Page 452 - Instrumentation Reference Book 3E

Measurement of d.c. and a.c. current and voltage using indicating instruments 435

measured accurately and gives such instruments a Table 20.4 Multimeter specification
typical minimum FSD of 1OV. When used as a
voltmeter the variation of the diode forward drop D.c. voltage
with temperature can provide the instrument with 8 ranges:
100mV. 3; 10, 30, 100, 300;
a sensitivity to ambient temperature. It is possible 600V, 1 kV
to design such instruments to provide an accuracy D.c. current
of 1 percent of FSD from 50 Hz to 10 kHz. 7 ranges:
5OpA, 300pA. 1, 10, 100mA.
20.2.1.4 Multimeters 1 A: and 10A
A.c. voltage
These are multi-range devices using a permanent 7 ranges:
magnet-moving coil instrument. They enable the 3, 10, 30, 100, 300; 600V, 1 kV
measurement of d.c. and a.c. current and voltage, A.c. current
and resistance. One of the most common instru- 4 ranges:
ments of this type is the AVO-Biddle Model 8 lOmA, 100mA, 1A; and 10A
Mark 6 (Thorn-EMI). Table 20.4 gives the specifi- Resistance
cation for this instrument and Figure 20.13 shows 3 ranges:
x 1:0 - 2 kR
the circuit diagram. The basic movement has a full- x 1QO:O - 200 kR
scale deflection of 50 PA, and therefore, this gives ~10 - 20MR
k:O
the instrument a sensitivity of 20,000QN on its Souvce,for resistance
d.c. voltage ranges. The three ranges of resistance
operate by measuring the current passing through One 15V type B121 battery
the resistance on applying a d.c. voltage supplied (for x 10 k range)
from internal batteries. A zero control on these One 1.5V type SP2 single cell
(for xl, xl00 range)
ranges, used with the instrument probes shorted Accuracy
together, enables compensation for changes in the D.c. &% fsd
emf of the internal batteries to be made. AS. (150 Hz)+2% fsd
Resistance $37'0 center scale
20.2.1.5 Electronic miltimeters Sensitivity
D.c. 20,000 RIV all ranges
By using the electronic input in Figure 20.14(a) it A.c. 100 nlV 3 V range
is possible to achieve a high input impedance irre- 1,000 n/V 10 V range
spective ofthe voltage range. This is used as shown 2,000 QIV all other ranges
in Figure 20.14(b) to measure current, resistance, Overload protection
and a.c. quantities. For current measurement the High speed electromechanical
maximum input voltage can be made to be the cut-out with a fuse on the
two lower
same on all ranges. Resistance measurements can resistance ranges
be made with lower voltage drops across the resis- Decibels
tors and with a linear indication. A.C. quantities - 10 to +55 using ax. voltage scale
are measured using rectification and mean or peak Voltage drop at
sensing. Table 20.5 gives the specification of such terminals
an instrument (Hewlett-Packard HP410C General D.c. 100 mV on 50 /*A range.
Purpose Multi-Function Voltmeter). approx. 400 mV on other ranges
A.c. less than 450 rnV at 10 A
Frequency response
a.c. voltage range
20.2.2 Moving-iron instruments (up to 300 v)
There are two basic types of moving-iron instru- < f3% discrepancy between
ment: the attraction and repulsion types shown 50 Hz reading and readings
taken between 15 Hz and 15 Hz
in Figure 20.15. In the attraction type a piece of
soft iron in the form of a disc is attracted into the
coil which is in the form of a flat solenoid. Damp-
ing ofthe instrument is provided by the air-damp-
ing chamber. The shape of the disc can be used to
control the scale shape. In the repulsion instru-
ment two pieces of iron, either in the form of rods
or vanes, one fixed and the other movable, are where L is the inductance of the circuit. The
magnetized by the field current to be measured. restoring torque, T,, is produced by a spring:
In both instruments the torque, T,+ generated by
the attraction or repalsion is governed by T,. = IcB