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Old 21st Mar 2016, 1:54 pm   #101
mhennessy
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Default Re: A precision voltage calibrator

Excellent - and it'll be good to check that the LM4040 reference survived the journey
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Old 21st Mar 2016, 2:05 pm   #102
Skywave
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Arrow Re: A precision voltage calibrator

Quote:
Originally Posted by mhennessy View Post
Rightly or wrongly, I'm actually ignoring all the readings from the AVOs

Ultimately, with the best will in the world, I'm not sure you'll ever get to the bottom of this particular mystery. The question now is simple: do you adjust all 3 of your Flukes to 2.503V, or do you seek a second opinion from someone else with an known-accurate meter or reference?
The last question above has now (unexpectedly) been addressed - as per the above posts.
As for the AVO readings, I understand why you have said that - and I am in agreement with the embedded thinking behind that remark.

In a post of mine above, I have reported my findings using a Iso-Tech DMM. On the grounds that:
1. That instrument is the newest of the DMMs I own;
2. It has only ever been owned by me (so its history is known);
3. I note its indications when connected to the voltage calibrator ;
4. The calibrator is new - and I know its history;
► I am going to adjust all 3 Flukes to the voltage as indicated on the calibrator.

Will I ever get to the bottom of this mystery? To be quite blunt, I've now reached the point where I care not: my mind is now totally focussed on the end result.

Finally, I hope to be able to accept the offer of assistance from Oldtestgear. Once that has been done, hopefully this entire matter can finally be laid to rest. Upon review, I had no anticipation that what started out as a 'simple idea' would mushroom into such a saga. But we are talking about 'electronics' here, and now and again, strange & totally puzzling incidents do arise. Perhaps it is knowing that which keeps us so committed in this field of technology?

Anyway, nuff said for now. My thanks to everyone who as contributed so far.

Al.
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Old 22nd Mar 2016, 10:39 pm   #103
Skywave
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Originally Posted by Oldtestgear View Post
There is only one way for Skywave to be clear what is happening & get his Flukes tested on all ranges using a decent calibrator.
Perhaps a trip to Torquay might give help? Anyway, if you are interested please let me know.
Yes, thanks for your offer: you now have a P.M.

Al.
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Old 23rd Mar 2016, 12:28 am   #104
mhennessy
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Default Re: A precision voltage calibrator

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In a couple of weeks time, I'll have 1 or 2 of them, so will add details here (along with how well they compare to my meters).
The first of mine has arrived - the plastic cased version with the pushbutton switch and a rechargeable battery. I have to say that it's a thing of beauty! There's a surprising amount of electronics on there, and I spent an hour reverse-engineering the circuit. To save drifting too far from the point of this thread, I'll put those details elsewhere in the fullness of time...

Anyway, how does it measure up?

The sticker states that the unit was measured with a HP 3458A (a bit of a beast!) at a temperature of 21C. I measured with my Keithley 2015THD, and checked the temperature (20.6C). Everything had at least 3 hours to warm up and stabilise.

Code:
Setting     Measured     Claimed       Error between measurements
-------     ---------    --------      --------------------------
 2.5V        2.49926V     2.49917V      0.0036%
 5.0V        5.00218V     5.00196V      0.0044%
 7.5V        7.50171V     7.50138V      0.0044%
10.0V       10.00420V    10.00374V      0.0046%
So, I'm pretty pleased with that

The IC (the KH variant, date code 0619) is well within spec, and the difference between my measurements and the manufacturer's measurements is pleasing - the pattern suggests that the measurements were genuinely made, rather than made up at random.

The basic specification of the 2015 is 0.003%, and the 3458A is an order of magnitude better again. To apparently be that close to their 3458A is pleasing, and further increases my confidence in my Keithley.

Next up, I'm expecting one like Jim's and Bill's, plus another variant that's even cheaper. I'm not sure if the latter will come with any sort of "calibration", but I'll be interested to see what the former comes with
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Old 23rd Mar 2016, 11:52 am   #105
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Default Re: A precision voltage calibrator

I quite often see errors of 'many millivolts' due to 'poor' contact between differing metals on 4mm connectors, especially noticeable when using a very high impedance voltmeter (e.g. 50G ohm). I usually need to wipe the surfaces with contact cleaner to get the same voltage with the probes reversed !

dc
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Old 4th Apr 2016, 11:33 pm   #106
mhennessy
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Originally Posted by mhennessy View Post
Next up, I'm expecting one like Jim's and Bill's, plus another variant that's even cheaper. I'm not sure if the latter will come with any sort of "calibration", but I'll be interested to see what the former comes with
My other 2 have arrived now. Both work well...

First up, the one already featured (see post #55). This uses the top-spec "LH" variant, as Jim's picture shows. Interestingly, mine has exactly the same date code as his (1015). I also noted that if I bring up the supply rail slowly on a bench PSU, I see the strange behaviour that was reported earlier - so I'll investigate changing that capacitor.

Even more interesting is the "calibration certificate". Yes, it's identical to Jim's! And Bill said that his is also identical. Hmm. So, for these particular units the calibration appears to be false (unlike the plastic-cased versions).

As before, here are my measurements. Temperature was 21C, same Keithley 2015 was used, and the supply was 15.00V, applied to the battery terminals. A minimum of 3 hours warm up time.

Code:
Setting     Measured     Claimed       Discrepancy     Absolute error 
-------     ---------    --------      -----------     -------------
 2.5V        2.49969V     2.49942V      0.0108%         -0.0124%
 5.0V        5.00156V     5.00037V      0.0238%          0.0312%
 7.5V        7.50213V     7.50042V      0.0228%          0.0284%
10.0V       10.00333V    10.00066V      0.0267%          0.0333%
It's not a million miles out, but compare this table with my previous results, where the mean discrepancy was only 0.0043%.

Fictitious certificate aside, the IC is comfortably within the "L" specification, so I'm happy with that...


Next, a type that hasn't featured yet - astonishingly cheap at just £2.89 delivered - this one uses the "JH" chip, so the least good of them. My sample has a date code of "9148". Pictures below...

There is no calibration data supplied with this one. Measurement conditions exactly the same as before:

Code:
Setting     Measured     Absolute error 
-------     ---------    -------------
 2.5V        2.49857V    -0.057%
 5.0V        5.00159V     0.032%
 7.5V        7.49998V    -0.0003%
10.0V       10.00259V     0.026%
The JH chip is only specified to 0.3%, so that's pretty good...

Hope this is of interest...

Mark
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Old 5th Apr 2016, 2:47 am   #107
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Default Re: A precision voltage calibrator

I looked at the cheapest one, and as they're both for peanuts I chose the first one for convenience. TBH for an Amateur as I am these days it's a bit of an indulgence anyway, but for Professional users it's a different matter. I trust my digital meters to stay sufficiently accurate, and it turns out they all did, at least at low voltages.
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Old 5th Apr 2016, 1:41 pm   #108
Skywave
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Arrow Re: A precision voltage calibrator

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Originally Posted by Skywave View Post
I hope to be able to accept the offer of assistance from Oldtestgear.
Contact was made and a meeting was arranged, but that was subsequently cancelled due to health problems. Hopefully, a meeting will be arranged at a later date.

Al.
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Old 8th May 2016, 11:01 am   #109
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Default Re: A precision voltage calibrator

In late March, I decided to "join the bandwagon", so I ordered two units as shown in #106. About 2 weeks ago, I realised they had not arrived, so raised the matter (thro Ebay). I had a reply confirming despatch, but then later traced as RETURNED to their warehouse. They apologised and promised replacements which arrived after about 5 days, something of a record for far East despatch. Alas, no "Calibration certificates", and so far no time to play, but when a few minutes appear, I will be creating my own "reference centre" which should at least act as a check on my Telequipment C3 calibrator.
Les.

Last edited by MotorBikeLes; 8th May 2016 at 11:02 am. Reason: Corrected #106 plus sp.
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Old 9th May 2016, 12:34 am   #110
mhennessy
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Default Re: A precision voltage calibrator

Someone on the EEVBlog forum recently received one without the certificate - perhaps the seller(s) have decided to stop sending out fake certificates now that it's public knowledge?
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Old 9th May 2016, 5:48 pm   #111
chriswood1900
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Default Re: A precision voltage calibrator

In my earlier post #32 mine only came with a label no certificate, but the label was pretty accurate. So no fake certificate in my case.
Chris
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Old 9th May 2016, 6:00 pm   #112
mhennessy
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Default Re: A precision voltage calibrator

Yes, all the hand-written labels I've seen have been pretty plausible. It's the printed "certificate" for the type with the 15V battery (unit #2 in my article) that appears to be fake
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