On 30 April 2018 at 10:45, Nigel Clarke via volt-nuts <volt-nuts@febo.com> wrote:
> -----Original Message----- > > From: Dr. David Kirkby <drkir...@kirkbymicrowave.co.uk> > To: gandalfg8 <gandal...@aol.com>; Discussion of precise voltage > measurement <volt-nuts@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. Dave _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.