Page 1
Model 8 Universal
AVOMETER
MK III
WO RK ING IN S TR U C TI O NS
L. TD
AVOCET HOUSE, 92-96 VAUXHALL BRIDGE ROAD,
LONDON, S.W .1. . ENGLAND
1
= -G RO U P
Page 2
T he Model 8
Univ ersal A vo m e te r Mk III
AVOtf τ
INSTRUCTIONS FOR USE
LTD
’ s ψc r o u p '
AVO CET HOUSE, 92-96 VA U X H A LL BRIDGE ROAD,
LONDON, S.W.1., ENGLAND
Te lep hon e: V ic t o ri a 3404 (12 lines) Tel egr am s: Avoce t, London, S.W.1
Page 3
FOREWORD
During the past forty years we have been engaged in the design
and manufacture of ‘ A VO ’ Electrical Measuring Instruments.
Thr oug hou t that time we have consistently pioneered the design
of modern multi-range instruments and have kept abreast of, and
catered for, the requirements of the epoch-making developments
in the fields of radio and electronics.
The success of our steadfast policy of maintaining high standards
of performance in instruments of such wide versatility, and mak
ing such instruments available at reasonable cost, is reflected
in the great respect and genuine goodwill which ‘ A VO ’ products
enjoy in every part of the world.
It has been gratifying to note the very large number of instances
where the satisfaction obtained from the performance of one of
our instruments has led to the automatic choice of other instru
ments from the 'A V O ' range. This process, having continued
over a long period of years, has resulted in virtual standardisation
on our products by numerous Public Bodies, The Services,
Railway Systems, and many Post Office and Telegraph Under
takings thr oug hou t the world.
Our designers have thereby been encouraged to ensure that new
instruments or accessories for inclusion in the ‘ A VO ’ range fit in
with existing ‘AV O’ apparatus and serve to extend the usefulness
of instruments already in being. Thus, the user who standardises
on ‘ A VO’ products will seldom find himself short of essential
measuring equipment, for, by means of suitable accessories, his
existing equipment can often be adapted to most unusual
demands.
It is with pleasure that we acknowledge that the unique position
attained by ‘ A VO ’ is due in no small measure to the co-operation
of so many users who stimulate our Research and Development
staffs from time to time with suggestions, criticisms, and even
requests for the production of entirely new instruments or
accessories. It is our desire to encourage and preserve this rela
tionship between those who use ‘ A V O’ instruments and those
who are responsible for their design and manufacture, and
correspondence is therefore welcomed, whilst suggestions will
receive prompt and sympathetic consideration.
3
Page 4
i
THE MODEL 8 AVOMETER Mk. Ill
4
Page 5
CONTENTS
Page
FO R EW O R D ................................................................................................ 3
INT RO DUC TIO N .................................................................................... 7
TA BL E OF R AN G ES .................................................................................... 8
GENERAL D ES C R I P T I O N........................................................................ 9
LIMIT S OF A C C U R A C Y ........................................................................ 9
GR APH OF FREQUENCY RESPONSE
DESIGN A N D CO NST RUC TION ................................................. 11
RANGE C O N T R O L S .................................................................................... 12
THE MOVEME NT .................................................................................... 13
SC AL IN G ................................................................................................ 13
REPLACE MENT OF INTE RNAL BA TTER Y, CELL A N D FUSE .. 13
REPLAC EMENTS ...................................................................................... 13
MOVEM ENT REVERSE CO NTRO L ................................................. 13
OV ER LO AD PROTECTION ............................................................. 14
OPE RAT ION OF I N ST R UM EN T ............................................................. 15
CURRENT ME ASUREMENT ............................................................. 15
VO L TA GE MEAS UREMENT ............................................................. 15
RESIS TANCE ME AS URE ME NT ............................................................. 16
IN SU LA TI ON RESIS TANCE MEASUREM ENT
LO W RESIS TANCE ME A SU RE MEN T................................................. 18
DECIBEL ME A S UR E M E NT
ACCE SSORIE S .................................................................................... 20
........................................................................ 18
.....................................
.........................
10
18
D.C. V O LT AG E MULTIPLIE RS 20
CURRENT TRA NSFOR MERS 20
D.C. SH UNT S 21
RESIST ANCE RANGE EXTENSION UNIT 21
REPLACEMENT BATTERIE S 21
C O N CL U S ION ................................................................................................ 22
CIRC UIT D IA G RA M OF THE MODEL 8 AVOM ETER Mk III .. 23
C O PY R IG H T
No i nf o rmat io n or dia g ra m s in w h o le or in p art m ay be cop ie d or
re p ro du ce d wit h o u t th e pri or pe rm is si on in w r iti n g of A v o Lim i te d.
5
Page 6
INTRODUCTION
Since its conception in 1923, the AvoMeter has maintained a
distin ct lead upon all its competitors, and can today quite rightly
be termed the most popular ins trument of its type in the world,
for in no other instrument can one find such a unique combina
tion of ranges and comprehensive automatic overload protection
in addition to a high degree of accuracy, reliability and simplicity
of use.
Much time and th ough t is continually devoted by our design
department to the improvement of our products and it is for the
Electronic, Radio and Television Engineer that this new instru
ment has been primarily produced. The Model 8 AvoMeter Mk. Ill
has a high d.c. voltage sensitivity coupled with improved tempera
ture compensation and frequency response. A useful feature
which has been retained is the push button change-over switch,
which ena blesthe direction of the current through the moving coil
to be reversed, th us obviating the necessity of changing leads
when working with d.c. voltages and currents which may be
either positive or negative in respect to a basic test position. The
excellent qualities of previous models including the ‘ A V O’ auto
matic c ut-out have been retained, and we have great confidence
that given a reasonable amount of care and attention, not
forgetting the removal of exhausted batteries, this instrument
will give lasting satisfaction.
7
Page 7
TA BL E OF RA NGES
D.C. Voltage
2,500 V.
1,000 V.
500 V.
250 V. 10 mA.
100 V.
25 V.
10 V. 50 μΑ .
2-5 V.
D.C. Current
10 A.
1 A.
100 mA.
1 mA.
2 5 0μΑ .
Resistance
A.C. Voltage A.C. Current
2,500 V. 10 A.
1,000 V.
250 V.
100 V.
25 V.
10 V.
2-5 V.
2 -5 A.
1 A.
100 mA.
0—200 megohms— with external voltage or external unit
0—20 megohms, (200,000 ohms mid-scale) Ί
0—200,000 ohms (2,000 ohms mid-scale) y self-contained
0—2,000 ohms (20 ohms mid-scale)
0—2-5 ohms (with external unit)
Page 8
The Model 8 Universal AvoMeter Mk. Ill
GENERAL D ES C RIP T IO N
The meter is extremely simple to use, range selection in general
being accomplished by means of two switch knobs. All tests,
except those on the 2,500 V. ranges, make use of the pair of
terminals at the base of the instrument.
A clearly marked 5 in. (127 mm.) scale has uniformly divided
graduations to match 100 and 250 scale markings, and in additon
there is an ohms scale and one for decibels. An anti-parallax
mirror permits readings of the knife edge pointer to be made with
great precision.
The meter is supplied complete with a pair of special rubber-
covered leads which are intended for attachment to the Avo-
Meter by means of its captive terminals. The remote ends of the
leads are fitted with spring clips, which may be interchanged with
the 'A V O ' Long Reach Safety Clips Mk. 2 supplied with the
instrument.
‘AV O ’ Long Reach Safety Clips Mk. 2 have been introduced to
enable connections for test purposes to be made at what are
normally inaccessible points on a chassis. Examination will
show that they are completely insulated with the exception of
the jaws at one end, which can be opened by compressing the
stem into the body of the clip. Rigid connections to wiring can
thus be made by this insulated device in complicated wiring
systems where other types of larger clip could not be attached,
or if fixed might cause short circuits.
L IM IT S OF A CC UR AC Y
Generally speaking, the highest percentage accuracy on current
and voltage ranges is obtainable at the upper end of the scale,
but on resistance ranges it is better towards the centre of the
scale. In the case of voltage measurements, which are more
frequently taken than those of current, successive ranges have
been closely chosen to obviate the need for taking readings on
very small deflections.
The instrument will produce its highest accuracy when used face
upwards, in which position it has been calibrated.
9
Page 9
% OF INSTRUMENT READ ING
FREQUENCY - CYCLES PER S ECO ND
Fig. 1. Typical frequency response of Avomete r Model 8 Mk III on 25 and 100V ranges
Page 10
D.C. Voltage. 2% of indication between full-scale and half
scale deflection. Below half-scale deflection, 1 % of the full-
scale value.
D.C. Current. 1 % of full-scale value over effective range.
A.C. Voltage*. Up to 250 V. 2-25% of full-scale value over
effective range (25-2000 c/s).
A.C. Current*. 2-25% of full-scale value over effective range.
The ‘effective range’ in accordance with British Standard
Specification 89/1954 is:
D.C.—from 0-1 of scale-range to full-scale value.
A.C.—from 0-25 of scale-range to full-scale value.
It will be noted that with the exception of the d.c. voltage ranges,
the instrume nt meets the requirements laid down in Section 6
of the British Standard Specification 89/1954 for 5 in. (127 mm,)
scale-length Industrial Portable Instruments. In practice, the
Model 8 is well within the above limits, due to the great care taken
in the manufacture of its various components, and to the fine
initial calibration.
*NOTE.
The instrument is calibrated for use at 50 c/s, Figure 1 shows the
frequency response of a typical Av oMeter Model 8 Mk.3on the
25V and 100V ranges. The change in readings, due to variation
in frequency (between 15 c/s and 15 kc/s) should not exceed 2%
on a.c. current ranges or on a.c. voltage ranges between 10V and
250V, on the 25V a.c. range the change should not exceed 4%.
Inasmuch as rectifier moving coil instruments give readings on
‘a.c.’ proportional to the mean and not the r.m.s. value of the
wave form with which they are presented, they depend for their
accuracy not only upon their initial calibration, but also upon
the maintenance of a sinusoidal wave form. Since the form factor
(r.m.s. value divided by mean value) of a sine wave is 1-11, this
has been taken into account in calibrating the meter which does,
therefore, indicate r.m.s. values on the assumption that the
normal sine wave will be encountered. Generally speaking,
considerable wave form distortion can occur without appreciably
affecting the form factor and resulting accuracy of measurement,
but the user should recognise the possibility of some error when
using distorted wave forms, squarish wave shapes producing
high readings, and peaky ones, low readings.
DESIG N A N D C O NS T R UC T IO N
The instrument consists of a moulded panel on the inside of
which are mounted the whole of the switching apparatus,
resistors, shunts, transformer, rectifier, etc., together with the
movement. The panel fits into a robust moulded case, the joint
being rendered completely dust proof, whilst a carrying strap is
provided to facilitate portability. The main switching is accom
11
Page 11
plished automatically by means of two knobs which indicate on
the engraved panel, the range in use. These switches are of
generous and robust design, the contacts being arranged to
‘ make’ before 'break' on adjacent rang es; a feature which provides
a factor of safety in use.
When the instrument is set for operation on d.c., the moving coil
is associated with a universal shunt and series multipliers, whilst
on a.c., rectifiers and a transformer are also introduced.
RANGE C ON T RO L S
The left-hand knob provides all the d.c. current and voltage
ranges (except 2,500 V.) and the right-hand knob the a.c. ranges
(except 2,500 V.) and also the resistance ranges. These knobs
are electrically interlocked so that d.c. readings can only be made
after the right-hand switch has been set to d.c., and the left-hand
switch to the range selected, a.c. readings call for the left-hand
switch to be set for a.c. (it must not be left at RESISTANCE)
and the right-hand switch at the range required. Resistance tests
require the left-hand switch to be set to RESISTANC E and the
right-hand one to the desired range.
If the switches are inadvertently left to actual ranges simultane
ously, there is no circuit through the meter, and it is thereby
safeguarded against accidental damage or misleading readings.
It is possible to determine whether a source is a.c. or d.c., since
a.c. will not produce pointer indication when the meter is set for
d.c. measurement. A small pointer indication, however, may
result if d.c. current is passed through an a.c. range, but no harm
can be done to the meter provided it is not at the same time
grossly overloaded.
The main ranges are engraved on the panel around the switches,
and arrow heads on the knobs indicate the actual range selected.
In the case of voltage, successive ranges are built up on the
ratios of 2:1, 2-5:1 and 4:1, but in the case of current, a wide
coverage has been chosen instead and the 10:1 ratio in general
is followed. The 2,500 V. a.c. and d.c. ranges are available by
means of the two special terminals so marked.
Extremely wide coverage in resistance has been achieved by
having a fundamental range as marked on the scale, together with
ranges of
x
100 and-|- 100 to supplement it. Before carrying out
resistance tests, the meter should be adjusted for the state of
the batteries. It is merely necessary to join the leads together and
adjust to zero in the follo wing sequence: ohms, oh m s ^ - 100,
followed by ohms
x
100, using in each case the adjuster to match
the range.
In addition, a 200-megohm range marked ‘IN S’ is available, using
an external d.c. voltage source o rth e Resistance Range Extension
unit described on page 21.
12
Page 12
TH E M OV EM EN T
The moving coil consists of an aluminium alloy former wound
with copper wire and supplemented with Constantan in order to
reduce temperature error. It is pivoted on hardened and highly
polished steel pivots between conical spring-loaded jewels, and
swings in a gap energised by two powerfully magnetised and
aged ‘A lco max ’ blocks associated with mild steel pole pieces.
Two phosphor bronze hair springs are fitted for the purpose of
conveying current to the moving coil, and to provide controlling
torque. A knife edge type of pointer is fitted enabling very fine
readings to be taken, whilst the whole movement is perfectly
balanced and reasonably damped so that the pointer quickly
comes to rest. Temperature compensation for the movement is
provided by a thermistor.
SC ALI NG
The scale plate has three main sets of markings, each of approxi
mately 5 in. (127 mm.) length, the outermost being for resistance
measurement and is marked 0-200,000 ohms. The second is for
current and voltage (both a.c. and d.c.) and is marked 0-100, with
divisions approximately 1£ mm - apart. The third scale, calibrated
0-250, has 50 divisions, and is so used for current and voltage
measurements. In addition, there is a decibel scale marked from
—15 db. to -f15 db., which can be used with any of the a.c. ranges.
RE PL ACE ME NT OF IN TE RN A L B AT T ER Y, CELL A ND
FUSE
Inside the cover, under the carrying strap is mounted a 15 V.
battery and a Η V. cell together with a 1 A fuse and a spare fuse.
The batteries should be examined from time to time to ensure
that the electrolyte is not leaking and damaging the instrument.
This condition will generally occur only when the cells are nearly
exhausted. If it is known that the meter is going to stand unused
for several months, it is preferable that these batteries should be
removed to prevent possible damage.
When replacing batteries, the 1^ V. cell and the 15 V. battery must
be inserted with the poles to match the markings of polarity
inside the battery box.
RE PL ACE ME NT S
1*5 V. cell, 1f in. dia.
Berec) U.2. 15 V. battery, l - ^ in.
x
2f in., such as Ever Ready (or overseas,
x
f in. X 1£ in., such as Ever
Ready B.121.
M OV EM E NT RE VERSE C O NT RO L
It sometimes happens that d.c. voltages may be required both
positive and negative to a reference point, or the direction of
flow may be reversed. In order to simplify the matter of lead
alteration, a movement reverse press button (REV. M.C.) is
13
Page 13
provided. It should be noted that the polarity marked on the te r
minals is for normal use and does not apply when the button is
pressed.
OV ER LO AD P R OT EC TI ON
Apart from the ability to do its job, one of the most attractive
features of the instrument is the provision of an automatic cut-out
which gives a very high degree of overload protection to the whole
of the instrument. The incorporation of this device will be found
to be of particular value when c onducting experimental work, for
it imparts to the user the feeling of mental ease and confidence.
When conducting experimental work with conventional moving
coil meters, these can be easily ruined by inadvertently applied
overloads, whereas the AvoMeter is so well protected that it can
withstand considerable mishandling.
If an overload is applied to the meter, the cut-out knob springs
from its normal position in the panel, thus breaking the main
circuit, and this knob has only to be depressed to render the
instrument again ready for use. It is important to note that the
cut-out should never be reset when the inst rument is connected
to an external circuit, whilst the fault which caused the overload
should be rectified before the meter is reconnected.
The mechanism is brought into operation by the moving coil
coming into contact with a trigger just beyond its full-scale
position. There is, in addition, a second release at the zero end,
so that the cut-out is tripped if the meter is overloaded in reverse.
Althoug h the overload mechanism gives almost complete pro
tection to the meter, it cannot be guaranteed to fulfil completely
its function in the very worst cases of misuse, such as the mains
being connected across the meter when set to a current range.
It should be noted that mechanical shock to the instrument will
sometimes trip the cut-out mechanism. The cut-out should be
reset, using direct pressure and witho ut twisting the button, the
instrument lying face upwards. Extra protection is given ο η ' Ω '
and the ‘ Ω - ^ - 100’ ranges by means of a 1A fuse located in the
battery box.
W AR NI NG
Special care must be taken when using the instru me nt to service
television receivers or other apparatus employing capacitors o f large
capacitance, for the inclusion o f such com ponents in a circu it may
mean that very heavy peak currents may flow when the apparatus
is switched on. Such surges produce a peaky wave form, and although
these peaks are of only a few mill i-seconds duration, they may,
never-the-less, puncture the i nstr um ent rectifiers.
14
Page 14
O PE RA TI ON OF IN S T R U M EN T
The meter is intended for use horizontally. Should it happen by
any chance that the pointer is not on zero, it may be so set by
means of the screw head on the panel.
The leads fitted with Long Reach Safety clips Mk 2 or clips, as
required, should be connected to the lower pair of meter terminals
in all cases except when measuring voltages over 1,000 V.
When measuring current or voltage, ensure that the instrument
is set to match the type of source to be measured (either a.c. or
d.c.) and then choose a suitable range before connecting up to
the circuit under test. When in reasonable doubt, always switch
to the highest range and work downwards, there being no neces
sity to disconnect the leads as the switch position is changed.
Do not, however, switch o f f by rotating either o f the knobs to a blank
position.
The instrument is flash tested at 6,000 V. a.c., but should the meter
be used with accessories on circuits in excess of 2,500 V., it
should be kept at the low potential end of the circuit (near earth
potential). If this procedure cannot be adopted other suitable
safeguards must be applied.
CU RR EN T M EAS UR EM ENT
To measure current, the instrument should be set to a suitable
a.c. or d.c. range, and then connected in series with the apparatus
to be tested.
Generally speaking, the power absorbed in the instrument is
negligible, but in cases of low voltage heavy current circuits, the
inclusion of a meter may reduce the current appreciably below the
value which would otherwise prevail. The potential drop atf.s.d.,
across the meter terminals is in the order of 500 mV. on all d.c.
ranges, except the 50 microamp range which has a drop of 125
milli-volts. In the case of a.c., it is less than 250 mV. on all ranges.
Standard meter leads have a resistance of 0 02 ohm per pair.
In certain cases, care should be taken to ensure that the circuit
is ‘dead’ before breaking into it to make current measurements.
VO LT AG E ME AS URE ME NT
When measuring voltage, it is necessary to set the appropriate
range of ‘a.c.’ or ‘d.c.’ and connect the leads across the source of
voltage to be measured. If the expected magnitude of the voltage
is within the range of the meter, but its actual value is unknown,
set the inst rument to its highest range, connect up and if below
15
Page 15
1,000 V. rotate the appropriate selector switch, decreasing the
ranges step by step, until the most suitable range has been
selected. If the voltage should exceed 1,000 V., the instrument
should be set to measure 1,000 V. as described above, but the
negative lead should be transferred to the appropriate 2,500 V.
terminal. Great care must be exercised when making connections
to a live circuit, and the procedure should be entirely avoided if
possible.
On d.c. ranges, the meter consumes only 50 microamps at full
scale deflection, this sensitivity corresponding to 20,000 ohms per
volt. In the case of a.c. ranges from 10 V. upwards, full scale
deflection is obtained with a consumptio n of 1mA. (1,000 ohms
per volt). The 2-5 V. a.c. range consumes 10 mA at full scale
deflection. The meter maintains a high degree of accuracy
for audio frequency tests up to 15 kc/s on ranges up to 250 V. a.c.
Whils t discussing the problem of measuring voltage, it would be
well to draw attention to the fact that in certain circuits where the
current is limited because of the presence of a resistance between
the source and the point at which a measurement is to be made,
it is possible fo r the actual voltage to be higher normally than when
the meter is connected. All current consumi ng voltmeters,
however sensitive, draw current to varying degrees from the cir
cuit under test, thus causing a higher volts drop in the resistances
mentioned, and thereby causing the voltage to fall at the point
of measurement.
Owing to the high sensitivity of the Model 8 on its d.c. ranges, this
effect is unlikely to be of importance except in a very few instances.
A practical example of where it might be taken into account is in
the measurement of e.h.t. voltage on a television set or the
tapping on a potential divider, where the resistances are so high
as to be comparable with the resistance of the meter on the
range in use. It is generally possible to use a meter on a higher
range than absolutely necessary, and in such a case the higher
meter resistance causes less disturbance than would otherwise
be the case. A t the same time adequate pointer deflection for
reasonable accuracy should be attained.
When it is essential to obtain an accurate indication of the voltage
developed across a high resistor it is sometimes preferable to
insert the meter in series with it, and to measure the current
flowing. The reading given upon the meter, in milliamps, multi
plied by the value of the resistance in thou sands of ohms, will
give the developed voltage.
RE SIS TA NC E M EA SUR EM EN T
There are three self-contained ranges covering from 0-5 ohms to
20 megohms, and provision is also made for both upward and
downward extension of these limits. The self-contained ranges
16
Page 16
make use of the usual series circuit, and successive ranges are on
100:1 ratio, which permits of very wide coverage with three
ranges.
On resistance ranges, the meter must not merely start from its
normal instrument zero, but must have, in addition, a resistance
zero corresponding to the full scale deflection of the meter.
Before carrying out tests for resistance a check and, if necessary,
adjustment should be carried out to ensure that when the leads
are joined together the meter actually indicates zero ohms,
irrespective of the condition of the battery (within the limits of
adjustment). The method of adjustment is described later.
Owing to the nature of the scale, it is not easy to define the
accuracy, but it should be within 3% of the reading about centre
scale, increasing up to about 10% of the indication around
deflections corresponding to 10% and 90% of full scale deflection.
Resistance test should never be carried out on components
which are already carrying current.
On three ranges which utilise the internal source of voltage, it
should be remembered that a positive potential appears at the
negative terminal of the inst rument when set for resistance tests.
This fact may be important because the resistance of some
components varies according to the direction of the current
through them, and readings, therefore, depend upon the direction
in which the test voltage is applied, quite apart from its magnitude.
Such cases include electrolytic capacitors and rectifiers.
When measuring the leakage resistance of an electrolytic capa
citor, the negative lead from the meter should be connected to
the positive terminal of the capacitor, and the ohms X 100 range
employed.
Before making resistance tests the pointer should be adjusted
to zero in the following sequence:
1. Set left-hand switch at 'RESISTANCE'.
2. Join leads together.
3. On the Ω range, adjust to zero by means of the knob marked
‘ZERO Ω\
4. On the ‘ Ω -j- 100’ range, adjust to zero by means of the knob
marked 'ZERO Ω-j- 100’.
5. On the ‘ Ω x 100’ range, adjust to zero by means of the knob
marked ‘ZERO Ω x 100’.
To test a resistance, set the right-hand switch at the range
required, the leads being connected across the unknown com
ponent.
Resistance is read directly on the ‘ Ω1 range, but indications
should be divided or multiplied by 100 on the other two ranges.
If on joining the leads together it is impossible to obtain zero
ohms setting, or if furthermore the pointer position will not remain
17
Page 17
constant, but falls steadily, the internal battery or cell concerned
should be replaced. It is important that a discharged unit should
not be left in the instrument, since the electrolyte might seep
through and cause damage to the meter. If it is impossible to
obtain readings on the Ω-j- 100 range, the 1 A fuse located in the
battery box should be checked.
NOTE. It can so happen that a 15-volt battery may age in such a
manner that although it indicates a potential of 15 volts, its
internal resistance has increased so much that some loss of
accuracy can occur on the high resistance range ( Ω x 100).
If the battery has been in use for some time, or if errors are
suspected on the high resistance range, it is worth while removing
the battery and checking its short circuit current on the 100 mA.
d.c. range. If the battery fails to give a reading greater than 5 mA.
it should be discarded.
IN SU L AT IO N R ES IS TA N CE M EA SU RE ME NT
Two courses are open, the first merely calling fo r a battery or
other source of d.c. voltage in the order of 130 V. to 160 V. The
left-hand switch should be set at ‘ RE SIST ANCE ’ with the right-
hand switch at ‘ INS' and the meter leads should be connected
to the battery. The pointer should be brought to zero o n t h e oh ms
scale by means of the adjuster marked ‘ZERO Ω x 100’. To test,
connect the unknown resistance in series with the meter and its
value will be that shown on the ohms scale multiplied by 1,000.
Resistances up to 200 megohms can, therefore, be read on this
range.
The alternative method makes use of the ‘Model 8 Resistance
Range Extension Unit,’ described later.
LOW RE SI STA N CE M EA SU RE ME NT
The meter setting marked L.R. is for use with the Model 8 Resist
ance Range Extension Unit. The method of use is described in
the section covering accessories.
DECIBEL M EA SU RE ME NT
The decibel scale can be used with any of the a.c. current or
voltage ranges. It has a logarithmic scale shape and is useful
in so far that it gives a measurement closely related to the impres
sion of aural intensity in sound reproduction apparatus. A differ
ence of one decibel is about the minimum difference which can
be appreciated when comparing two intensities. For convenience,
the scale is marked in decibels both positive and negative from
a reference point. The difference in level between a negative
value on the db. scale and a positive one is the sum of the two,
i.e. the difference between — 5 db. and +6 db. is 5 - f 6 = 11 db.
18
Page 18
It will be appreciated that when changing from one current or volt
age range to the next higher, the pointer indication will fall,
although input is kept constant. For a current or voltage range ratio
of 2J:1 this corresponds to a reduction of 8 in the indication on the
db. scale. It follows, therefore, that 8 should be added to the
reading every time an increase of 2 \ times takes place on the
range. In the same way, 12 should be added for an increase of
4 times on the range, or 8+12=20 db. for an increase of
2 \
x
4 = 10 times in the range ratio.
The following might serve as an example: Suppose that the
meter is connected on the 25 V. a.c. range across the primary of
an output transformer and that a reading of +9 db. is indicated
(corresponding to 12*5 V. on this range). If now the output
increases to say 40 volts, necessitating a change to the 100 V.
a.c. range, the pointer will indicate + 7 on the db. scale.
The 4:1 increase in the voltage range calls for an addition of
12 to the db. indication, so that its true value represents + 19 db.
The increase over the original reading is 19—9 = 10 db.
19
Page 19
ACCESSORIES
D.C. V OL TAG E M UL TIP LIE RS
10 ΑΙΛ D.C. Multiplier
A 10kV. d.c. Multiplier has been developed mainly to enable
tests to be carried out in television circuits. The multiplier should
be connected in series with the meter on its 2,500 V. d.c. range,
in which state maximum consumption on measurement cannot
exceed 50 microamps, and may be considerably less. It is recom
mended that the meter is kept as near earth potential as possible,
and the multiplier used at the high potential end, e.g., when
measuring an e.h.t. voltage where the negative line is earthy, the
multiplier should be connected between the point of positive
potential and the positive terminal of the meter, the negative
lead being connected to the terminal marked 2,500 V. d.c.—. We
do not recommend, in such cases, connecting the multiplier to
the 2,500 V. d.c.—terminal and pressing the moving coil reverse
button, notwithstanding the fact that the meter is at the earthy
end of the circuit.
25 ΑΙΛ D.C. Multiplier
A 25kV. d.c. Multiplier is available for use in series with the meter
set to its 10 V. d.c. range, readings being made direct in kV. on
the 0-25 scale. It is most important to ensure that the meter is
kept in the earthy end of the circuit and the multiplier connected
to either the positive or negative terminal whichever is at high
potential. This method of connection to get forward pointer
indication with the meter earthy is recommended as we do not
think it desirable to use the moving coil reverse button when
measuring high voltage.
In general we recommend that neither the meter, multiplier nor
leads are handled whilst high voltage tests are in progress, and
a special lead is provided with the multiplier for connection to the
high potential point.
NOTE. The 2,500 V. d.c. range is not employed when using this
multiplier.
CUR RENT T RA N SF OR M ER S
T ransformers as used on the Model 7 Mk. II Avo Meter are equally
applicable for use with the Model 8, Mk. Ill when set to 100 m A. a.c.
It is necessary to connect the meter up to the secondary of the
transformer before current is passed through the primary, and
care should be taken that the cut-out is in position. If this course
is not followed, quite a considerable voltage will appear at the
secondary terminals, if current passes through the primary.
Transformers for 50 amp., 100 amp., 200 amp, 400 amp., and 250/50
amp., are available.
20
Page 20
D.C. S H U N T S
The Shunt should be connected by means of its two main
terminals in series with the circuit to be measured. The meter,
set to its 50μ Α (125mV) d.c. position should then be connected
to the two small studs on the shun t end blocks.
The Avom eter when so set, consumes only 50μ Α at full-scale
dsflection, a value which is negligible in comparison with the
full-scale current of the shunt. The millivolt drop across the
shunt is directly proportional to any current which may flow
thro ugh it and since the deflection on the meter is directly
proportional to the millivolt drop across its terminals, the instru
ment indicates correctly over its entire scale length.
Shunts available:— 50A, 100A, 200A, 400A.
R ES IS TA N CE RAN GE E X TE NS ION U NI T
This accessory enables the meter to be used for both high and
low resistance measurements. It is complete with batteries
(except in some instances) and switching to facilitate tests. The
device should be connected to the lower terminals on the meter.
For high resistance the meter is set to the ‘ Ω x 100’ position, the
Unit switch at the ‘SET’ position and the unknown resistance
should then be connected to the ‘H igh’ terminals. Adjustm ent
to full scale deflection should be performed by means of the
‘ZERO Ω x 100’ knob. The Unit switch should then be rotated
to ‘ TE ST ’, and the reading on the ohms scale noted. Its value is
that shown multiplied by 1,000 corresponding to a range of 200
megohms.
On the low range the Unit switch should be placed at ‘SET’, the
unknown resistor connected to the ‘L ow’ terminals and
adjustment to full scale deflection carried out by means of the
‘ZERO Ω ’ knob. The Unit switch should then be moved to posi
tion marked ‘T ES T’ and the pointer deflection on a uniformly
divided scale noted. Full scale deflection corresponds to 2\ ohms.
In order to avoid discharging the batteries, immediately tests have
been completed the test leads and resistor should be removed
from the unit, its switch set to the ‘low set’ position, and the unit
disconnected from the meter.
RE PL AC EM EN T BA TT ER IE S
1 ·5 V. cell—1f in. dia. x 2f in. such as Ever Ready (or overseas,
Berec) U.2. Four 30 V. batteries— 1 ^ >n · x 4 ·η · x ·η ·. such as
Ever Ready B.123.
21
Page 21
CONCLUSION
Due to the high operational standards maintained th roug hout
our organisation, and the close limits within which we work,
breakdowns are comparatively rare, and can often be traced to
transit damage or careless handling, for which the Company
cannot be held responsible. Should you at any time have to return
your instrument to the Company for repair, pack it carefully and
enclose a note informing our engineers of the faults which you
have found.
22
Page 22
TA BLE OF RANGES
A.C. Switch (SA)
Position
1.
2.
3.
4.
Range
Ins.
Ω x 100
Ω
Ω 100
5. LR
6.
7.
8.
1000V
250V
100 V
9. 25 V
10.
11.
12.
13.
14.
15.
16.
10V
2.5 V
100mA
1 A
2.5A
10A
DC
D.C. Switch (SB)
Position
1.
Range
AC
2. 10A
3.
4.
1 A
100mA
5. 10mA
6. 1mA
7.
8.
9.
2 5 0μΑ
50μΑ
Resistance
10. 2.5V
11.
12.
13.
14.
15.
10V
25V
100 V
250V
500V
16. 1000 V
Page 23
LEAF S W IT C H C O N T A C T S
A.C. Switch Cam— Outer Segment
Range
LR, Ω -+100 and Ω
All a.c. ranges
Contacts
f a and b open
^ a and c open
Ld and e open
f a and b closed
^ a and c closed
[ d and e closed
f a and b closed
All d.c. ranges
and Ω x 100
A.C. Switch C am—Inner Segment
Range Contacts
Ins and Ω x 100
a and c open
^d and e open
/ f and g open
\j and h open
Ω -j- 100
All d.c. ranges
and Ω
D.C. Switch Cam
Range
Resistance
A.C. range
d.c. voltage
and current
/ f and g closed
\h and j closed
/ f and g closed
\ h and j open
Contacts
f o and p closed
k and I closed
and n closed
f o and p open
{ k and I open
l^m and n open
f k and I open
^ m and n open
^o and p closed
Page 24
TA BLE OF C OM P ON E N TS
Circuit Ref.
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13
R14
R15
R16
R17
R18
R19
R20
R21
R22
R23
R24
R25
R26
R27
R28
R29
R30
R31
BY1
BY2
FS1
M1
MR1
MR2
SA
SB
SC
T1
TH1
C1
Value
4k Ω
1.5M Ω
750k Ω
250k Ω
99k Ω
24.2k Ω
9.2k Ω
406 Ω
406 Ω
222 Ω
0.05 Ω
0.45 Ω
4k Ω
600 Ω
SW A MP
4.5 Ω
2.8 Ω
42.2 Ω
450 Ω
300 Ω
1.2k Ω
8k Ω
3.3k Ω
47.5k Ω
150k Ω
300k Ω
1.5M Ω
3M Ω
5M Ω
10M Ω
30 Μ Ω
15V B121
1.5V U2
1 A Fuse
3 7 . 5 μ Α (3333 Ω) with swamp
OA95
OA95
a.c. Range Switch
d.c. Range Switch
Rev. M.C.
Internal Transformer
Thermistor VA1039
0.015-xF
Page 25
D.C. SWITCH S B
R 31
D.C . + 4 A C. C OM M ON D.Cr-4 A .C. SW ITCH RAN GE S
CIR C UI T D IA GR A M OF T HE MODEL 8 MK. Ill A VO M ET ER
Γ 4
I
Page 26
Loading...