[volt-nuts] Cropico DO4A Digital Ohmmeter

Dr. David Kirkby drkirkby at kirkbymicrowave.co.uk
Mon Apr 30 07:38:46 EDT 2018

On 30 April 2018 at 10:45, Nigel Clarke via volt-nuts <volt-nuts at febo.com>

> -----Original Message-----
> From: Dr. David Kirkby <drkirkby at kirkbymicrowave.co.uk>
> To: gandalfg8 <gandalfg8 at aol.com>; Discussion of precise voltage
> measurement <volt-nuts at febo.com>
> Sent: Sun, 29 Apr 2018 22:18
> Subject: Re: [volt-nuts] Cropico DO4A Digital Ohmmeter
> I see this is a low-resistance (10 u ohm resolution) ohmmeter. Have you any
> idea how you are going to check the calibration? I just bought a Simpson
> 444, which has 1 u ohm resolution. I have not got it yet, but are wondering
> how I am going to check the calibration.
> 0.005% resistors are available from Farnell in a fairly limited number of
> values, but certainty not in the range of values needed for a milli/micro
> ohm meter.
> Dave
> ---------------------
> Whoops, whilst checking the calibration is one thing, actual self
> calibration on the DO4A is perhaps not looking quite so straightforward,
> it's an automated calibration process that requires six set resistor values
> of 4mohm, 40 mohm, etc up to 4Kohm, and a quick check online doesn't show
> high tolerance versions of these values exactly falling out of the woodwork!
> Nigel, GM8PZR

They are annoying values! Often one see 100 u ohm, 1 m ohm, 10 m ohm, but
most I have seen have resistances starting with the number 1.

The Simpson 444 I bought can only read up to a maximum of 2 ohms, which is
a bit limiting it must be said. The lack of GPIB, (or any other computer
interface), is another major inconvenience too.

I feel the low resistance meters that I have seen, all have some annoying
limitations. I've contemplated whether it is possible to make something
better, which outputs 0-10 V to be read by a normal multi-meter. Issues
seem to be

* Some are AC only, which makes them useless for inductive components.
* Some are DC only, which makes them suspeptable to thermal EMF
* Some use high currents, which can damage sensitive components
* Some can use fairly high voltages, which can break down oxides. (The
Simpson 444 does not output more than 100 uV, which seems quite unique in
this way)
* Some don't have GPIB
* Some like my Simpson 444, has a very limited maximum resistance - in this
case just 2 ohms.
* The Keysight 34420A has an impressive 100 n ohm resolution, but achieves
this by having a 1 ohm range with a 7.5 digit meter.

A combination of issues leaves me wondering if something better could not
be designed. But the only sensible way I can see would be an add-on for a
normal bench multimeter.

Calibration seems a non-trivual task, even for a commerical unit like
yours, so would be quite tricky for a home-brew unit, unless one spent a
fortune on calibration standards.


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