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AVO multimeters - which one, and why?

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Jo:
I used to have a very nice Mk 8 but my ex-husband felt the need to take it with him  :wack: 

As Andrew says there are times when an analogue meter is better than a digital read out to enable you to see a moving value - like with the rev counter and speedo in a car.  I have been able to use one of those cheap as chips (<£4) chinese digital meters for everything for the last 20 years, even though I have a much more expensive Fluke digital carefully squirrelled away somewhere  :scratch:

Jo

Pete W.:
According to my Senior Engineer when I was a mere Junior, regarding comparison of the AVO 7 and the AVO 8:

The AVO 8 is more sensitive because its movement's full scale deflection (FSD) is 50 μA (microamps).  It consequently needs to 'rob' only a little current from the circuit under test.  This will minimise the difference between the indicated and true voltages when the circuit under test presents a high source resistance.   This low value of FSD requires high Ohmic values for the AVO 8 multiplier resistors and other internal circuitry.  These high Ohmic values require, for reasons of economical manufacture, the use of metal film or hi-stab carbon resistors which are less intrinsically stable than wirewound resistors.

The AVO 7, on the other hand, is less sensitive because its movement has an FSD of (if I remember correctly) 1 mA (milliamp).  This means that the AVO 7 needs to 'rob' more current from the circuit under test leading to more disturbance of the circuit and a greater difference between true and indicated values, the more so if the circuit under test presents a high source resistance.  However, the higher FSD of the AVO 7 movement leads to lower Ohmic values for its multiplier resistors and other internal circuitry.  These lower Ohmic values are consistent with the economic use of wirewound resistors.  These are intrinsically more stable than metal film or hi-stab carbon resistors so the AVO 7 is more precise than the AVO 8.

Joules:
Hmmm, lets throw in some controversy.  The AVO time is long gone, modern DMM with a bar graph gives you a fast acting visual clue, with the added benefit of much higher impedance.  Your AVO will only indicate an average, if you care about the signal change in detail you should be using an oscilloscope.   These days we have a vast range of affordable DMM's and scopes to hand, sorry guys the AVO age is over, says he who is building a Commodore PET, to relive his YOOF....

awemawson:
I have a modern 'clamp' equipped Fluke meter that is extremely versatile in that it measures DC current as well as AC giving a true RMS value - it was expensive but is very useful.

I got it when I needed to set up the servo motor on my 4th axis that I added to my Beaver Partmaster CNC milling machine - it did what I asked it to do.

I also used it to good effect setting the current / voltage relationship curve for the field coil driver on the HUGE DC motor on my Beaver TC20S CNC lathe.

So why didn't I use my Avo 8? Well I was convinced by 'those in the know' that the Avo would not give a true RMS reading as the currents involved were complex wave forms - so I laid out the dosh for the Fluke.

Being the sort of chap that I am, in both cases I (once things were set up) put the Avo in series so it was measuring EXACTLY the same variable as the posh digital Fluke and guess what - it indicated PRECISELY the same value - in fact iirc I posted pictures of same here when I was setting up the Mawdsley DC motor on the Beaver lathe.

. . . did I need to buy the Fluke . . go figure !

AdeV:
Thanks for all the replies, appreciated :)

Yes, I do miss having an analogue MM, although most of what I use it for is digital circuitry, testing resistors (because those colour rings are tiny, and my eyes aren't that good any more!), checking that point X has power to it... and for those uses, a digital is fine.

However... what I really wanted to know was - why an AVO rather than Brand X (Fluke?) analogue MM; and if there's a compelling reason to get an AVO, which one is best - a Mk3, 5, 7 or 8? All of which are available on eBay for varying amounts of wedge...

Ta!

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