Electronics > Metrology

Null meters still in production? How to replace a Keithley 155 null meter?

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We use the Fluke 752A Reference Divider which needs balancing every now and again. So far, the only thing I know capable of doing the balancing is a null meter. We use Keithley 155 which is still working...for now. Fluke says their 8588A DMM does not have capability to be a null meter. When I try to balance the 752A using that DMM, it's no where as good as a Keithley 155.

So now we're using a product that's no longer supported and repaired to support something critical. As far as I know, no one is making null meters anymore. So what's replacing the null meter? What other options can I use to balance the Fluke 752A divider?



--- Quote from: ttttrigg3r on May 18, 2022, 06:00:26 pm ---Fluke says their 8588A DMM does not have capability to be a null meter.

--- End quote ---

You've linked to a paper from Fluke actually telling you that certain DMMs (including the 8588A) are workable substitutes for a null meter and explaining how to do it.

What issues are you having using a DMM and what bias current are you measuring?

If you prefer the somewhat deceptive smoothness and stability of the analog null meter, you're not alone.  But you'll have to obtain and maintain one yourself, they're obsolete as far as the OEMs are concerned.

Look for TEGAM AVM-2000 Null Meter

Interesting picture of the Tegam.  Current price about $6000 USD, and more sensitive than the Fluke 845.
When did manufacturers of such instruments delete the front-panel Ground post?
My classic Fluke 845AB has Input High, Input Low, and Guard on the left, but Output High and Ground on the right.
I prefer connecting my shield braids and boxes to Ground, while connecting Guard as required.

Conrad Hoffman:
All the old null meters are subject to problems. I've got two HP 419 and the Fluke 845 and all of them need service... when I get around to it. The paper referenced isn't very helpful because we can't all afford an 8.5 digit reference meter. Makes me want to go back to the old L&N "top hat" galvanometers! Point a laser pointer at it and put a paper target a few hundred feet away.


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