or
the measurement circuit. Some typical sources
circuit impedance is high). Suppress as many
are:
sources as possible.
• Static electric fields.
• Fluorescent lighting.
• Inadequate shielding, filtering or grounding of
power lines.
• Always keep interconnecting leads as short as
possible, especially unshielded lengths.
• Run leads together as twisted pairs in a common
shield to reduce loop pick-up area, but beware of
leakage problems and excessive capacitance.
• Transients from local switching.
• Where both source and load are floating, connect
• Induction and radiation fields of local E-M
transmitters.
Lo to ground at the source to reduce common
mode voltages.
• Excessive common mode voltages between
source and load.
The disturbances can be magnified by the
• If an external measurement ground connection has
been made, select External Guarding at the
Multimeter in Voltage and Current functions and
deselect External Guarding in Ohms and PRT
capacitance of the user’s hand. Electrical
functions.
interference has greatest effect in high
impedance circuits. Separation of leads and
creation of loops in the circuit can intensify the
disturbances.
• When connecting to a Multifunction Calibrator such
as the Fluke 5720A or 5520A, follow the above
Guarding and Grounding advice and deselect
External Guard at the Calibrator.
Lead Resistance
The resistance of the test leads can drop
• Keep all leads as short as possible.
significant voltages between the source and
• Use conductors with a good margin of current-
load, especially at high load currents.
carrying capacity.
• Use External Guard or 4-wire connections where
necessary.
Lead Insulation Leakage
This can cause significant errors in measurement
circuits at high voltages and high resistances.
Some insulating materials suffer greater losses
than others, e.g. PVC has more leakage than
PTFE.
Choose low loss insulated leads - PTFE is preferred
to PVC. When running leads together in shielded
pairs, avoid large voltages between leads in the same
screen, especially if using PVC insulation.
3-4
To Next Page
Loading...
