(a) General. The quality (accuracy) of the instrument is related
to the errors that it produces. The better instrument costs more than the
poorer one. There are also many instrumental errors caused by the way that
the operator used the instrument.
(b) Leveling.
Many instruments read as well in one position as
another; however, others must be carefully leveled in order to yield their
best results.
(c) Calibration.
Unless an instrument has been recently and
properly calibrated, there is no way to know whether it is telling the truth
or not, even after its correction factor has been applied. If it has been
subjected to overload or physical shock since its last calibration, there is
a good chance that it is off.
(d) Adjustment.
Many instruments require a zero-set, power-set,
electrical zero, or other compensation adjustment prior to use. This is a
source of error if not done properly.
(e) Wear and tear. As instruments get old or after they have had
much use, they tend to wear out.
Bearings wear out of round and become
loose; friction changes or becomes erratic; dust and corrosion accumulate;
magnets weaken; springs stretch; and electrical resistance, capacitance and
inductance change.
All maintenance checks of these things create errors
that are difficult to determine except by calibration.
(f) Loading.
Loading errors must be considered as errors
resulting from the type of instrument used, the quality and type of the
instrument usually determines the amount of loading.
It takes energy
(current) to move the needle of a voltmeter. This energy must come from the
circuit under test.
Thus we change the load on the circuit with a
consequent voltage drop. It should be remembered that a meter movement
Figure 2.
Effect of Loading
143
Previous Page
