[volt-nuts] Just back from cal

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Fri Apr 8 00:47:23 EDT 2016

On 8 Apr 2016 04:21, "Joseph Gray" <jgray at zianet.com> wrote:
> In February, I sent my HP 3457A in for cal. In late March, I sent my
> EDC VS330 DC Voltage Reference in for cal. Today, I just got the VS330
> back and thought I'd compare the two instruments.

Did you send them to the manufacturers or a third party lab? I am
personally VERY skeptical of the competence of a number of so-called
calibration labs. A few months back there wss a 40 GHz vector network
analyser on eBay for auction which included a cal certificate.  The
certificate listed the items used for the calibration.  The calibration kit
used was a 6 GHz economy type N kit made by HP. The N connector will not
work properly at 40 GHz, but even if it did,  the use of a calibration kit
is insufficient to calibrate a VNA.

I have seen countless examples of this sort of thing. I would not
personally consider getting any of my HP kit calibrated by anyone other
than Keysight.

> Both instruments have been on for a couple of hours. It is about 25 C
> in the room.

That is quite warm.  Are both specified at that temperature?

> Setting 10.00000 on the VS330, I measure 9.99891 VDC on the 3457A.

I would try to find a local volt-nut with a 3458A.

I would also keep a log of the differences between your voltage standard
and multimeter.  If the difference is changing significantly over time, it
would tend to indicate one or both are unstable.  If however the difference
remains very similar then I would be more suspicious that one is
incorrectly calibrated.

> The report on the 3457A, dated 16 Feb says the 30 VDC range read
> 29.9991 VDC. The report on the VS330, dated 31 Mar says 10 VDC read
> 10.0001 VDC. Both instruments required adjustment.

I have heard several reports of 3457As being in spec many years after they
were last calibrated.  The fact your unit required adjustment seems a
little unusual.

If the VS330 was out by 1  on the last digit,  that suggests to me it
didn't require adjustment.

> These numbers would lead me to believe that I should be getting a lot
> closer agreement to 10 VDC for both of these instruments. Am I
> expecting too much?

I have not checked your numbers,  but would agree if both units have been
recently calibrated,  the agreement between the two should not be just
inside the limits if you take the worst case of each. I would expect much
closer agreement.

If the two appear to be drifting with respect to each other the the
purchase of another 3457A, even if uncalibrated,  would probably allow you
to determine what one of your

> Joe Gray
> W5JG

Dave, G8WRB.

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