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Old 30th Dec 2011, 9:11 pm   #1
2E1CIH Mike
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Default Fluke AC/DC Differential Voltmeter

Hello,

I have recently bought a "job lot" of test equipment and components from a chap who wanted to have a clear out. The main attraction to me, within this assortment, was a Variac. This was the main reason I travelled to him.

I was very glad I made the effort to go as a Gould AF signal generator, Heathkit RF signal generator and a Fluke Differential Voltmeter were included in the sale.

However, my knowledge is limited, and I'm not sure if the "Fluke" is likely to be of any use to me.

I have looked on the internet, but can't find much information, other than it seems to be an extremely accurate, laboratory type of instrument.

"Is my ownership wasteful," I'm wondering? For the purpose of just doing voltage checks on valve radios, is it likely to be any better than a multi-meter? Would I need such accuracy? Can it be adapted for any other use?

Many thanks.....Mike
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Old 30th Dec 2011, 9:36 pm   #2
Craig Sawyers
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Default Re: Fluke AC/DC Differential Voltmeter

Hi Mike

The Fluke Diff voltmeters were what was available before DVM's. They were capable of all sorts of different measurements like resistance ratio, but their main application was as a voltmeter.

First you switched the unit to TVM, which was a straightforward voltmeter. That enabled you to find out the rough level you were trying to measure accurately. You then set up the range and switched digits to about the correct voltage as previously read using the TVM function. The unit then compares the test voltage with an internal voltage reference, divided down by a Kelvin Varley voltage divider controlled by the digit switches.

The meter now reads the error between the instrument set up and your test voltage - usually up to 1kV on the highest range. You then adjust the switches to give the best null, and the final switch digits give a direct reading of the test voltage.

The key attribute, even now, is that when the meter is nulled, absolutely no current flows from the test voltage - it is true infinite input resistance (limited only by leakage currents, of order picoamps).

Fluke made several sorts - some DC only, and some AC/DC. And the highest resolution ones are very accurate - five or six digits. The most accurate was the 895A, which had an absolute accuracy (after traceable cal) of 0.0025% plus 0.0001% of range plus 5uV from 0 to 1100V DC input.

Some folks have used these mainly as a source for a darned good Kelvin Varley for another application (such as for a volage reference). However, with the 895A, switching to RATIO puts the KV across rear panel binding posts, so it is externally accessible.

So keep it, love it, use it. It does things that no other instrument does.

Craig
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Old 31st Dec 2011, 4:09 am   #3
dinkydi
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Default Re: Fluke AC/DC Differential Voltmeter

Hi, That's a very good overview Craig. I must admit to being partial to old Flukes myself, particularly the high-resolution types.

Fluke made many models over the years. What model is yours, Mike?

Many of the Flukes are battery powered and could still have the original batteries installed! I always remove the batteries immediately, however, some Flukes (usually multimeters) unfortunately require the battery to be present even though you may only wish to use mains power. Always check the battery compartment (and circuitry!) for corrosion and repair if necessary.

If you poke around the circuitry, look but don't touch. Scrupulous cleanliness must be maintained for high accuracy - it is a bit different from an old radio!

The manual is most likely readily available on the net, if not, I may be able to source it for you.

Should you keep it? All Flukes are quality instruments and deserve preservation, if not everyday use. After checking out/restoration you could use it as a voltage standard for your other equipment or you could pass it on to a worthy recipient who respects our electronic heritage.

Peter
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Old 31st Dec 2011, 3:36 pm   #4
Craig Sawyers
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Default Re: Fluke AC/DC Differential Voltmeter

Quote:
Originally Posted by dinkydi View Post
Many of the Flukes are battery powered and could still have the original batteries installed! I always remove the batteries immediately, however, some Flukes (usually multimeters) unfortunately require the battery to be present even though you may only wish to use mains power. Always check the battery compartment (and circuitry!) for corrosion and repair if necessary.
That is a very good point. The mains power supply will always put out more voltage than the batteries. And usually there is a current-limiting resistor in series. A quick calculation suggests that without the battery pack present, you could get 50% or more voltage increase, which runs the risk of damaging the circuitry.

That can lead to a bit of a quandry. I have two Fluke 540B thermal transfer standards, which use 20 tabbed odd-size NiCd's (2/3C or some such). Replacing those cost more than the cost of the instrument, in both cases.

Craig

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Old 31st Dec 2011, 8:52 pm   #5
2E1CIH Mike
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Default Re: Fluke AC/DC Differential Voltmeter

Hi Craig, Peter,

Many thanks to you both for giving me some clarity on this instrument.
I'm not sure if I will have a use for it, but appreciate that it is an interesting piece of kit.

Craig, thanks for the detailed description.

Peter, thanks for the tip about the batteries....I will check this (and be careful re cleanliness). Also, thanks for the offer to find the manual, but I'll have a go first.

Mine is model 883AB.

2012 approaches (Peter, yours is with you.....I saw the fireworks in Sydney in the TV) so best wishes to you both.

Regards.....Mike
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