Author Topic: UNI-T UT71E calibration question (Read 55511 times)

pklawit · « **Reply #50 on:** June 14, 2011, 05:58:42 pm »

Hi Kiriakos and All,

I'm updating the topic for the last time. I promise.

I got reply from post-sales support Engineer working at Uni-Trend China.
He says, that both problems (non-zero on the mV range and faulty temperature readings)
have the same reason - faulty capacitor C19.
He's sent me also the board layout, pointing where this capacitor is located - I will attach it here.

Unfortunately I have sent my VC-940 to the seller yesterday. Will ask them to fix the meter
or ship it back to me again, so I will replace it by myself.

I have to say, that exactly as Dave stated few times in his video blogs - I'm very pleased
and impressed, that the Uni-Trend company responded.
Sure, such thing as bad component should not happen, but the willingness to help one single
buyer (which does not even have a warranty nor receipt) is really priceless.

Regards,
Piotr

Kiriakos-GR · « **Reply #51 on:** June 14, 2011, 06:12:03 pm »

Oh my this schematic its a true treasure, for any one with UT71E or similar.
Thanks for your insistence to deal with it, and Yea UNI-T acted professionally all the way.

I just wish to had the same schematic also for the Fluke 28II that I also favor allot.

Even so you got in a lot of trouble with this ebay sale,
I wish you from the bottom of my hart to have an " Happy End " with it.

Lightages · « **Reply #52 on:** July 28, 2011, 06:36:22 am »

I ordered and received a DMM Check from Doug at http://www.voltagestandard.com. I was finally able to check and adjust my UT71E. It was reading low on the 40 scale so I adjusted it to get a 5.000 reading with the DMM Check. I then checked the meter against my reference meters and it seems that the base adjust VR2 affects all ranges as well. So I appear to have an accurate meter. I am awaiting for some more reference equipment to confirm the the results.

The meter seems to read correctly now, but for how long?

Kiriakos-GR · « **Reply #53 on:** July 28, 2011, 10:58:32 am »

Quote from: Lightages on July 28, 2011, 06:36:22 am

The meter seems to read correctly now, but for how long?

I would worry more for the cheapo reference item that you got.
The DMM it is made in a serious environment with serious tools and serious parts.

Lightages · « **Reply #54 on:** July 28, 2011, 03:32:02 pm »

Quote from: Kiriakos-GR on July 28, 2011, 10:58:32 am

Quote from: Lightages on July 28, 2011, 06:36:22 am
The meter seems to read correctly now, but for how long?

I would worry more for the cheapo reference item that you got.
The DMM it is made in a serious environment with serious tools and serious parts.

Well that is just silly. You have no basis to denounce the accuracy of the DMM Check from voltagestandard.com. It is brand new and from a reputable person and business.

I have a very good reason to doubt the accuracy of the UT71E as it did not match other meters when I got it. I recently just acquired a UNI-T UT33C for a very good price here in Chile, actually cheaper than available online from China! Anyway, this new meter, one from the same company, shows 5.00V when connected to the DMM Check. So the UT71E was less accurate out of the box than all the "inferior" meters I had, and also the cheaper meter from the same company.

UNI-T is not perfect. I am not impressed with fanboyism.

Bored@Work · « **Reply #55 on:** July 28, 2011, 05:46:59 pm »

Quote from: Lightages on July 28, 2011, 03:32:02 pm

Well that is just silly.

He usually talks out of his arse to get attention. Ignore him.

Kiriakos-GR · « **Reply #56 on:** July 28, 2011, 07:35:08 pm »

Quote from: Lightages on July 28, 2011, 03:32:02 pm

Well that is just silly. You have no basis to denounce the accuracy of the DMM Check from voltagestandard.com. It is brand new and from a reputable person and business.
..............
UNI-T is not perfect. I am not impressed with fanboyism.

If you become jumpy so easy, there is no need to say more.
You came to post and brag, well done mission successful.

saturation · « **Reply #57 on:** July 28, 2011, 08:23:23 pm »

Great, that's what that cal board is for, and luckily having only one pot to adjust will make it easy for you to maintain its accuracy. Old style pot DMM from even Fluke required only a few pot adjustments to do the cal; today you have to set a constant for every range, which is more work. Luckily they don't go out of cal often, if at all.

The Uni-T should hold its cal if the parts used are good quality. There are many reasons it could still drift, an unaged internal reference is one, and temperature stresses is another common one. DMM shouldn't drift much with humidity and barometric pressure as the variation is often in the uV, but with lesser DMMs you never know. The biggest drift from internal reference aging occurs in the 1st 200 hours, and its prudent to preage internal references for use if you want to minimize this phenomenon. For temperature stress, put the DMM in a refrigerator for ~ 40F exposure and summer sunlight to your regular temperature, typically 100-120F. Just cover the unit so UV light won't accelerate wear on the the plastic or the paint labels. If the unit is rated to those temps it should still read within spec and when it cools down to room temp it should not have drifted from your initial adjustment. There are many reasons thereafter for what parts could be suboptimal to cause the drift, a common one being the adjustable pot, the wiper may nudge just a little, contracting with cold, and expanding with heat, but when back at room temp it should be at the baseline setting.

The voltagestandard.com board seems to hold its factory cal very well. There are no formal studies just anecodotes from time-nuts, volt-nuts and the hp_agilent_equipment yahoogroup; the calibrated output has remained stable >= 3 years, IIRC. You can find comments about them in the archives.

I have a Geller SVR board that is now just 1 year old, its holding its original cal of 10.000 00 VDC very well.

Quote from: Lightages on July 28, 2011, 06:36:22 am

I ordered and received a DMM Check from Doug at http://www.voltagestandard.com. I was finally able to check and adjust my UT71E. It was reading low on the 40 scale so I adjusted it to get a 5.000 reading with the DMM Check. I then checked the meter against my reference meters and it seems that the base adjust VR2 affects all ranges as well. So I appear to have an accurate meter. I am awaiting for some more reference equipment to confirm the the results.

The meter seems to read correctly now, but for how long?

Kiriakos-GR · « **Reply #58 on:** July 29, 2011, 12:30:35 am »

I love challenges ..

Enjoy : David and Goliath ( Full HD with Dolby surround) LOL

https://www.eevblog.com/forum/index.php?topic=4001.msg52785#msg52785

nukie · « **Reply #59 on:** July 29, 2011, 02:31:43 am »

In defence of high precision low cost DIY voltage reference, a lot of work has been done in the Chinese community to recycle these precious parts. Especially those of which utilize the SZA263/LTFLU same reference device found in Fluke DC voltage standards. More popular reference such as LTZ1000 is also available. Once aged, this device is reported to be 0.05ppm/C and has a 1yr drift of 1ppm. All these are tested & monitored by individuals but in a rather controlled and scientific way. Why are these people shooting out for such a device when they have reference DMM standards such as a 8-digit HP DMMs?

I have three of these LTZ1000 boards and two of the SZA263 which are still waiting to be built. I can testify for their performance.

Below are a few more websites you can visit
http://www.eefocus.com/lymexbg2vo/blog/09-12/181353_9b971.html
http://www.ourdev.cn/bbs/bbs_content_all.jsp?bbs_sn=3908863

These devices are actually quite useful. For $25/$35 you are buying a voltage transfer standard and a piece of hardware. It might not hold voltage as well as those LTZ1000 powered boards but as long as you know it's spec and behavior you can work out if your meter is in or out of spec. Performance can be improved by grouping a few units as one.

Your meter might be 1% accurate at room temperature but have you tested it in a freezing climate? Or maybe perhaps you should try your meter in South Australia 48c degrees hot days?

Kiriakos-GR · « **Reply #60 on:** July 29, 2011, 11:45:36 am »

I do not know anything about the Fluke parts.
But by looking the pictures with the LT1013 and the PCB for it, I believe that the guy who did those PCB ,
he has no idea of what he was doing.

The precision reference LT1013 or similar, needs just one capacitor, if you follow the suggestion of the manufacturer.
And the point is, that if some one is unable to solder one simple IC with 8 contacts (With only three in actual use), on one small PCB and add few wires to it.
He does not need to have a DMM or working with electronics at all.

saturation · « **Reply #61 on:** July 29, 2011, 01:51:57 pm »

Nice, I know those boards, were did you get yours since those are all salvage from China. Did you go to China or buy it mail order? Those ICs are nicely aged. If you search on my name and voltage reference you'll find added links to more DIY standards.

Quote from: nukie on July 29, 2011, 02:31:43 am

In defence of high precision low cost DIY voltage reference, a ..

nukie · « **Reply #62 on:** July 30, 2011, 03:11:14 am »

Quote from: Kiriakos-GR on July 29, 2011, 11:45:36 am

The precision reference LT1013 or similar, needs just one capacitor, if you follow the suggestion of the manufacturer.

LT1013 is not a precision reference.
Refer to LTZ1000 datasheet for 7v Positive reference circuit page 7.

saturation - I guess your interest by your posts regarding those HP boat anchors. I never have the space for them so I settle for LM399 powered 34401s & Keithley 2000(also LM399 variant). A dear friend of mine whom I never met, help me to forward parts from China. Many sellers will send abroad but these DIY community are not traders so they don't sell abroad. They make these by batches once finish that's it. Some of them are still selling bare PCB you can search on taobao.com, you can get agents to buy it for you they usually charge 10%. Some parts are used, therefore the reference boards are very affordable, they were made available for the sake of the community, interested person can have a go at it. So statistic data can be shared and analysed, which is cool until someone decides to jump in and comment on an unused pin which is not soldered.

I think this is OT but I would love to share with everyone(what's going on in the East) since this thread is related to calibration. There are many variant of these LTZ1000 reference boards designed by different individuals but they are loosely based on the first 10V reference board by lymex/bg2vo he's also a camera enthusiastic / laser diode man.

East meets the West.

Lightages · « **Reply #63 on:** July 30, 2011, 04:12:06 am »

nukie:

Your posts and the like about all these voltage references are not exactly on topic, but in the context of calibrating my meter they are welcome posts.

I get the impression that these Chinese voltage standard enthusiasts are close to be fetishists with their projects. Its like standards porn. It also reminds me of the audiophiles who spend tons of money to eek out that last .001% of sound quality from their systems.

Anyway, I have decided that one voltage reference is not enough and I now have a Geller SVR on the way too. I will calibrate and adjust all my meters but one. My oldest and most stable will remain at its current slightly out of adjustment readings. It is 0.2% out of true reading after 25 years. Best to leave it alone and use it as a drift check for everything else.

nukie · « **Reply #64 on:** July 30, 2011, 04:16:17 am »

I agree for not adjusting the 25 years old meter. Since you know it's drift number you can offset the results by that number. For 0.2% it aged really well. I have a UT71D it drifted quite a bit over the years since I purchased it. I hope the drifting will slow down.

And I made a mistake, I should say "high accuracy" instead of precision.

Kiriakos-GR · « **Reply #65 on:** July 30, 2011, 01:39:25 pm »

A bit of drifting was a common problem for any DMM that it was using potentiometers as adjustment.

The latest and expensive ones, they do not use potentiometers.
By having in mind that there is no potentiometers, the precision sources has lesser or no use at all.

alm · « **Reply #66 on:** July 30, 2011, 02:55:58 pm »

Quote from: Kiriakos-GR on July 30, 2011, 01:39:25 pm

A bit of drifting was a common problem for any DMM that it was using potentiometers as adjustment.

Most, if not all, high-accuracy bench meters used mechanical trimmers before closed-case calibration became feasible, and had excellent stability, at least an order of magnitude better than most modern high-end handhelds.

Kiriakos-GR · « **Reply #67 on:** July 30, 2011, 03:39:30 pm »

Quote from: alm on July 30, 2011, 02:55:58 pm

Quote from: Kiriakos-GR on July 30, 2011, 01:39:25 pm
A bit of drifting was a common problem for any DMM that it was using potentiometers as adjustment.
Most, if not all, high-accuracy bench meters used mechanical trimmers before closed-case calibration became feasible, and had excellent stability, at least an order of magnitude better than most modern high-end handhelds.

Well if you feel the need to say the truth, you will have to revile it all, with out cutting corners.

The old bench-top they have inside huge mechanical trimmers, an over kill in size in comparison with the application that they was perform.
The handhelds due space limitation they use tiny ones, and those can be easily loose their adjustment.
By all this in mind, now it is more understandable than before, the reason of why all those large rubber holsters become as standard accessory in the handhelds DMM.

saturation · « **Reply #68 on:** July 30, 2011, 03:45:53 pm »

Hello nukie,

My only boat anchor is the HP3456a; but is not that big, its nicely flat to use as a table for stacking other devices, so it doesn't occupy that much space; its also very easy to service [ if needed] and most parts are generic. I have 3 stacked on each other and the other gear on top, my 'test gear rack.'

The only issue with the DIY standards is how to define its accuracy over time. You can subtract seasonal variation due to fluctuations in climate conditions and keep your existing lab ambient conditions tightly controlled, identical to the original cal date, the standards are accurate for over much a year. Temp & humidity swings will cause change to mV, and ambient pressure in the uV. Its very difficult to control ambient pressure. Seasonal phenomena are well described in the metcal literature.

The variations are so predictable you can calculate an offset for your particular board, and apply a correction, its a bit complicated to get the data at first such as when multiple season conditions interact.

Quote from: nukie on July 30, 2011, 03:11:14 am

saturation - I guess your interest by your posts regarding those HP boat anchors. I never have the space for them so I settle for LM399 powered 34401s & Keithley 2000(also LM399 variant). A dear friend of mine whom I never met, help me to forward parts from China. Many sellers will send abroad but these DIY community are not traders so they don't sell abroad. They make these by batches once finish that's it. Some of them are still selling bare PCB you can search on taobao.com, you can get agents to buy it for you they usually charge 10%. Some parts are used, therefore the reference boards are very affordable, they were made available for the sake of the community, interested person can have a go at it. So statistic data can be shared and analysed, which is cool until someone decides to jump in and comment on an unused pin which is not soldered.

Lightages · « **Reply #69 on:** November 27, 2011, 06:45:11 am »

By request in a PM, I am posting the calibration instruction files from an earlier post in this thread. I beleive they did not survive the server transition.

Two of the files are the originals I posted, and one is an additional with more complete information but for a rebadged Voltcraft 960.

fmaimon · « **Reply #70 on:** November 27, 2011, 03:21:40 pm »

Quote from: Lightages on November 27, 2011, 06:45:11 am

By request in a PM, I am posting the calibration instruction files from an earlier post in this thread. I beleive they did not survive the server transition.

Two of the files are the originals I posted, and one is an additional with more complete information but for a rebadged Voltcraft 960.

Thank you Lightages. Unfortunately, I still can't see the files...

Can anyone see them or it's just me?

Lightages · « **Reply #71 on:** November 27, 2011, 03:28:00 pm »

Resent in my post. I don't know why they didn't show up.

fmaimon · « **Reply #72 on:** November 27, 2011, 05:16:15 pm »

Thank you again.

saturation · « **Reply #73 on:** November 28, 2011, 05:59:11 pm »

Good job! I can see it but I have to rename the file to zip. Thanks for making it available.

Quote from: Lightages on November 27, 2011, 03:28:00 pm

Resent in my post. I don't know why they didn't show up.

Lightages · « **Reply #74 on:** November 28, 2011, 06:28:16 pm »

Yes I have noticed lately that all the files on this forums are named "index.php" when you try to download them. I will ask Dave what is going on.

And I am glad to help whenever I can.


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Author Topic: UNI-T UT71E calibration question (Read 55511 times)

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