Electronics > Metrology

Null voltmeter

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doktor pyta:
I'm starting a topic which might be interesting for volt nuts.
I'm going to share some thoughts on null voltmeters as I own three units.
For me there are a beautiful example how the engineers in the 60's managed to overcome the lack of high precision components.

Repairs:
In meters that use neon bulbs most common failure are the neon bulbs :) They get metalized inside and their flash threshold voltage gets much higher. They start to flicker. The flickering can be seen off course however You should remember about the 'dark effect' in neon bulbs (dim the lights in the room while observing).
Off course electrolytic capacitors and leaking batteries are a common issue.


Fluke 845AB- difficult access to the components because of it's mechanical construction . Batteries inside can cause problems due to leaking electrolyte. Goes down to 1uV full scale.

Fluke 845AR- easier to repair, no batteries inside. Goes down to 1uV full scale.

For both Flukes, notice two 47nF capacitors between LO- Guard and between Guard- mains earth. If You connect directly a output of an OPAMP to LO terminal it is highly probable that it will start to oscillate at high frequency. I warned You!

HP419A- very good design. Goes down to 3uV full scale. Seems to have lower noise than Fluke models. Watch out when disassembling and assembling because the screws have different lengths (about 3mm and 5mm ) and if You put the longer one in place where it shouldn't be you will have a short between LO terminal and mains earth (it will disturb floating measurements). The meter utilizes a Mercury battery as a floating voltage source, but You can use the meter without this feature as well.
If the meter is operated without batteries, two Zener diodes should be soldered in place of the batteries to create a low impedance ground path.

Keithley 155- I heard that it is a very good design. If there is any owner, please write an opinion.

Let's also consider using modern good digital multimeters with the autozero function at 100mV range. What are the pros and cons?

HAL-42b:
With 1uV full scale you obtain about 2nV resolution.

Makes me wonder, how cheaply can we obtain  2nV resolution with modern off the shelf equipment.

ManateeMafia:
The Keithley 155 is based on a solid-state chopper. Some parts are hard to track down. I have one functional and one non-functional Keithley. Another plus is that it can run from four 9-volt batteries. If you are in the market for one, try and find one with the external power pack. It allows the meter to run from mains. If not, an external ps will need to be attached so it is easier to replace the batteries as needed.

Edwin Pettis pointed out to me that ESI used the HP and Keithley null meters in the 242's. The 801 came mostly with the HP but it appears on the last model (801C) it was replaced with the K155. It looks like Fluke's design may have been too big to fit.

I also have the 845AR and 845AB. I recommend any model that will null after warming up. Any that are unstable most likely need new neon bulbs. A member of the volt-nuts group uploaded an LED mod for the 845 to replace the neons. It located on the ko4bb website .

doktor pyta:

--- Quote ---With 1uV full scale you obtain about 2nV resolution.
--- End quote ---
Nope. Usable resolution is something about 50nV due to the low frequency noise.

TiN:
I'm new to null-meters (was under false impression that K2002 or 3458A can replace nullmeter), but already have F845AB in the mail my way.
Should be working, but we know how it goes.

Reason why I hit that evil "bid" button, is the fact that nullmeter can run from battery, floating from rest of circuit, which none of long-scale DMMs are able to do. With proper setup, I'm hoping to very low leakages.
And since I have Keithley 182-M with lowest 3mV range and battery powered front-end amp, would be interesting comparison.

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