Precision weight scales Kern KB 650-2NM - Page 1
Forum section: electronics / general

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 11, 2024, 02:34:00 pm
content: A friend of mine dropped his precision scales, I've been asked to see what I could do.He'd already taken them apart and apparently flicked the calibration switch to the adjustment position which may be my main problem now.After their warm-up time the scales display "==================" which means insufficient weight to measure.Hitting "TARE" has no effect in zeroing the display.Adding weight to the platter has no effect until the a point where the display comes up with "==Overload=="The PW6CC3MR 3kg load cell has an output of 110µV with no load (bare, all platter supporting parts removed) and an output of 1130µV when the display switches to overload. (I'll have to check what weight that was...)Knowing they will need readjustment in the end anyway, I'm trying to get them through the calibration to see if that will return any measurements at all, the scales ask for a reference calibration weight, I've tried random objects beneath the overload limit but always get a "wrong weight" message.Anyone know the weight expected for the calibration procedure on this model?

author: HighVoltage
author_rank: Super Contributor
timestamp: February 11, 2024, 02:41:23 pm
content: This is a "low cost" scale from KERN and a few years ago I also had a very similar model that was broken and needed repair. I gave up, when the low quality wires broke at the solder points.I was very disappointed in the quality of this Kern scale. These low cost scales have nothing to do with the good quality scales that Kern is building.Nevertheless I think you need a 500 gram weight for calibration. And when you start the calibration process, it will ask to add this weight and then to remove it again.

author: Kean
author_rank: Unknown
timestamp: February 11, 2024, 02:51:14 pm
content: Check out the operation manual athttps://dok.kern-sohn.com/manuals/files/English/572-573-KB-DS-FKB-FCB-KBJ-BA-e-1673.pdfOn page 13 it lists the KB 650-2NM and shows a recommended adjustment weight of 600g (F1 grade).The adjustment procedure starts on page 25. The procedure seems a little unclear to me.

author: mendip_discovery
author_rank: Super Contributor
timestamp: February 11, 2024, 03:16:48 pm
content: Usually the procedure is basic terms is unloaded platten, it will beep to say it's got the zero point, load with 600g then it will beep again to say it's got the load reading. Then it resets and all is ok.Issues are that dropping that range of scale could have caused it to overload the load cells.It's not accurate as a f1 weight but it will get you close is to use a measuring jug with 600ml of water.

author: Kean
author_rank: Unknown
timestamp: February 11, 2024, 03:35:17 pm
content: Yes, a simple two point calibration of zero and 600g makes sense, but the document mentions a few things that make the process unclear.- "Recommended adjusting weight F1 (not supplied) 600 g" - Presumably this is a set weight for this model.- "Adjustment points 200/500/600 g" - So can you actually do a 4 point calibration?- "With verified balances, this precision adjustment is not possible." - I assume this is referring to certified scales for trade.Section 6.8.1 tell you to enable adjustment and then refers you to 7.2 (page 31) which I see now points with an arrow across to the next page (page 32) which shows the button sequence.If you cannot TARE at the start then I doubt you will be able to proceed any further with calibration.

author: mendip_discovery
author_rank: Super Contributor
timestamp: February 11, 2024, 06:10:31 pm
content: Quote from: Kean on February 11, 2024, 03:35:17 pmF1 is a grade of weight. It just means it is 600g +/-2.5mg. E1 is better at 0.25mg but they are expensive.https://www.norfolk.gov.uk/-/media/norfolk/downloads/business/norfolk-calibration-services/maximum-permissible-errors-for-weights-chart.pdfQuote from: Kean on February 11, 2024, 03:35:17 pmIn some cases yes, but it might also be that it allows you to calibrate with a choice of these weights incase you don't have a F1 600g.Quote from: Kean on February 11, 2024, 03:35:17 pmSuspect so as adjustment voids the certification.Quote from: Kean on February 11, 2024, 03:35:17 pmThis is because they are trying to use one manual for a range of devices, and secondly, because there are some translation issues. It took me a good 30mins of fiddling to get a set of scales to adjust on Friday, it would error out when asked to set, turns out you need to set the display from the last digit of 1 to 5 then you can adjust, then set it back to 1. I think the customer read the manual and thought they could get an extra decimal point and got it wrong so it ended up in a error state.

author: Kean
author_rank: Unknown
timestamp: February 11, 2024, 06:25:49 pm
content: I own a set of F2 grade weights up to 5kg, but those are primarily for verification.I am not using them for calibration, at least not for others, but I do design and build weighing platforms for wildlife.With the 3 weight choices, I was just wondering if it prompts as part of the process, or if it has predefined bounds for each one. Having implemented this stuff myself just make me curious.Translation is definitely an issue with this kind of guide, so it does make sense the way it has been documented. Even so, I've experienced similar issues where a calibration process isn't clear and you spend ages fiddling with the device trying to follow the steps and worrying that you've just screwed it up in a non-recoverable way...

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 12, 2024, 07:04:25 pm
content: Thank you all for your input.Yes the engineering put into this is really deceiving considering the retail price...I tried with 500g and 600g weights... as Guessed by Kean no way to get through a coarse calibration.I just don't understand what has hit these scales. AFAIK the load cell makes sense unloaded but I'm not sure about slope.The latest measurements are approx 50µV no load for 5.025V cell bias voltage and "overload" point 1140µV.That's approx 1090µV delta for 333g delta.The load cell output is sent to a CS5532BS ADC on AIN1+/-Load cell bias voltage seems also to be monitored by the ADC on AIN2+/-ADC Vref differential measured at 5.017VAlthough I haven't looked at the actual data values the ADC's SDO changes with weight.So unless the STM microcontroller has decided something had changed too suddenly between consecutive power-ups (it does seem to say it writes to NVRAM every time it is switched off) or data has corrupt on a wild power outage I'm at a loss for an explanation.

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 13, 2024, 12:07:45 am
content: Played around with the RS232 a little in case that would teach me anything, no luck.I wish I could factory reset or rewrite the MCU flash being convinced nothing is wrong with the hardware. A quick look at Kern's website gives me the feeling there's no hope in obtaining anything from them other than a large bill.I'm afraid I'm putting this back together and forgetting about it.+1 expensive paperweight to the collection.I hope they get the reputation they deserve for selling this kind of product

author: HighVoltage
author_rank: Super Contributor
timestamp: February 13, 2024, 09:39:48 am
content: Quote from: shakalnokturn on February 13, 2024, 12:07:45 amThat was exactly my feeling a few years ago.Kern used to build only high quality scales but that has changed.I am not even touching them with a stick.These days I have concentrated on Mettler Toledo and Sartorius, they still make good quality scales.

author: vindoline
author_rank: Unknown
timestamp: February 13, 2024, 01:57:41 pm
content: Quote from: HighVoltage on February 13, 2024, 09:39:48 amAgreed. I would also add Ohaus

author: r6502
author_rank: Regular Contributor
timestamp: February 13, 2024, 07:08:33 pm
content: Hello,I also designed some scales, useed to fill a defined amout of liquid with help of a pump in small glas vials. During design process, we have boken a few load cells, for example by dropping the load cell from the table on the floor.The loadcells where all defect after they have been droped. Most error we saw, that the dropped load cells where no longer stable, both in the zero point and with a weight on the scale. It is just driting over time - here 600 g load cell driftts by up to 0,5g in a time of about 10 sec. But we had also seen, that the loadcell was that mutch stressed when it was dropped, that the unloaded output voltage was out of the sprcification of the load cell. Typical the scale has a defined range for zero load, and when the load cell is out of this range the scale generates an error.We used the following load cells from Tedea-Huntleigh, now part of Vishay:http://www.vishay-loadcells-intertechnology.com/pdf/tedea/single-point/1004.pdfCan you replace the load cell?GuidoEdit: link added, spelling

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 13, 2024, 07:38:13 pm
content: Thanks Guido, that's interesting.The load cell can be found separately although not that cheap so I wouldn't be buying one just to see. Besides that I'm suspecting that even if the original is damaged it won't be as simple as just replacing it and going through the calibration sequence.What I did try was shorting the load cell's output terminals while leaving it connected to the electronics. Still there was no way to tare the display to zero.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 13, 2024, 08:00:04 pm
content: From about 2004, I have a KERN KB 800-2, 810g max, d=0.01g and an M1 mass set for FS adjustment.Latter is not possible for calibrated scales, KERN calls this "verified scales", for commercial transactions and alike. This - 2NM refers that it is "verifiable", which drives the price upwards.My scale does not have this "adjusting (enabling) switch" on the bottom side.The adjustment process requires only two steps, first "Calibration Zero", then placing 600g for "Calibration Span".I'm very satisfied, as it was quite cheap, linear (0.02g), and does a good job for the low price.The error description might hint to a different fault.Only if I press TARE, then the display briefly shows: "================" .A missing weighing table displays: "=^=====vv=====^=",but following the manual, an "Underload" should display "= ==============", i.e. with a missing "=".So what happens if you remove the weighing table?That might indicate that something in the electronics, or the keyboard is faulty.I just discovered, that it might feature an internal adjustment weight..do you have a photo of the interior?I don't know how this works, but maybe this weight is flapping in the breeze.Anyhow, if the scale really fell down hard, then the weighing cell is defect, and this would be BER, i.e. not useful to change the cell.Why would you blame KERN that a scale does not withstand such a mis-treatment?I guess, any other brand would also be damaged under such circumstances, but the loss would be much higher.Frank

author: HighVoltage
author_rank: Super Contributor
timestamp: February 14, 2024, 10:23:47 am
content: Quote from: Dr. Frank on February 13, 2024, 08:00:04 pmI was not aware that this scale fell down. That of course can not be blamed on a scale manufacturer.Kern used to make really good scales.And they probably still do on some high end models.And your Kern scale might be one of the good products.But when I got a broken KERN scale a few years ago, that needed repair, I was really shocked of what was inside. It looked more like a cheap chinese made calculator than a real "Made in Germany" product. Everything was made for a one time only assembly. Too bad I did not take any pictures.The total opposite was true, when I repaired some similar priced Mettler Toledo or Sartorius scales.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 14, 2024, 11:20:38 am
content: Quote from: HighVoltage on February 14, 2024, 10:23:47 amHello HighVoltage,that was said in the first sentence: "A friend of mine dropped his precision scales,..."Those "cheap" KERN scales, like mine as well, very probably are designed-to-cost, since 20 years already unchanged, maybe.Therefore, I assume that this KB 650-2NM is really very similar to my old 800-2, especially concerning the display.I agree with you, that the quality is - of course - not comparable to the Mettler Toledo and Sartorius.Latter also use the magnetic force compensation method, instead of the deflection method with strain gauges, as probably used in all KERN scales.I also opened my scale, of course, but also forgot to make photos.What do you assume as being "quality" scales from KERN?These precision µg analytical scales?I guess, there is no basic design difference, apart from having much better sensitivity / resolution and hysteresis / repeatability characteristics with special strain gauge cells.Do you have any additional information about this class of KERN scales, i.e. on what design features you categorize the "high end models"?These "verifiable" scales might cost over 1000€, but probably have the same interior like my 400€ non-verifiable scale.Even their analytical scales with 10µg resolution cost < 10k€. Let's compare that to those other brands.I always found those pure mechanical µ-scales very fascinating, especially at the chemical practical-courses, we had to join during physics diploma programme @ RWTH Aachen university.Frank

author: HighVoltage
author_rank: Super Contributor
timestamp: February 14, 2024, 02:23:25 pm
content: Hello FrankI wouldassumethat a €10k analytical scale from KERN would be build in high quality mechanical way, using a magnetic force compensation method, instead of the deflection method. But I do not know. And since my disappointment was so high in KERN products, I will probably never fin out.On the other hand I have repaired some high end (100 ug resolution) Mettler Toledo scales and when you open them up, it is a total eye candy surprise, just by looking at great mechanical quality of parts.Next time I repair one or open one of them, I will take some pictures.What is overlooked many times, when using a precision scale is the air weight displacement by the part that is being measured. You need to account for the Buoyancy correction.What I also find amazing with a precision scale, is to use a calibration weight and then bring the scale one floor up in the house and notice the difference in gravity.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 14, 2024, 03:33:06 pm
content: For their precision scales, they use their "single cell", which I copy here from their video.That's obviously high grade, but still of the deflection type cells.Either they use strain gauge sensors, or a capacitive method for readout.They still have that high repeatability or hysteresis parameter of 3 x sensitivity, compared to the other methods.Frank

author: David Hess
author_rank: Super Contributor
timestamp: February 14, 2024, 04:24:19 pm
content: Quote from: r6502 on February 13, 2024, 07:08:33 pmOverload will obviously cause a zero shift and damage the load cell.What was never clear to me is that shock without overload can also damage a load cell. Some load cell designs are more resistant to shock, and I think it has to do with the gauges and adhesive, but I never found an answer as to how or why.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 15, 2024, 09:45:26 am
content: I just remembered that I bought a broken Mettler Toledo scale as a parts donor and leftover parts were still around, so I took some pictures.This is Model Mettler Toledo AB204-S Classic- Max 220 g- Min 10 mg- e = 1 mg- d = 0.1 mgHere is a picture of principle of the working mechanism

author: HighVoltage
author_rank: Super Contributor
timestamp: February 15, 2024, 09:49:20 am
content: More picturesThe electrical connecting wires for the coil seem to be made out of pure gold.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 15, 2024, 09:53:39 am
content: This is the internal calibration weight and the loading / unloading system

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 15, 2024, 12:12:14 pm
content: The engineering is definitely nothing comparable to HighVoltage's photos...The scales I have were dropped so it does make sense that the load cell would be damaged if anything. I wasn't around when the fall happened but there are no cracks or dents to be found in the plastic.Quote from: Dr. Frank on February 13, 2024, 08:00:04 pmI can't get any display corresponding to what you describe as an underload or missing platter condition these scales are binary "==========" or "=Overload="There is no internal calibration weight in this model, I'll take some photos IF I re-open them.I don't blame Kern that precision scales would not withstand a drop, that isn't making normal use of them.I do find the build low quality considering their retail price.I'm also bothered by the idea that the scales may react to a supposedly damaged load cell by the software possibly locking out any possible use at all, this makes some sense as it's wise to at least tell the user that the supposedly calibrated instrument is suddenly completely wrong, if that's what they're doing it should be documented somewhere.I wouldn't mind replacing the load cell if I was certain that there was nothing more to it like a possible service mode unlock that I have no access to.How could I tell? Simulate the Load Cell with an external voltage divider?As it is the zero-load output voltage seems within tolerance, note that this is not scales unloaded but load cell stripped from platter and platter supporting parts.If I'm estimating this the right way from my previous measurements and datasheet the excitation voltage shouldn't change much from 5V, with a 2mV/V (+/-0.2mV) sensitivity and maximum load-cell capacity of 3kg that's in the 3.7µV/g range.Going back to my previous measurements of 1090µV for 333g that would be somewhat off the expected value that should be between 1110µV and 1332µV but then again this doesn't take into account the DMM 10MOhm loading, my 333g are far from accurate (cheap kitchen scales) and excitation voltage is also grossly rounded.How would you metrology wiser people go about this ?

author: guenthert
author_rank: Frequent Contributor
timestamp: February 19, 2024, 02:00:53 pm
content: Quote from: HighVoltage on February 14, 2024, 02:23:25 pmAnd if you return it to its former place, it'll show another value yet again ... but not due to gravity. If you really can measure weights with an uncertainty of less than one part per trillion, the BIPM wants to hear from ye. ;-)

author: David Hess
author_rank: Super Contributor
timestamp: February 19, 2024, 05:25:48 pm
content: Quote from: guenthert on February 19, 2024, 02:00:53 pmStrain gauge load cells can be sensitive enough to pick up variation of gravity as you move around the surface of the Earth, but not from moving vertically within a building. We once hired someone with a gravitometer to measure the local gravity were we calibrated our load cells.