This document is the operating instructions manual for the AVO AVO Meter Model 8 Mark 5, a versatile electrical measuring instrument.

Function Description

The AVO Meter Model 8 Mark 5 is a multi-range electrical measuring instrument designed for horizontally oriented use. It is capable of measuring direct current (DC) voltage, alternating current (AC) voltage, DC current, AC current, and resistance. Additionally, it offers features for insulation resistance measurement, decibel measurement, and E.H.T. (Extra High Tension) measurement with appropriate accessories. The instrument incorporates an automatic cut-out for overload protection, which provides a high degree of protection and confidence to the user.

Important Technical Specifications

The AVO Meter Model 8 Mark 5 provides a comprehensive range of measurements:

DC Voltage Ranges: 3V, 10V, 30V, 100V, 300V, 600V, 1000V, 3000V.
DC Current Ranges: 50µA, 300µA, 1mA, 10mA, 100mA, 1A, 10A.
AC Voltage Ranges: 3V, 10V, 30V, 100V, 300V, 600V, 1000V, 3000V.
AC Current Ranges: 10mA, 100mA, 1A, 10A.
Resistance Ranges:
- 0 to 200Ω (2Ω centre scale) - self-contained
- 0 to 200kΩ (2kΩ centre scale) - self-contained
- 0 to 20MΩ (200kΩ centre scale) - self-contained
- 0 to 200MΩ (2MΩ centre scale) - with external voltage
Accuracy:
- DC Voltage & Current Ranges: ± 1% of full scale deflection (f.s.d.)
- AC 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, not greater than 0.15% per degree C.
Flash Test: 7kV proof.
Overload Protection: High speed electro-mechanical cut-out with a fuse on the two lower resistance ranges.
Response Time: Typically 1 second to full scale.

The instrument's movement is a robust AVO centre pole movement type ACP1, featuring spring mounted jewelled bearings and an impregnated coil. It has a full scale deflection of 37.5µA. A knife edge pointer ensures fine readings, and the movement is balanced and damped for quick settling.

The scaling plate has three main sets of markings: one for resistance measurement (0 to 2000 ohms), the second for current and voltage (both AC and DC, 0 to 10), and the third for current and voltage measurements (0 to 3 with 60 divisions).

Usage Features

General Operation: The meter is intended for horizontal use. If the pointer is not at zero, it can be set using the screw head on the panel. Leads with Long Reach Safety Clips Mark 2 or clips are used for measurements, connecting 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, the instrument must be set to an appropriate AC or DC range. In case of doubt, always switch to the highest range and work downwards. There is no need to disconnect leads when changing the switch position. The instrument should not be switched off by rotating either of the knobs to a blank position.
Current Measurement: To measure current, the instrument should be set to a suitable AC or DC range and connected in series with the circuit under test. The voltage drop at the meter terminals is 400mV on the 10A DC range, dropping to 100mV on the 50 microamp range. For AC ranges, it is less than 450mV. Standard meter leads have a resistance of 0.02 ohm per pair.
Voltage Measurement: When measuring voltage, the appropriate AC or DC range should be selected, and leads connected across the source. If the voltage is unknown, set the instrument to its highest range and connect up. If below 1000V, decrease the range step by step until the most suitable range is selected. For voltages above 1000V, the instrument should be set to measure 1000V, and the positive lead transferred to the appropriate 3000V terminal.
Resistance Measurement: The meter offers three self-contained ranges covering 1 ohm to 20 megohms. To test resistance, the right-hand switch should be set to OFF during transit, and to D.C. for operation. The left-hand switch should be set to OFF during transit, and to Ω for operation. Resistance is read directly on the Ω x 1 range, but readings should be multiplied by 100 and 10,000 on the Ω x 100 and Ω x 10k ranges respectively. The accuracy is 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 not be carried out on components already carrying current.
Insulation Resistance Measurement: High resistance measurements are made using an external DC voltage of 140/160V. The left-hand switch should be set to Ω with the right-hand switch at INS, and the meter leads connected to the battery. The pointer should be brought to zero on the ohms scale by means of the adjuster marked Ω x 10k.
Decibel Measurement: A decibel conversion graph is provided on page 7 to determine dB values corresponding to r.m.s. voltage values across a 600 ohm resistive load.
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, a polarity reverse press button (REV. M.C.) is provided.
Overload Protection: The instrument features an automatic cut-out that provides a high degree of overload protection. If an overload occurs, the cut-out knob springs from its normal position, revealing the main circuit and the red portion of the cut-out knob. This knob must be depressed to reset the instrument.
E.H.T. Measurement: For E.H.T. (Extra High Tension) measurement, the combined 10kV/30kV Multiplier or 30kV Probe Multiplier can be used. The Multiplier should be connected to the AVOMETER set to the 10V DC range. The meter is kept at near earth potential, and the Multiplier is connected to the high potential end.

Accessories

The manual lists various accessories available for the AVO Meter Model 8 Mark 5:

Test Leads: 1 metre (approx. 42in.) with 4mm plug end (Pt. No. 6220-003/4, pair), 1.8 metres (approx. 72in.) with hook end (Pt. No. 209130/Q/R, pair), 3 metres (approx. 120in.) with hook end (Pt. No. 20913L/M, pair).
Test Prods: Long reach safety clips (red and black) (Pt. No. 6220-007, pair), (Pt. No. 6220-008, black), (Pt. No. 6220-009, red), Standard AVO prods (red and black) (Pt. No. 6120-012/3, pair).
Test Clips: Bulldog clip (Pt. No. 6120-003, each), Insulated clips (Pt. No. 14319 C/D, pair).
Carrying Cases: Standard leather case (Pt. No. 6320-002), Ever-ready leather case (Pt. No. 6320-052).
Current Shunts: 30 Amp (Pt. No. 6220-015), 100 Amp (Pt. No. 6220-016), 300 Amp (AVO 8 mk5 only) (Pt. No. 6220-017), 600 Amp (Pt. No. 6220-018).
Current Transformer: AVO Multirange transformer for a.c. currents up to 600A (Pt. No. 45869).
Voltage Multipliers: 10/30 kV D.C. box type (Pt. No. 6310-094), 30 kV d.c. probe type (Pt. No. 6220-039).

Maintenance Features

Replacement Batteries: The instrument uses a 1.5V cell and a 15V battery. These are easily accessible by turning the ¼ turn fastener on the back of the instrument until the slot is vertical. The batteries should be examined periodically to ensure the electrolyte is not leaking. If the meter is to be stored unused for several months, the batteries should be removed. Suitable replacement batteries include:
- Ever Ready SP2 (or equivalents, Burgess 210 or 230, Ever Ready HP2, IEC R20, Leclanché R20S, Ray-O-Vac 13 or 3D, Varta 232) (Pt. No. 25511-013).
- Ever Ready B121 (or equivalents, Burgess U10, IEC 10F20, Leclanché 215G, Ray-O-Vac 208, Varta 71) (Pt. No. 25511-181).
- Ever Ready B154 (or equivalents, Burgess Y10, IEC 10F15, Leclanché GB15, Ray-O-Vac 220, Varta 74).
Replacement Fuse: The instrument uses a 1¼ inch Cartridge Fuse – 1A (Pt. No. 6120-299, Pack of 5). This fuse is located on the two lower resistance ranges for overload protection.
Plug-in Type Leads: When ordering replacements, the full description and Part numbers should be quoted.
AVO Repairs Department: AVO Limited maintains high standards for its instruments. In case of damage or careless handling, the company cannot be held responsible. However, if the instrument requires servicing or calibration, customers should contact AVO to obtain the correct spares item, identify the component required, and state its position on the instrument. When applicable, the circuit reference number should be provided. For repairs, pack the instrument carefully and enclose a note detailing the faults. Customers outside the U.K. should contact their local AVO representative.

The manual also includes a detailed "Table of Components" and a "Circuit Diagram" for comprehensive servicing and repair.

AVO AVOMETER 8 - Page 16

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