IET Labs, Inc. 1864-1644, Руководство пользователя и обслуживания

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1864-1644 Positive Polarity Megohmmeter
APPLICATIONS
17
4.6 Guarded 3-Terminal Measurements
In many cases it is necessary to measure the resistance
between two points in the presence of resistance from
each of these points to a third point. This third point
can often be guarded to avoid error caused by the
extraneous resistances.
This situation can be shown diagrammatically as a
three-terminal resistor (Figure 4-3). Here, R
X
is the
quantity to be measured in the presence of R
A
and
R
B
. If the junction of R
A
and R
B
is tied to a guard, R
A
is placed across the power supply and has no effect
if it is greater than 500 k
Ω
. R
B
shunts R
S
and causes
a much smaller error than that which would be pres-
ent if no guard were used. The error is approximately
-R
S
/R
B
x 100%, where R
S
equals the value shown in
Table 4-1 for the various ranges. If a choice is pos-
sible, the higher of the two stray resistances should
be connected as R
B
.
The guard terminal can be used whether the GUARD
or the - UNKNOWN terminal is grounded, but note
that if the -UNKNOWN terminal is grounded, the
GUARD terminal will be a high positive voltage
level. Often the terminal to be guarded is a large chas-
sis and it is, therefore, safer to ground the
GUARD
terminal. If this third terminal is true ground then the
GUARD terminal
must be grounded
.
4.7 Remote Shielded Measurements
Measurements can be made on components that are
some distance from the instrument if care is used to
prevent leakage between the connecting leads and
to avoid the shock hazard. A convenient way to do
this is to use a shielded cable. If the unknown can
be measured ungrounded, make the connection to
the
+UNKNOWN terminal with the shielded lead,
tie the shield to the GUARD terminal, and connect
the
GUARD terminal to the panel ground with the
connecting link. If one side of the unknown must
be grounded, connect the grounding link to the
+UNKNOWN terminal, shield the +UNKNOWN
terminal, and tie the shield to the GUARD terminal.
In this instance, the shield is not at ground potential
and should be insulated.
4.8 Measurements Under Humid
Conditions
The 1864 Megohmmeter has been designed to operate
under conditions of high humidity but, nevertheless,
a few simple precautions should be taken to ensure
accurate measurements. These precautions are:
1. Allow several minutes warmup (internal heat
will reduce humidity inside the instrument).
2. Clean the binding-post insulation with a dry,
clean cloth.
3. Use ungrounded operation (tie the GUARD
terminal to the panel ground).
To determine the presence of errors due to humidity,
measure the resistance between the binding posts
with no external connections. Note that with the
-UNKNOWN terminal grounded, breathing on the
terminals will cause a meter de fl ection because leak-
age from the insulator of the +UNKNOWN terminal
to the panel is measured.
Actually, this problem is somewhat academic because
the unknown to be measured is usually much more se-
verely affected by humidity than is the megohmmeter.