34401a range issue

[dohyun]

I was contemplating that (waiting) but I was also figuring that if they cal my meter and then the standard goes off to be cal'd then I would find out reasonably soon that the cal job they did on mine was in spec (as the end point of the two time points of calibration)

Though I am new to the precision and calibration world so I could be totally off base

[Kleinstein]

The freshly checked cal instruments can be slightly better than just before the end of calibration cycle. So calibration just after the check might be slightly more accurate. However for the 34401 the calibration instrument should be really much better than the meter. So this should not matter that much. Even at the end of the cycle there instrument should be way better than the 34401. The weak point there is more the disturbance / drift of the 34401 due to shipping and warmup time. It would be a different if you send in an 8 digit meter, where the quality of the reference could be an factor.

Having the calibration just at the beginning also has some downsides: there instrument may still recover from shipping disturbance and they may be in a hurry because of a backlog of waiting instruments.

If at the end they find out there instrument was way off (which is very unlikely), they should in principle give you a note and maybe offer corrections (new calibration). So you have timely check for the unlikely case that there instruments where out of spec.

[dohyun]

I heard back from the cal lab. Sounds like initially the zero offset was out of tolerance on both the front and rear terminals as well as DCI.

They made adjustments and re-ran the tests this time passing.

The lab noted that there is still some drift with the unit, and that it is not uncommon for the age of the DMM to have that happen. They also mentioned if I get anymore 34401A's to avoid ones with serial numbers starting in 3146A as those are starting to show more signs of issues.

That is a bit worrisome, in part because I had done the zero offset adjust with the Fluke shorting plug prior to sending it in and it was dead on. Hopefully it won't drift outside of tolerances for a good while.

If it does, I guess I am back to trying to find out what is causing the drift and fixing it.

[bitseeker]

Also note that the early 34401A (panel assembly 34401-66502) have unobtanium VFD driver ICs and cannot be replaced by the newer assembly due to firmware incompatibility, which is another reason to get the later units (firmware 06-04-02 or higher), if you buy another.

On the other hand, the VFD itself is the same for both assemblies and new VFDs are available from China, which is a nice benefit of having a 34401A vs. some other model.

[dohyun]

Thanks for the info on the VFDs! Definitely good to know.

I got the unit back today, will be checking it out and seeing if the things which were out of tol before start drifting at all (zero offsets and DCI).

[ManateeMafia]

Check the 2W ohms ranges, especially the lowest range. There have been a few 34401A owners that have found issues with the front/rear switch . Typically the 2W ohms tests will show a higher than normal offset and can be seen by exercising the switch a few times.

[Kleinstein]

If drift is a possible problem, I would check the input bias current, as one possible source of drift could be excess leakage (e.g contamination) which will show as excess leakage. Transport could have caused condensation / humidity build up.

[dohyun]

So far so good.

I've gone over the calibration as-received and as-left data and it looks like the main issues were with zero offset on front and rear 100mV range, zero adjust front and rear on 2W 100R and 1KR ranges. DCI gain verification was also out of tol in the 10mA and 100mA ranges.

As left everything was back in tolerance. As far as I've been able to test on my side things are still looking good after the journey from the lab to me.

Thank you again for the help on this! I will keep an eye on the ranges which were out of tolerance in case anything starts drifting heavily. (Though I need to figure out something for a precision current source

Also before the next calibration I probably will swap out the VFD itself as it is on the dim side.

[bitseeker]

Sounds good. Regarding the VFD, plenty to choose from:

http://www.ebay.com/sch/i.html?_sacat=0&_nkw=34401a+%28vfd%2Cdisplay%29&_sop=15