AVOMETER Model 8 Mark 5

Introduction

This document provides comprehensive operating instructions, specifications, and servicing information for the AVO Model 8 Mark 5 multimeter. It details the instrument's features, measurement capabilities, and operational procedures.

Specifications

Table of Ranges

Accuracy

D.C. Voltage & Current Ranges: ± 1% of f.s.d.

A.C. Voltage & Current Ranges (50Hz): ± 2% of f.s.d.

Sensitivity

D.C. Voltage Ranges: 20,000Ω/V

A.C. Voltage Ranges: 2,000Ω/V (above 10V)

Frequency Response

Variation from reading at 50Hz, on A.C. voltage ranges up to 300V and A.C. current ranges is not greater than ±3%, between 15Hz and 15kHz.

Temperature Range

Operation: 5°C to +35°C

Storage: -40°C to +50°C

Temperature Effect

Variation due to temperature change, 0.15% per degree C.

Flash Test

7kV proof

Overload Protection

High speed electro-mechanical cut-out on the two lower resistance ranges.

Response Time

Typically 1 second to full scale.

Operation

General

The meter is intended for use horizontally. If the pointer is not on zero, it may be set by using the screw head on the panel.

The leads fitted with Long Reach Safety Clips Mark 2 or clips, as required, should be connected to the lower pair of meter terminals except when measuring voltages over 1000V. Long Reach Safety Clips are not suitable for high current.

When measuring current or voltage, ensure that the instrument is set to either AC or DC as appropriate and a suitable range before connecting up to the circuit under test. When in doubt, always switch to the highest range and work downwards; there is no necessity to disconnect the leads as the switch position is changed.

Do not switch off by rotating either of the knobs to a blank position.

The instrument is flash tested at 7000V AC but if used with accessories on circuits in excess of 3000V, it should be kept at the low potential end of the circuit (near earth potential) or other suitable safeguards must be applied.

Switch Transit Position

During transit, the left-hand switch should be set to OFF. The right-hand switch should be set to D.C.

Current Measurement

To measure current, the instrument should be set to a suitable AC or DC range and then connected into the circuit under test. The voltage drop at the meter terminals for the 10A DC range is approximately 100mV. For AC ranges, it is less. Standard meter leads have a resistance of approximately 0.02 ohm per pair. Ensure that the circuit is broken to make current measurement.

Voltage Measurement

When measuring voltage, it is necessary to select the appropriate range of 'AC' or 'DC' and connect the instrument to the source of voltage to be measured. If the voltage is unknown, set the instrument to its highest range and if below 1000V, decrease the range until the most suitable range has been determined. If the voltage should exceed 1000V, the instrument should be set to measure 1000V as described above, and the high potential lead should be transferred to the appropriate terminal.

Great care must be exercised when making measurements to a live circuit, and the procedure should be followed precisely if possible.

For high AC and DC voltages (say above 800V), if the common negative terminal is not earthy or connected to earth, errors will be introduced if the instrument is touched during a reading.

Resistance Measurement

There are three self-contained ranges covering from 1 ohm to 20 megohms, with provision for upward extension of these limits. On resistance ranges, the meter must have, in addition to the normal instrument zero, a resistance zero corresponding to the full scale deflection of the meter. Before carrying out tests for resistance, a check should be carried out to ensure that the meter actually indicates zero ohms irrespective of the condition of the battery (within the limits of adjustment described later).

The accuracy 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 tests should never be carried out on components which are already carrying current.

On three ranges which utilise the internal source of voltage, a positive potential appears at the negative terminal of the instrument when set for resistance tests. 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 capacitor, the negative lead from the meter should be connected to the positive terminal of the capacitor, and the ohms x 10k range employed.

Before making resistance tests, the pointer should be adjusted to zero in the following sequence:

1. Set the left hand switch at Ω.
2. Join the leads together.
3. On the Ω x 1 range, adjust zero by means of the knob marked Ω x 1.
4. On the Ω x 100 range, adjust zero by means of the knob marked Ω x 100.
5. On the Ω x 10k range, adjust zero by means of the knob marked Ω x 10k.

To test a resistance, set the right-hand switch as required, the leads being connected across the component. Resistance is read directly on the Ω x 1 range and should be multiplied by 100 and 10,000 for the Ω x 100 and Ω x 10k ranges respectively.

If on joining the leads together it is impossible to obtain a zero ohms setting, or if the pointer position does not remain constant but falls steadily, the internal battery cell concerned should be replaced. It is important that a discharged unit should not be left in the instrument as it might cause damage to the meter. If it is impossible to obtain readings on the Ω x 1 and Ω x 10k ranges, the fuse located in the battery box should be checked.

NOTE: A 15V battery may age in such a way that though it indicates a potential of 15V, its internal resistance has increased so much that some loss of voltage may occur on the high resistance range (Ω x 10k). If errors are suspected on the high resistance range, check the battery and its short circuit current on the DC current range. If the reading is below than 2mA, the battery should be discarded.

dB Measurement

The scale can be used to determine the dB relative to r.m.s. voltage values across a 600Ω load. A dB value is defined as the number of times the power is below a reference level of 1mW in 600Ω. 0dB therefore, would indicate a power of 1mW, +10dB, 10mW, +20dB, 100mW. Because dB is relative to a 600 ohm load, power levels can be converted to voltage levels. Decibels can be measured in AC voltages across a 600 ohm resistive load. 0.775V r.m.s. indicates 0dB and 7.75V r.m.s. indicates +20dB.

Whilst these measurements must be made with waveforms conforming to avoid waveform error, any frequency used within the range of the Model 8 is acceptable, and ear response curves have their reference at 1kHz.

Power levels can be read along the top of the graph. If the r.m.s. voltage is measured across a resistive load other than 600 ohms, the correction factor given below must be added algebraically to the dB values read from the graph. The following formula should be used for determining the correction factor:

Correction Factor = 10 log₁₀ (600 / R)

where R is the load in ohms. If R is greater than 600 ohms, the Correction Factor is negative.

(Refer to the graph on page 9 for dB measurements.)

Polarity Reverse Control

If DC voltage is required both positive and negative to a reference point, or the direction of current flow may be reversed, in order to simplify the matter of lead alteration, a polarity reverse press button (REV. M.C.) is provided. It should be noted that the polarity marked on the terminals is for normal use and does not apply when the red section of the REV. M.C. button can be seen.

EHT Measurement

CAUTION: It is recommended that neither the meter, multiplier, nor leads are handled whilst high voltage tests are in progress.

A combined 10kV and 30kV DC multiplier is available for use with the instrument. The multiplier should be connected to the AVOMETER set to the 10V DC range. It is recommended 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 EHT voltage when the negative line is earthy, the multiplier should be connected between the point of positive potential and the positive terminal of the meter. In addition, the low potential end of the meter must be connected to the low potential of the supply being measured using the low potential lead provided. A cap is provided which should always be in position over the high voltage terminal not in use.

A resistor connected permanently across the multiplier terminals prevents the full voltage being present should the meter be disconnected.

Using the 30kV Probe Multiplier: A 30kV DC multiplier is available for use in series with the meter set to its 1000V DC range, readings being made direct in kV on the 0-100 scale and multiplied by 300. 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.

A special lead is provided with the multiplier for connection to the high potential point.

Heavy AC Current Measurement

AC Currents between 10A and 600A may be measured by use of the AVO Multi-range Current Transformer (Part No. 45869). Full operating instructions for its use are packed with the optional accessory.

Heavy DC Current Measurement

DC currents between 10A and 600A may be measured by use of the appropriate AVO DC Shunt accessory. When used with the AVOMETER 8 Mk 5, the shunts produce a voltage drop of 100mV when passing current at their maximum rating. The shunt should be connected by means of terminals in series with the circuit to the meter set to its 50μA (100mV) DC positive terminal, connected to the two small studs on the shunt. The AVOMETER when so set, consumes only 50μA at full scale deflection, a value which is negligible compared with the full scale current of the shunt. The voltage across the shunt is directly proportional to the current which may flow through it and since the resistance of the meter is directly proportional to the mV drop across the terminals, the instrument indicates current over the entire scale length.

Non-Sine Waveforms

Inasmuch as rectifier moving coil instruments measure on 'AC' proportional to the mean and not the RMS value, the waveform with which they are presented affects their accuracy, not only upon their shape but also upon the maintenance of a sine wave. Since the form factor (RMS value divided by the mean value) of a sine wave is 1.11, this has been taken into account when calibrating the meter, which does, therefore, give values on the assumption that the normal waveform encountered. Generally speaking, considerable distortion can occur without appreciably affecting the form factor and resulting accuracy of measurement. However, one should recognise the possibility of some severely distorted waveforms, squarish waves, high readings and peaky ones, low readings and troughy ones.

Features

Design and Construction

The instrument comprises a moulded panel on which are mounted the switching apparatus, the movement, and other components. A carrying strap is fixed to the rear case. The front panel is fitted with a dust-proof joint. The switches are of robust design, the contacts being arranged to 'break' on adjacent ranges; a feature which ensures safety in use.

The left-hand knob is set for operation on DC, controlling the moving coil with a universal shunt and series multiplier. Diodes and a transformer are also introduced.

The right-hand knob provides all the DC current and voltage (up to 1000V) and the right-hand knob the AC ranges (up to 1000V) and also the resistance ranges. These are electrically interlocked so that readings are not taken after AC or DC measurement and range selection.

Resistance tests require the left-hand switch set to Ω and the right-hand one to the desired range selected. The 3000V AC and DC ranges are available at the two 3000V terminals.

The Movement

The meter movement in the Model 8 Mark 5 is a robust AVO centre pole movement type ACP1, fitted with spring-mounted jewelled bearings and impregnated coil. The meter has a full scale deflection of 37.5μA. A knife-edge pointer enables very fine readings to be taken, whilst the whole movement is balanced and damped so that the pointer quickly comes to rest.

Scaling

The scale plate has three main sets of markings. One is for resistance measurement and is marked 0 to 2000 ohms. The second is for current and voltage (both AC and DC) and is marked 0 to 10, whilst the third scale, calibrated 0 to 3, has 60 divisions and is used for current and voltage measurements.

Servicing Your Avometer

Replacement of Batteries and Fuse

By turning the ¼ turn fastener on the back of the instrument until the slot is vertical, the 15V battery, 1.5V cell, 1A fuse, and spare fuse are easily accessible. The batteries should be examined from time to time to ensure that the electrolyte is not leaking.

If the meter is going to stand unused for several months, the batteries should be removed.

When replacing the 1.5V cell and the 15V battery, they must be inserted in the correct polarity.

Suitable replacement batteries are listed on Page 3.

Plug-in Type Leads

When ordering replacements, the full description and Part numbers should be quoted.

AVO Repairs Department

Due to the high standards maintained by the organisation, breakdowns are comparatively rare and are often traced to transit damage or careless handling, for which the company cannot be held responsible. If your instrument should require servicing or a check, AVO has the knowledge and facilities to repair your instrument to the highest standards.

For customers who feel they have the necessary skill and equipment to carry out repairs themselves, spares are available from AVO. To obtain the correct spares item, identify the part required and state its position on the circuit diagram. In the applicable state, the circuit reference number should be quoted. If at any time you have to return your instrument to AVO for repair, pack it carefully and enclose details for our engineers of the faults which you have observed. For customers outside the U.K., please contact the AVO representative for their territory.

Table of Components

Circuit Diagram

A detailed circuit diagram is provided within this document, illustrating the internal connections and components of the AVOMETER Model 8 Mark 5. It includes reference coding for resistors, capacitors, and electrolytic capacitors, along with specific values and types. The diagram shows the arrangement of the movement, printed resistors, shunts, batteries, and other electronic parts.

(Refer to the visual diagram in the original document for detailed schematic representation.)

Manufacturer Information

Avo Limited

Dover, Kent CT17 9EN

England

Tel: Dover (STD 0304) 202620

Telex: 962

Cables: Measurement, Dover

A member of the Thorn Measurement and Automation Group.

Printed in England.