Precision AVOmeter
s/n 441-567
Nato part 6625-99-104-5783

Precision AVOmeter s/n 441-567 working

This instrument was given to me by Ian Rea in 2013. He rescued from his place of work some years ago when the calibration department was closed. The other stuff went in the bin but he didn't have the heart to chuck this as it's such a beautiful item! It is housed in a highly polished wooden box which in turn is protected with a translucent plastic cover. Five leads are contained in the hinged compartment in the lid of the meter.

It was designed to provide an accurate and stable local reference or transfer standard for industrial and laboratory engineers. It meets the requirements of Section 6 of BSS 89/1954 for precision grade instruments ie:
D.C. Voltage ranges 0.3% of f.s.d.
D.C. Current ranges 0.5% of f.s.d
The accuracy on a.c. voltage and current ranges is 0.75% of FSD (form factor of 1.11) This accuracy is maintained for audio frequency tests up to 1 kHz on ranges up to 3OOV.
Temperature correction:
true value= 1+.0003(t-20) where t is ambient temperature in degrees Celsius

Range selection is accomplished by means of two controls mechanically interlocked with the third central control to select voltage or current as required. Two 7 inch scales calibrated 0 to 150 and 0 to 30 cover both voltage and current measurements. A test certificate is provided for each instrument.

DC  1.5V to 1500V and 1.5mA to 15A
AC   3V to 1500V and 3mA to 15A
Sensitivity 100 Ohms/Volt on DC and AC except for 3V AC range which is 50mA fullscale

Dimensions and weight
155x315x365 mm (not incl. feet) and weight 9.2kg.

The detachable lid houses the connecting leads and the test certificate dated 17/12/1973

The internal construction has echoes of the more workaday AVO meters.

The wire wound resistor boards are supported in a mahogany wood frame and the switches are ingeniously interlocked, the current and voltage selectors cannot be turned until the centre switch is correctly positioned.

The serial number indicates that this meter was made in May 1967.

 Adrian Almond  who has a  Mk II version (709-670) which is now in full working order after fixing a couple of mechanical problems with the interlock mechanism and the associated range switch has sent me some pictures of his instrument. It was made in 1970 and is very different inside although externally there appears to be lmuch the same. Clearly a much less labour intensive manufacturing process. Andy Gilham has advised that the development of the resistor board (with the blue Welwyn type resistors) for the mark 2, was December 1966 so it was possibly not in production for a further 6 months to a year. This means that my instrument may have been one of the last with the wire wound resistors.

Rare Instrument?
Judging by the dates and serial numbers that I have been able to find perhaps up to 100 of these meters were manufactured each year:

441-567 (this one)
709-670 (Mk2 below)



The blue resistors have a 0.05% tolerance


Another view of the interlocking

For further information have a look at my pages on other Avometers see here

mation have a look at my pages on other Avometers see here