UNI-T UT71E calibration question - Page 1
Forum section: electronics / general

author: Lightages
author_rank: Unknown
timestamp: April 04, 2011, 07:12:50 am
content: Hello, first post but long time electronics dabbler here.I recently purchased a Unitrend UT71E meter and I suspect that it is not as close to accurate as it could be. It seems to be within spec, but right at the limits. After seeing the youtube videos on someone calibrating the meter using a built in routine and a Fluke referenceI see that the values they keep entering for calibration are multiples of 19. Are these multiples of 19 needed or is it just needed to have values at X.X000 or XX.000 or X00.00 for examples?Any help would be appreciated.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 04, 2011, 11:40:49 am
content: QuoteThe limits gets always effected , even by the temperature of the environment that you live.For instance the winter-summer difference in temperature it can cause different readings.As about the question in what voltages the calibration should be made,there is profile-forms for its instrument , and the one who owns the calibrator ,follows the directions of the profile-form of the specific instrument.

author: saturation
author_rank: Super Contributor
timestamp: April 04, 2011, 03:26:34 pm
content: Its common for DMMs to use the maximum input for a full scale reading during calibration. For example, to calibrate the 2.0000 scale, the input voltage required is 2.0000. But it depends on whatever the maker wants, the Fluke 87V uses mainly 6: 600mV, 6V, 60V etc., while the Agilent 1252a uses multiples of 5 for full scale performance tests, but 3 for calibration. I don't know specifically what the UT71E requires, but chances are that's it.If you're not sure of your DMMs accuracy, you can do a performance test. You'll measure the maximum input for a particular range to see that it meets the published specifications. For an example, download the Agilent 1252a or Fluke 87V service manual and just what they consider a performance check.

author: Lightages
author_rank: Unknown
timestamp: April 04, 2011, 08:27:45 pm
content: It does look like they use multiples of 19 for almost all of the calibrations, and in OHMs and ACV too. I have an email waiting for reply from UNI-T on this. Maybe they will reply with useful information. MY meter seems to be a low on every range I can compare with my other meters by around 0.4%. Now I would not normally trust old meters against a new one but when three meters I have all say that the UT71E is reading 0.4% low then I do need to question the claim of 0.025% basic accuracy and 0.05% accuracy on the ranges I am using (ignoring the +5 digits). The reason I purchased this meter was the claimed accuracy and expected it to not be a order of magnitude out of spec. My expectation was that it would be with 0.1% accuracy at least.Now I suppose that all my other meters have drifted all equally off the mark by almost exactly the same amount even though they are different designs and years, but it seems highly implausible.Now the question is; Where do I get this thing re-calibrated? It is not easy to find such a service here as I am currently living in Chile. Perhaps I can build a reference for the voltages of 1.9V and 19V easily enough but for 190V and 700V I am getting into expensive territory. A high voltage OP amp and precision resistors might get me to 190V but at what accuracy?

author: saturation
author_rank: Super Contributor
timestamp: April 04, 2011, 08:41:19 pm
content: Sorry to hear Lightages. One issue with equipment makers with a less established reputation, is that they overstate [ or lie!] about specs since the sheet is the basis for comparison against other DMMs and its price. Only a review by a 3rd party, like our sysop Dave started and others have posted on eevblog, brings it to light. Maybe its just a quality control issue, and they don't calibrate them properly before shipping them out, but regardless sloppy engineering simply doing a calibration could mean other things are equally haphazard.https://www.eevblog.com/forum/index.php?topic=533.msg7695#msg7695To recalibrate it yourself without the right gear, you need the calibration sequence from the service manual. To do this on the cheap by yourself, you also need at least one good calibrated DMM that is equal to or greater in accuracy than the meter you are going to calibrate. You also need adjustable AC, DC and ohms sources that are stable for even just a few minutes so you can compare the calibrated DMM against the device under test.Digital DMM can be a pain to calibrate without the right equipment because you need to set the cardinal value precisely by what the meter wants, you can't input 1.901, it must be precisely 1.900 or the meter will be incorrectly set. Also the values cannot be any other value, you can't substitute a stable 1.700 for 1.900 because the meter is going to store that value in a set of tables to use to offset measurements by. In analog instruments, you can set the calibration literally to anything you want, preferrably as close to, but not necessarily the exact value, specified in the manual.

author: tekfan
author_rank: Frequent Contributor
timestamp: April 04, 2011, 10:19:53 pm
content: WOW! That's one helluva Fluke DC calibrator! Hear those relays click! Music to my ears!

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 05, 2011, 12:34:49 am
content: Quote from: Lightages on April 04, 2011, 08:27:45 pmyour concern about accuracy means you are embracing expensive territory. 0.4% at 19V is what? ±0.076V so your dmm read 18.924V? for that big number i usually just round off to 19V. at most electronics say 5V, err = ±0.02V 3.3V err = ±0.0132Vas saturation said. in addition, it maybe they are not lying, its just their interpretation on how to get the claimed figure. they maybe choosing the max accuracy dmm (instead of averaged or mod) from their testing and they probably have large standard deviation, meaning you get more chance of getting the lemon (out of spec) from your purchase.i never characterize my UT71A properly, since its reading very close/comply to my other $20 DMM, so i'm happy. i dont really care much if its .1 out in 1st decimal, and i believe i'm blessed if the error is in 2nd decimal place. because i know... accuracy % = reciprocal $

author: Lightages
author_rank: Unknown
timestamp: April 05, 2011, 04:46:07 am
content: Yes I am concerned about accuracy. I have been all my conscious life. I am managing a number of solar power installations and knowing voltages accurately is necessary for the health of the batteries. The comparison of readings below is a mixture of real world values I need to monitor mixed in with other AA cell values and some stacked 9V battery values. It is obvious that the UT71E is reading low compared to my other meters.UT71E : 83.82 54.01 44.91 35.428 13.558 9.18 3.0786 1.5136Reference meter #1: 84.2 54.2 45.1 35.6 13.61 9.22 3.09 1.52Reference meter #2: 84.2 54.1 45.0 35.49 13.60 9.18 3.110 1.527I use a battery monitor that measures a Kelvin shunt to determine the state of charge and discharge of the batteries. The accuracy of the setup of the monitor is crucial to the accurate monitoring of the state of the batteries. Large deep cycle batteries used in these situations are VERY expensive and the small changes in voltages that relate to their state of being requires equipment that can read them accurately.I know, why did I take a chance on anything other than the big names? Well mainly because of the relatively favorable results I saw on the EEVblog and other reviews. The price also seemed right and not ridiculously cheap at the same time.

author: Bored@Work
author_rank: Super Contributor
timestamp: April 05, 2011, 06:15:08 am
content: Quote from: Lightages on April 04, 2011, 08:27:45 pmAskCode:DICTUC S.A. National Custodian LaboratoryVicuna Mackenna # 4860 - MaculCasilla 306 (correo22)Santiagoif they do UNI-T's or know someone. If they do, expect to pay a bunch of money for a calibration.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 05, 2011, 06:42:15 am
content: Well , there is always the option to build something like this ..https://www.eevblog.com/forum/index.php?topic=1148.0

author: Lightages
author_rank: Unknown
timestamp: April 05, 2011, 06:44:47 am
content: Thanks for the information. I will try to call them. Usually my searches for anything in Chile result in 100s of results pointing to web portals of useless information.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 05, 2011, 10:28:33 am
content: Quote from: BoredAtWork on April 05, 2011, 06:15:08 ami think Fluke DMM is a cheaper option.

author: saturation
author_rank: Super Contributor
timestamp: April 05, 2011, 02:40:18 pm
content: Eyeballing this values, it seems the Uni-T is consistently off ~ 0,4%, so its either just in need of a calibration or the reference meters are off. An important thing is the Uni-T has not proven its long term stability, so even after you calibrate it can drift into inaccuracy shorter than the claimed calibration period or it will drift with environmental conditions up or down, averaging to the proper value when the conditions are similar to the calibration conditions. This will give you erratic readings on the same test point over time that is climate dependent, very annoying.A simple check for the kind of precision and accuracy you need is to get a voltage reference to make a spot check. Since they are far more accurate than your meters, even if they drift in 1 year, it will still check the state of your meters, even if limited to just one range.https://www.eevblog.com/forum/index.php?topic=650.45See this thread, both Malone and Geller have the accuracy you need. Simpler still if you can find someone nearby in Chile with a known calibrated DMM and check those meters against it.Quote from: Lightages on April 05, 2011, 04:46:07 am

author: saturation
author_rank: Super Contributor
timestamp: April 05, 2011, 02:51:29 pm
content: The problem I find mecha, see the links on the post of slburris,https://www.eevblog.com/forum/index.php?topic=533.msg7695#msg7695is that on that thread and on this link, Uni-T overstates its accuracy. If they quote their statistics as you say, that is still misleading. At least Fluke, Agilent, HP, B&K, and all other reputable makers I know either understate their specs not exaggerate them, so you know what you are getting. If it was calbrated properly but drifted, or they are quoting statistics improperly, its still not excusable. Further, those are new machines, not years old from calibration date.But you are right, real accuracy, that not only is spot on but stable over time, is not cheap, but I can say with great confidence of you buy a known reputable make like Fluke or Agilent it will be what the spec sheet says it is; many really old Fluke 80 series DMM are still in spec,decadessince it was last calibrated [ I have 4 of them].Accuracy is important when measuring electrical phenomena related to natural events: physiology, chemistry, etc., because in many instances the thresholds are truly finite, not relative. The cutoff voltage for NiMH cells is ~ 1.0 Vdc and to go lower risks damage to the cell. OTAH, deep discharging them close to this is a good way to prolong their service life, so the margin for error for good performance is narrow.If your DUT provides 1.01V, meter should be 10x more accurate or measure to 0.001 or 0.1% to resolve 0.01V unambiguously. For example at 0.5% 1.01V becomes 1.005-1.015 and is more uncertain compared to 0.1% which provides 1.011-1.008.Quote from: Mechatrommer on April 05, 2011, 12:34:49 am

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 05, 2011, 07:22:01 pm
content: cant you open it up and try to find any potential trim pot to adjust, resistor devider etc etc. if it broke while tinkering, maybe a good excuse to get a Flukejust a 2cnt suggestion.

author: Lightages
author_rank: Unknown
timestamp: April 06, 2011, 05:46:32 am
content: Quote from: Mechatrommer on April 05, 2011, 07:22:01 pmThere might be a trim pot, or maybe ten, but what does it do and which one is correct? If I do find the right adjustment pot, how do I calibrate the meter without a standard 10x better than the meter, never mind anything the same accuracy. There has to be a specific procedure such as described in the video that should be used and used with a good calibration source.The point is I purchased something as being a certain specification with a one year guarantee that should be close to spec if not right on, not an order of magnitude out.My emal has been answered by Unitrend, but so far they are only asking what country I am in. More to follow as I get more information from them

author: saturation
author_rank: Super Contributor
timestamp: April 06, 2011, 11:59:39 am
content: Alas, from the video it looks like close case calibration, which is done by inputting values into memory locations. That's how most modern DMM calibrate these days; old style is with trim pots [ which is better for DIY calibration!].You may want to see this new coincidental post that is relevant to our topic, but it shows what you pay for with Fluke and Agilent:https://www.eevblog.com/forum/index.php?topic=3050.msg40602#new

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 06, 2011, 01:57:48 pm
content: Quote from: Lightages on April 06, 2011, 05:46:32 amthere are, you just mentioned it, in the beginning of this thread (2nd post?). thats how you know your 71E reads lower.

author: Lightages
author_rank: Unknown
timestamp: April 06, 2011, 09:17:14 pm
content: Quote from: Mechatrommer on April 06, 2011, 01:57:48 pmI suspect my meter is lower. I do not have proof but rather very convincing evidence. I purchased a meter to gain accuracy, not just merely repeat the values I can rwad with my 20 year old meter. So I await the response from Unitrend. Perhaps they will pay to have it calibrated in Chile as their specs are guaranteed for a year.

author: Lightages
author_rank: Unknown
timestamp: April 06, 2011, 09:21:05 pm
content: Quote from: saturation on April 06, 2011, 11:59:39 amI have read that post and it does show the quality control on the Unitrends are not all that great. But when I see Dave's reviews on multimeters the Unitrends seem to be OK in his accuracy tests. I might be naive, but when a big company states a certain spec and then also provides a year guarantee to cover it, I expect the specs to be close, not an order of magnitude out.

author: Unknown
author_rank: Unknown
timestamp: April 06, 2011, 09:41:40 pm
content: Dave only tested a single sample of each, you need lots of samples to get any meaningful statistics about quality control. Dave also didn't test long-term stability (takes lots of time, obviously).Quote from: Lightages on April 06, 2011, 09:21:05 pmBig companies are just as likely to produce crap as small ones. You might expect quality brand products to easily meet their specs, but Uni-T is not at all in the same class as the likes of Agilent, Fluke and Gossen. There also seem to be fewer (no?) complaints about the stability/accuracy of brands like B+K Precision and Extech, although this may be because they're less popular.

author: Lightages
author_rank: Unknown
timestamp: April 09, 2011, 05:02:19 am
content: For those of you still interested, I have a reply from UniTrend on how to calibrate the meter."Thanks for your feetback very much,I want to know the real reason of the "lower" value in order to how prevent from it happen again,but very sorry,I can't open the video of your website.I notice that there are some precise calibration source in the picture of your website,so I believe that you should be able to calibrate the UT71E if you know how to adjust it.I send the calibrating istruction for UT71E as an attachment ,For details, pls see the appendix.ThanksFlyhone Tsang"I have attached the file so that it can be of use to others. If adjusting the basic level trimpot gets rid of the offset, then I will be completely happy with the meter. I will be buying a voltage reference as I probably should have one anyway.And I have have now included the file in Open Office or Libre Office format for those of you who prefer it in that format.

author: saturation
author_rank: Super Contributor
timestamp: April 09, 2011, 09:29:28 am
content: Amazingly simple; it has a trimmer and close case calibration? This calibrates all ranges, including Vac, ohms etc.,? If just Vdc that's adequate, but if all ranges, that's impressive.Studying this, thanks for the files will report later. What most important is if you have limited access to test gear in Chile, being able to calibrate DIY in your own lab gives you a way to compensate for the Uni-Ts faults. Even before you get the voltage references, you could adjust the Uni-T to match your trusted meters and see how it holds across the range.Quote from: Lightages on April 09, 2011, 05:02:19 am

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 09, 2011, 11:01:26 am
content: great job! thanx for sharing.

author: Unknown
author_rank: Unknown
timestamp: April 09, 2011, 12:57:26 pm
content: Quote from: saturation on April 09, 2011, 09:29:28 amLooks like they have no ability to adjust VAC, resistance or current, these are just performance checks (although they don't actually document these, only VDC is mentioned).Quote from: saturation on April 09, 2011, 09:29:28 amThis does appear to require a stable source with the exact values (within accuracy specs / 5 or so) required.

author: saturation
author_rank: Super Contributor
timestamp: April 11, 2011, 06:43:53 pm
content: On the cheap, its possible to adjust any meter with non-metrological sources so long as the source is stable for a few minutes, and the reference meter is equal to or better than the DUT. For accuracy, the reference meter needs to be calibrated. Its not a preferred way because its non-standardized and difficulty to deploy to metrology houses.For example, if you have a 5.00V source but if you measure it down to the uV its stable for 5 minutes say e.g. 5.0015V, you can measure it with reference meter, then DUT, adjust, measure again with DUT, reference etc., and repeat the cycle to insure at any time, the reference meter value = DUT value regardless of what the source is showing say 10 minutes later; you've now transferred the reference meter's accuracy to the DUT for this range. If you have a calibrated meter with lesser resolution, and the DUT has higher resolution, you'll transfer the accuracy of the lesser meter. If the source is too unstable at the uV level, but stable at mV level, then you're limited to the resolution of the source that is stable, that is mV, regardless of the reference meter.The whole point of this exercise is to prioritize accuracy over resolution.

author: Lightages
author_rank: Unknown
timestamp: April 11, 2011, 07:44:19 pm
content: Because I live in the middle of a tech vacuum here, it is best that I have a couple of voltage standards to check my own meters. With this in mind I will be buying the necessary standards to do my own calibrations up to the 40V scale. The higher scales will be calibrated using additional measured smaller voltages (measured by the now partially calibrated meter) in series. I know this will not be ideal, but it will work for me.

author: saturation
author_rank: Super Contributor
timestamp: April 11, 2011, 08:47:53 pm
content: Yes, that's precisely it. When I lived in the Pacific, we had next to nothing [ worse in the 1980s] and getting gear took weeks or months before it ever showed up [if it didn't get stolen or damaged in transit].I finally saw it 'suggested' when I received my Agilent 1252a, its actually in the calibration page of the manual. I don't often see it mentioned anywhere. Here the Agilent folks suggest the 3458a, probably one of the last 8.5 digits around, as the 1252a has a resolution of 1uV, but it may be possible to use a lesser meter for the other scales.Quote from: Lightages on April 11, 2011, 07:44:19 pm

author: Unknown
author_rank: Unknown
timestamp: April 11, 2011, 10:32:04 pm
content: Quote from: saturation on April 11, 2011, 06:43:53 pmSure, that works, as long as short-term stability and noise is good enough and you can generate the value it wants. This can actually be tricky at higher voltages and for AC. Do you have a stable 1000VDC source? Or something like a stable 300VAC 10kHz source? I've actually adjusted the 30V range of a multimeter with something like 7Vrms because that was the max. output of my only stable function generator. Not great for accuracy, but the previous cal constants had a slope of zero (result was 10V, regardless of input), so at least the accuracy got much better.Quote from: saturation on April 11, 2011, 06:43:53 pmI've done this, I agree it works fine (although I wouldn't consider that meter calibrated). I don't like to invest much in calibration sources because they are basically single-purpose devices. Something like a multimeter is much more versatile, so I've always had to improvise. The more resolution the meter has, the more interesting this exercise becomes. Lab supplies are not very stable if you're measuring down to the µV level, and 1/4W resistors are completely unstable (not even for a few seconds) due to self heating from the test current. But fortunately I don't think the UT71E is in that territory.The issue I see with the Uni-T meter is that the supplied calibration procedure suggests to me that the trimmer is only for the lowest VDC range, and the other ranges just require a certain input value. It doesn't show the option of entering an arbitrary value it should expect. In that case, you have to generate the 5.0000V +/- .03% or whatever the spec is, at least for a few minutes. Good luck doing that with some random LM723-based lab PSU. Even getting a potmeter to sit exactly on that value is hard, most pots have a fairly limited settability, even the multi-turn ones.Quote from: saturation on April 11, 2011, 08:47:53 pmThe 8.5 digits may not actually be necessary (a 6.5 digit ~20ppm meter should be enough), maybe they just recommend it because most cal labs tend to have a reference multimeter like that around. But yes, extremely linear multimeters have largely replaced the old way of dividers and null meters. A stable source (although they probably don't abuse a lab supply to do it) and an accurate multimeter is all you need in that case. Sometimes even the pros struggle with accuracy, I think the calibration procedure for the HP 3468A 5.5 digit bench meter states that the reference multimeter (I think it was an HP 3455A) should be calibrated within 24h before calibrating the 3468A, because the 90 days accuracy spec was not good enough.

author: nukie
author_rank: Frequent Contributor
timestamp: April 12, 2011, 06:54:18 am
content: Thank you very much for sharing the calibration instructions. I followed the youtube clip and calibrated my UT71D meter along side with my Fluke 8840A and now it is spot on. I only have a 60V power supply at this time so I only did the 1.9V and 19V, you will notice the reading changes as soon as you press the hold button. You don't have to mess with the RANGE key as you increase the voltage to 19v, the auto-ranging will take care of it. I pressed hold many times at the 1.9000v it doesn't affect the higher ranges. The meter is quite clever not to store 1.9000v into the 19.000v calibration.The low range VDC measurement is now *calibrated* and higher range voltages remained the same. I didn't have any issues with the higher range voltages so I am going to leave as is. It's quite fun, now I can calibrate it as regularly as I wish.There are different instructions for ACV and also Ohms calibration on youtube.

author: saturation
author_rank: Super Contributor
timestamp: April 12, 2011, 01:24:49 pm
content: Yes, its not doable on all meters; the Fluke 87V for example, will not allow partial calibrations: doing only one or 2 ranges ... its all or none. However, older Flukes in the series with pot adjustments you had free reign in choosing not only which range but what reference voltage to use. The meter's linearity took care of the rest. After the DIY calibration you can check values across the entire range of the meter from mV to kV to insure it matches your reference meter. But truly, Fluke 80 series work well in heat and humidity, it never needed adjustment in 10 years, the only problem was performance checks, who was off, the Fluke or the reference voltage? In the Pacific temp would swing from 75F to 120F daily, in the shade.High frequency and voltage AC sources are not easy to come by, and its tricky doing the high current calibrations on AC and DC. The good news, accurate AC isn't as critical in my need as DC. But in the olden days we'd use AC voltage regulators [ typical in the boonies to run gear like PCs] and under no load provides stable 120-240Vac output at 60Hz typically down to 100mVac, most stable at 1Vac; that's what was available easily.You can get stable low cost kV DC sources by looking for surplus electrophoretic power supplies, they deliver 3-6kV, old ones to 600Vdc and stable to the mV. I got one on eBay for $10. The stability is important because protein electrophoresis requires a stable reliable voltage source or the migration will be off; these power supplies are dumped by biological labs every year, they cost $1K-10K each new, and well, that's bioresearch!Many new batteries, and my favorite now is eneloop NiMH LSD, have stable voltage outputs to the 10uV, even 1uV for a few minutes, particularly when using very high input impedance meters. Fresh charge them, let them 'rest' for a day before using them as a votlage reference to transfer measurements between 2 meters. You can series them to go from 1.2Vdc to XVdc but it gets noiser as the contacts and uV errors magnify with each series cell.Precision resistors are stable to the 1-10 uV level, for many minutes, stock up on them when you find < = 0.01% and the higher the wattage, the less likely to drift due to thermal and electrical heating. For best results use as low a voltage as possible for generating mA you need for cal. Values in the mid Kohm are best, as megaohm types are noisiest and single digit ohms require too much power.As for using resistors for precision voltage dividers you can too, but the weak link is the quality of the connection between any 2 resistors, soldering is the best bet.Quote from: alm on April 11, 2011, 10:32:04 pm

author: pklawit
author_rank: Contributor
timestamp: June 10, 2011, 11:08:24 am
content: Hello Experts!I have bought a Voltcraft VC940 DMM recently. According to what I have found over Internetand to the label on the circuit board it is in fact UT-71E manufactured by Uni-Trend.I would be very happy about that DMM as comparing to my old, 20 years old Metex.But, there are two (or probably only one) problems, that I have found:1. It does not show 0 on the mV range, when the probes are shorted.Actually it shows -14,34mV instead of 0,0.2. On the temperature measurement with K probe connected it displays -363,42 degrees Celsius,or about -630 degrees Fahrenheit. I believe this error comes from the first one,as the K probe generates milivolts.I have found and tried to use the calibrating procedure described in this thread,but the problem is, that it is not valid for the mV range, I believe.The procedure says, that on the 400mV range the VR2 resistor should be used to set the upper limit (199,V)and then for higher ranges, the "software" calibration should be used.But the procedure does not say anything, how to reach 0.0 readings on the 400mV range.Anybody knows how to do that?Problem is only on the 400mV range and temperature.For other ranges DMM shows correctly 0.0 when the probes are shorted.Regards,Piotr

author: pklawit
author_rank: Contributor
timestamp: June 10, 2011, 11:22:41 am
content: Hello again,I have found kind of "workaround" for the 400mV range :-)When I short the probes and it shows the buggy reading, I can press the REL button,so from now it shows 0,0 and all readings are relative to the zero-offset.Unfortunately this trick does not work for the temperature measurements :-(Regards,Piotr

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 10, 2011, 02:23:40 pm
content: Piotr do a favor to your self and read the Users manual .And if you have lost it , download the one for the UT-71E.

author: pklawit
author_rank: Contributor
timestamp: June 10, 2011, 05:09:55 pm
content: Hi Kiriakos,Nice try.I have read the user manual. I have the original delivered with VC940.Should I look for anything particular there?Piotr.

author: pklawit
author_rank: Contributor
timestamp: June 10, 2011, 05:18:08 pm
content: Hi Kiriakos, All,I did addidional test.I have set bench power supply to 0,190V, so 190mV.Then I have connected Fluke 73 and the VC960 to the supply in parallel.Fluke set to Volts range shows 0.190 VDCVoltcraft set to Volts range shows 0,1903 VDCFluke set to milivolts range shows 190,3 mVVoltcraft set to milivolts shows 175,4 mVSo, the Voltcraft substracts exactly the 14mV offset, that I have noticed shorting the probes.So in my opinion it really needs some adjustment for the zero setting on the milivolt range.Regards,Piotr

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 10, 2011, 06:09:26 pm
content: PiotrThis thread is all ready three pages long.And the key point is that if you do not own, an reference DC source.You have nothing in your hands as comparison point.And all your current tests haves no point at all.If you do not spent money about building something like this ..https://www.eevblog.com/forum/index.php?topic=1148.0or buy this ..https://www.eevblog.com/forum/index.php?topic=1032.0Forget the calibration process.

author: Lightages
author_rank: Unknown
timestamp: June 10, 2011, 10:18:53 pm
content: With your offset, it appears that you might be able to remove that offset by adjusting the trimpot. I still have not tried any calibration or adjustment of mine until I have a way of checking things correctly.Although Kiriakos might be a little blunt, he is correct in that adjusting blindly to something else that is not known to be correct too is just fooling yourself.

author: Unknown
author_rank: Unknown
timestamp: June 11, 2011, 02:16:14 pm
content: Adjusting it to a random bench supply may not be a good idea, but these tests clearly show that there's something funny going on. The results of a second DMM connected in parallel show that the source is stable down to at least the 0.001mV.Kiriakos is clearly in denial if he claims that these tests are worthless. Doing a performance verification with inferior equipment is possible. You will not be able to verify the performance down to the specified accuracy, but where not talking about 0.1% differences here.Stupid question: they didn't happen to specify the mV accuracy as +140 digits, did they?I would complain to Conrad and try to get it repaired/replaced, either the calibration is out of whack or it is defective, most likely the latter. In DCV mode, shorting the probes should result in a value very close to zero.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 11, 2011, 06:27:39 pm
content: Well alm it is known that I always enjoy one good pillow fightAnd so I will not miss even this one.I bet that by reading the word Fluke 73, you did melt like an ice cream.Memories and pride chocked you to the point, so to send at the hospital the fresh low cost one,instead the old beaten hand held. (condition unknown)Personally it took me long time in this forum,to self educate my self about performance verification standards.And I did spent some cash in the way about getting this chance of even touching the magic stuff.And today you are coming back to say that all that I need is a cheap LM7805 ?Today, by looking the active DHL tracking of the Agilent U1272A that comes as freeby,I had become for an third or fourth time an believer !!!There is a God out there , helping the ones who shows denial to use the LM7805 as reference standard.https://www.eevblog.com/forum/index.php?topic=3232.0The pillow fight ended ( Remove your helmets )

author: pklawit
author_rank: Contributor
timestamp: June 11, 2011, 06:43:54 pm
content: Thanks to all for the conclusions/suggestions,First of all, this is not a new unit, bought on Polish ebay.I may return it, but the price was quite attractive and I would not be able to buy anotherone for similar price. And apart of the mV and temperature it is perfect. Looks like new.Second of all I thought, that if the problem is only with the offset on the mV range, shorting the probesand using proper pot to get 0.0 on the display would solve everything, as the offsetseen for 0 and max of the range is the same.Third of all, I'm working for a big company that has a department responsible for periodiccalibration of all used meters (the Fluke that I was comparing readings is calibrated by them each year),and maybe they could help me with the VC-940 calibration, but I would like not to mess every rangeand only use the pot responsible for the zero offset on mV range.There are 5 pots on the board, VR1 to VR5.VR2 is for setting the upper limit on the mV range. So there are 4 remaining.Nobody has the schematic of the UT-71 ?Thanks and greetings,Piotr

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 11, 2011, 07:20:04 pm
content: Piotr just answer to me this simple question.What stopping you from contacting directly UNI-T by email, and request the info ?At the page 2 , another person did just that, and got plenty of help by the company.Even so next to every VR there is markings on the PCB .According to my magic sphere by looking an similar one,you should have there markings likeDCVACVACV0CAPJust by looking at them, you can tell what its one does.

author: pklawit
author_rank: Contributor
timestamp: June 11, 2011, 07:23:37 pm
content: Look at this :-) :http://www.uni-trend.com.cn/feedback.asp?yy=%D6%D0%CE%C4&lmm=&lmbs=&page=359Google translator did great job here:Moderators Reply to: 2006-12-11 8:47:03Hello:UT70B drawings have email to you, please find aDebugging components in this table is as follows (if no professional calibration equipment, do not secretly debug):DC V VR1/2KAC V VR2/10K? low temperature VR6/1K VR4/3K? low temperature VR7/1K VR5/3KUNI-T Customer Service CenterWould be good to have at least such list of VRs for the UT71 series.Regards,Piotr

author: pklawit
author_rank: Contributor
timestamp: June 12, 2011, 09:29:24 am
content: Hi Kiriakos,Unfortunately there is no any additional info near the pots, just the VR1 - VR5.I have already emailed Uni-Trend twice - once via email on their WEB pageand second time on their forum. Ok, I will wait a bit longer, maybe they'll respond.Greetings,Piotr

author: pklawit
author_rank: Contributor
timestamp: June 12, 2011, 07:08:32 pm
content: Ok, I will answer my own question, maybe it will be useful for somebody else.I've found this in the calibration procedure for Voltcraft VC920/940/960:VR1 - ACA, to set 1,0A current, 60Hz on the 10A rangeVR2 - DCV, to set 190mV on the 400mV rangeVR3 - ACV, to set 2,0V voltage, 60Hz on the 4V rangeVR4 - temperature - to set ambient temperature when probes are shortedVR5 - ACV, to adjust AC voltage readings less than 50 digitsUnfortunately none of them is responsible for setting the zero-offset on the DC milivolts range.I hope that on Monday Uni-Trend will respond to my email.Regards,Piotr

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 12, 2011, 08:04:09 pm
content: Piotr I have the impression that you are a bit confused.Why in earth one DMM with relative function to need an pot so to set the DCV0 ?Why you trust more one DMM with out relative mode ?QuoteIt could subtracts as much it needs so bring down any interference measured as voltage at the range of 0mV.Your old Fluke does not subtract anything.The only way to test one fresh multimeter are with another fresh one next to it.I was hoping that some else will jump in so to inform you,but I did not see any one.My prediction is that UNI-T will say the same.That the setting of DCV to Zero is an automated process , no pot or setting needed.

author: pklawit
author_rank: Contributor
timestamp: June 13, 2011, 08:25:55 am
content: Hi Kiriakos,Well, I just don't think, that setting the DMM to zero on the mV range with the REL functionshould be a normal task before each measurement.In fact the resolution on the Volts range is just 10 times worse than on the mV range:- on the Volts range when probes are shorted it shows 0,0000 V (so with 0.1mV resolution),- on the mV range it shows -014,26mV (so 0.01mV resolution)Why then on the Volts range it goes properly to 0.0000V and on the mV range it shows-0.01426V?.I do not have to use REL function to get 0 on the Volts and I think it should be the same on the mV.And the second problem - the temperature readings - in my opinion it comes exactly from thezero-offset on the mV range, thus using the REL function on the mV range will not solvethe temperature readings (-360 Celsjus degree - this is a joke I believe).PiotrStill waiting

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 13, 2011, 11:43:34 am
content: Quote from: pklawit on June 13, 2011, 08:25:55 amIf you do have electrical interference in your test lab or room, yes it is.And only the sensitive DMM will detect it.

author: pklawit
author_rank: Contributor
timestamp: June 13, 2011, 12:21:55 pm
content: I have called UNI-Trend service in Poland.According to them when probes are shorted, on the mV range meter is allowed to bouncefrom -0.02 to +0.02mV. Something like -14mV is unacceptable.They have told me, that some calibration data like the zero offset are stored in the processor'smemory and can only be changed by connecting programmer to the processor.In their opinion the memory has been erased or corrupted by electric shock for example.In Poland they do not have equipment to fix it and the only thing they do is to replace the meterif still under warranty.So, I'm returning my to the seller, as there seems to be no chance to get it fixed by simple calibration tricks.Regards,Piotr

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 13, 2011, 04:31:48 pm
content: Quote from: pklawit on June 13, 2011, 12:21:55 pmDo not forget to give a nice red dot to the seller, and write the reason and the DMM model on his feedback.Your story sounded fishy from the start, but it was not your fault.I am sorry for this turn out, many kids they sale on ebay their half damaged goods.Take care.

author: pklawit
author_rank: Contributor
timestamp: June 14, 2011, 05:58:42 pm
content: 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 meteror 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 pleasedand impressed, that the Uni-Trend company responded.Sure, such thing as bad component should not happen, but the willingness to help one singlebuyer (which does not even have a warranty nor receipt) is really priceless.Regards,Piotr

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 14, 2011, 06:12:03 pm
content: Oh my this schematic its atrue 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.

author: Lightages
author_rank: Unknown
timestamp: July 28, 2011, 06:36:22 am
content: I ordered and received a DMM Check from Doug athttp://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?

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 28, 2011, 10:58:32 am
content: Quote from: Lightages on July 28, 2011, 06:36:22 amI 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.

author: Lightages
author_rank: Unknown
timestamp: July 28, 2011, 03:32:02 pm
content: Quote from: Kiriakos-GR on July 28, 2011, 10:58:32 amWell 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.

author: Bored@Work
author_rank: Super Contributor
timestamp: July 28, 2011, 05:46:59 pm
content: Quote from: Lightages on July 28, 2011, 03:32:02 pmHe usually talks out of his arse to get attention. Ignore him.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 28, 2011, 07:35:08 pm
content: Quote from: Lightages on July 28, 2011, 03:32:02 pmIf you become jumpy so easy, there is no need to say more.You came to post and brag, well done mission successful.

author: saturation
author_rank: Super Contributor
timestamp: July 28, 2011, 08:23:23 pm
content: 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

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 29, 2011, 12:30:35 am
content: I love challenges ..Enjoy : David and Goliath ( Full HD with Dolby surround) LOLhttps://www.eevblog.com/forum/index.php?topic=4001.msg52785#msg52785

author: nukie
author_rank: Frequent Contributor
timestamp: July 29, 2011, 02:31:43 am
content: 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 theSZA263/LTFLUsame 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 theseLTZ1000 boardsand 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 visithttp://www.eefocus.com/lymexbg2vo/blog/09-12/181353_9b971.htmlhttp://www.ourdev.cn/bbs/bbs_content_all.jsp?bbs_sn=3908863These 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?

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 29, 2011, 11:45:36 am
content: 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.

author: saturation
author_rank: Super Contributor
timestamp: July 29, 2011, 01:51:57 pm
content: 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

author: nukie
author_rank: Frequent Contributor
timestamp: July 30, 2011, 03:11:14 am
content: Quote from: Kiriakos-GR on July 29, 2011, 11:45:36 amLT1013 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 aremany variantof these LTZ1000 reference boards designed by different individuals but they are loosely based on thefirst 10V reference boardby lymex/bg2vo he's also a camera enthusiastic / laser diode man.East meets the West.

author: Lightages
author_rank: Unknown
timestamp: July 30, 2011, 04:12:06 am
content: 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.

author: nukie
author_rank: Frequent Contributor
timestamp: July 30, 2011, 04:16:17 am
content: 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.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 30, 2011, 01:39:25 pm
content: 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.

author: Unknown
author_rank: Unknown
timestamp: July 30, 2011, 02:55:58 pm
content: Quote from: Kiriakos-GR on July 30, 2011, 01:39:25 pmMost, 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.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 30, 2011, 03:39:30 pm
content: Quote from: alm on July 30, 2011, 02:55:58 pmWell 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 insidehugemechanical 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.

author: saturation
author_rank: Super Contributor
timestamp: July 30, 2011, 03:45:53 pm
content: 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

author: Lightages
author_rank: Unknown
timestamp: November 27, 2011, 06:45:11 am
content: 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.

author: fmaimon
author_rank: Unknown
timestamp: November 27, 2011, 03:21:40 pm
content: Quote from: Lightages on November 27, 2011, 06:45:11 amThank you Lightages. Unfortunately, I still can't see the files...Can anyone see them or it's just me?

author: Lightages
author_rank: Unknown
timestamp: November 27, 2011, 03:28:00 pm
content: Resent in my post. I don't know why they didn't show up.

author: fmaimon
author_rank: Unknown
timestamp: November 27, 2011, 05:16:15 pm
content: Thank you again.

author: saturation
author_rank: Super Contributor
timestamp: November 28, 2011, 05:59:11 pm
content: 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

author: Lightages
author_rank: Unknown
timestamp: November 28, 2011, 06:28:16 pm
content: 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.