mg scale - interesting effect - Page 1
Forum section: electronics / projects

author: JBeale
author_rank: Frequent Contributor
timestamp: March 23, 2019, 09:32:42 pm
content: Somewhat tangential to electronics but... I wanted to build a FMES seismometer and the instructions called for a 0.05M K2HPO4 solution as part of the electrolyte, implying an accurate scale to weigh out a small amount of the dry salt to add to the solution.Amazon has 50g x .001 (milligram) scales around $17 and it turns out, no surprise, that the last digit has no real meaning, although the firmware tries to make it appear realistic when near zero or a standard test weight value. So I stepped up to a $43 ebay "200x0.001g Analytical Balance Scale" with a possibly-effective-looking boxy draft shield around the pan. At first this appeared to work, BUT the reading would randomly jump from 0.000 g up to several hundred mg, then return to zero, even with the draft shield in place and the top-load hole covered. It would also consistently sense the presence of my hand as a ~ 100mg force even just hovering 5 cm above the shield without touching anything. Some tests showed it was not stray light, not vibration and (maybe) not RF. I think it is either electrostatic force or 60 Hz capacitive pickup through the plastic case. I simply wrapped aluminum foil around three sides and the top except for the loading hole, and that completely fixed the problem. FWIW I then tested it for a while sequencing through 100 mg, 0 mg and 110 mg test weights and so far this scale is repeatable at the +/- 2 mg level although its firmware also "cheats" within 10 mg of zero to hide the zero drift.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 23, 2019, 10:43:58 pm
content: Heh, what do you expect, they aren't going to do EMC susceptibility tests at that price. Or if they do, their pass criterion is "didn't catch fire".Those are usually build on a load cell with a very precise (sub-microvolt) amplifier to sense it; it doesn't take much interference to upset the measurement. A well designed one will be insensitive to AC, or well enough filtered to not mind ambient noise. Or it could equally well be noise in the digital logic, leading to flipped bits (which could explain the seemingly jumpy response).Yeah, if foil is making it behave, that's not a terrible solution. Tape it down and make it permanent?Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2019, 10:50:58 pm
content: Sensitive scales can react to some odd effects. However 100 mg is a lot - the odd things happen at 1 mg or less.The hand for example could cause a local convection flow from it's heat and this can look like some weight.Electrostatics can also be tricky, especially in a dry environment, when glass looses it's surface conductivity.Temperature gradients and resulting air currents are tricky too.Paper can also be tricky, as is can change it's humidity and this way gain or loose weight.Depending on the construction variations in air pressure (e.g. wind gusts and closing doors) can be a problem too - even for larger weights.I remember a precision scale having some +0 and -0. So the interval for 0 reading was twice as large as it should have been. So it was best used with some offset. One could also see this as a mild form of cheating.

author: helius
author_rank: Super Contributor
timestamp: March 23, 2019, 11:36:20 pm
content: Do you think the EM susceptibility could be improved by coating the glass shields with a conductive layer (say ITO or the like)?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 24, 2019, 12:26:08 am
content: They used to make mechanical balances that could resolve milligramshttp://collection.sciencemuseum.org.uk/objects/co13992/torsion-balance-to-weigh-up-to-500-mg-balance

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 24, 2019, 12:44:48 am
content: They used to make them way crazier than that.Some of the Mettler single-dish weights from the 50's would have a mechanical BCD lever system to lift and lower wights, and the last digit would be reflected trough a lens in the weighing beam as the torsion or flex in the beam.It might be worth opening one of those ebay scales and filling the plastic enclosure with adhesive copper foil.For solutions however, a better solution (haha) is probably to weigh out your salt for a 100x as strong solution and then dilute x10 twice. Dilution can be made incredibly accurate if done right.

author: Mr. Scram
author_rank: Super Contributor
timestamp: March 24, 2019, 12:50:06 am
content: Quote from: T3sl4co1l on March 23, 2019, 10:43:58 pmTo be fair, not catching fire is a good test if it's going to be the only one.

author: coppercone2
author_rank: Super Contributor
timestamp: March 24, 2019, 01:07:05 am
content: Quote from: ChristofferB on March 24, 2019, 12:44:48 amI have one of those 1 microgram resolution mettlers. It is a dog to use, if anything its a interesting display of how everything is vibrating.

author: Mr. Scram
author_rank: Super Contributor
timestamp: March 24, 2019, 01:26:41 am
content: Quote from: coppercone2 on March 24, 2019, 01:07:05 amIf everything's vibrating, you may want to lay off the coffee.

author: coppercone2
author_rank: Super Contributor
timestamp: March 24, 2019, 01:36:24 am
content: did you ever see one of those scales? If your anywhere but the basement and on a stiff table the mettlers will wobble in the 10 microgram division. It's like averaged envelope noise.It's an optical one, not a digital one. It can pick up heart beat if you press your hands on the table I am pretty sure. It's a big mechanical DAC. It's built with armor (considering what it is) and thick glass planes to block out air etc.They have sapphire knives that sit on top of each other, jeweled orifices, all that good stuff.

author: JBeale
author_rank: Frequent Contributor
timestamp: March 24, 2019, 01:42:37 am
content: Hah, likely "no fire" was indeed the extent of their EMC testing. I had a peek inside this ebay thing and sure enough just a load cell and the top cover is entirely plastic so no surprise there is some pickup. Since it's working OK now with the foil, I'd still say it's a reasonable value for the price, though they should maybe include some foil in the box :-)

author: coppercone2
author_rank: Super Contributor
timestamp: March 24, 2019, 01:43:41 am
content: higher quality ones have equally shitty construction in terms of EMI (200+ $ range)Not china at fault (at least they pretty much visually copied what you can get from America with lower quality plastics). "real' ones will have a common mode torroid around the input DC though.The good ones will also have a much thicker stiffer 'webbed' glass reinforced nylon base (like a cast iron saw table) that connects to the load cell. That one looksveryanemic for 1 miligram. The good ones kinda look like the mothership from Independence Day.http://vignette3.wikia.nocookie.net/id4/images/6/6f/Escape_saucer_and_the_colony_mothership.png/revision/latest?cb=20160821201325The plastic bit in the higher price ones internally is very abrasive and glassy (enough to make it uncleanable with a towel if you spill something on it, you need to use a brush). They make it with a high glass content. It feels similar to fibrous wraught iron if you ever disassembled REALLY old tools before.If the bed flexes you get error. They give you a metal cover plate that you set on top and it rests on the nylon. It's not adhered or fastened in any way.

author: SenseofScale
author_rank: Contributor
timestamp: March 24, 2019, 01:55:58 am
content: Decided to search microgram scales and came across this. The guy should have rehearsed his lines a few times, but I can only guess how awkward I would sound. Interesting nonetheless.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 24, 2019, 02:27:54 am
content: Transplanting the electronics and load cell+steel plate into a largeish low-profile hammond box, tapping holes for the adjustable feet, adding a much needed round bubble level (a balance like this needs to be level!) might actually be worth wile. There'd be room for PSU filtering too. I'm almost tempted to give that a go now.

author: JBeale
author_rank: Frequent Contributor
timestamp: March 24, 2019, 02:50:46 am
content: not visible in the photo as it is in the rear, there actually is a bubble level on this one and it has 4 screw-in adjustable feet. Maybe I was helped by using only very small loads, while it was sitting on a granite countertop but after the foil fix I really did observe +/- 2 mg repeatability (vendor claims +/- 3 mg). I'm not equipped to speak to accuracy but it did read my cheap 10 mg - 100 g test weight series within 1% of marked value.btw I was kind of just using the chemistry experiment as an excuse to Buy More Tools but I also did not have much of the salt to start with, it's a little pricey, so it was going to be a small total weight anyway.The video showing an old panel meter as a magnetic force balance for micrograms is a neat trick!

author: coppercone2
author_rank: Super Contributor
timestamp: March 24, 2019, 03:06:32 am
content: just keep in mind you can do all the electronic stuff right but you still need a stiff enough table. It does not look right.For 50G a decent American scale would have about 3-4 times more material in the Part Being Weighed and the Load Scale. It looks like junk unless the industry changed (doubtful). It also uses the chassis as stiffener. The chassis of a real scale is wobbly compared to the 'anvil'. Your talking a heavy glass plastic part not some little sheet of whatever and some dull zinc assembly.If you got a quality consumer scale and then put it in a hammond box it would be worth it, because they have similar EM problems.Also I have NO idea of the quality of those load cells. It actually looks decent (mainly because its painted) but you better hope its not like a Bochen 10 turn pot. But seriously the load cell interface assembly looks like its not worth dealing with.

author: JBeale
author_rank: Frequent Contributor
timestamp: March 24, 2019, 03:27:19 am
content: This scale is labelled for 200 g although I've only gone as far as 100 g. My +/- 2 mg observation was for sub-gram weights, there is a little more variation at 100g. I have two physical 100g test weights and just now I tried them A - 0 - B - 0 and repeat for six trials. The scale could tell them apart even though they are only about 10 mg different. So maybe the repeatability is +/- 2 mg plus +/- 0.01% of the reading.Code:# Weight A Weight B------------------------1 100.108 100.1232 100.105 100.1183 100.104 100.1184 100.106 100.1155 100.105 100.1146 100.106 100.121

author: coppercone2
author_rank: Super Contributor
timestamp: March 24, 2019, 03:28:32 am
content: thats pretty bad. A good one will be repeatable to like 1mg (and this is kinda shady) with a 50G test weight.Are you letting it run for a while and putting a plastic bowl on top around the weigh plate (or a tupperware over it) to block air interference? did you level it correctly with a bubble center level?between 5 and 6 test B you have more then 6mg

author: JBeale
author_rank: Frequent Contributor
timestamp: March 24, 2019, 03:39:32 am
content: I did note the built-in bubble level and first levelled it on the granite countertop adjusting all four feet so there was no wobble. I am just using the square windshield enclosure that came in the box, with the aforementioned aluminum foil. I put on the weight, waited for the "stable" indicator flag, removed and waited for it to return to zero. It seems to have an automatic "tare to zero" feature if you're within say 10 mg of 0. This feature (?) sort of hides the non-ideal behavior of the load cell, but to be careful I should probably wait much longer at 0 load in between weighing larger loads like 100g that have more effect on the zero setpoint.For sure it is not a true mg-accurate scale, but for sub-gram weights their advertising is true ("repeatability: +/- 0.003 g") and for me it was a value at the $43 price. It is noticeably better than the even-more-mislabelled "50g x .001" scale Amazon has around $18.

author: David Hess
author_rank: Super Contributor
timestamp: March 24, 2019, 04:26:36 am
content: Quote from: Kleinstein on March 23, 2019, 10:50:58 pmLack of + and - zero used to be a premium feature with integrated ADCs. TI helped earn their bad name by releasing a new integrated ADC which was advertised to fix this problem ... except that it did not and they continued selling them for years after they were notified.Quote from: ChristofferB on March 24, 2019, 12:44:48 amI hated those things. When I was going to Cal Poly Pomona, one year everybody in the quantitative chemistry lab failed because some jerk had deliberately twisted the weight selection control too quickly on the lab balances causing some of the weights to fall off inside the mechanism. The teachers knew but elected to just fail entire classes of students (1) which is California schools and politics at its finest.It got better. I happened to work for a load cell company at the time so had access to calibrated weight standards. Bringing the problem up with the teachers and proving it did not go well for me.Quote from: T3sl4co1l on March 23, 2019, 10:43:58 pmIt is more like it takes a poorly designed one to be susceptible to interference. RF decoupling is required to prevent the input circuits from rectifying RF to produce an offset and the input should be integrated over a whole number of power line cycles which is a good idea anyway to produces the needed noise free resolution. A novice design can be identified by the lack of ratiometric operation. The last digit should never show more than 2 digits or +/-0.5 counts.In a good design, the load cells themselves contribute a majority of the error and there is really no excuse for more error than that.(1) Quantitative analysis depends on accurate numbers.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 24, 2019, 07:24:19 am
content: Maybe you can buy one of these just for funhttps://www.ebay.ca/itm/17Pcs-Set-211-1g-10mg-100g-Grams-Precision-Calibration-Weight-Test-Jewelry-Scale/202568482404?hash=item2f2a05be64:g:JTEAAOSwL-1cQCxh

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 24, 2019, 02:06:10 pm
content: I own an old Mettler (H51ar) that can resolve 10 micrograms and use a modern solid state one at work with the same resolution. They're affected by just about everything and the electronic one is worthless if the room temperature is changing or with any kind of forced air heating or cooling system. The Mettler seems a bit more resistant. The fancy $5k one at work also has an option to zero out periodically if the reading is under some small value because drift is a fact of life. FWIW, the old Mettler is about as accurate as the fancy electronic one. There are all manner of "How to use an analytical balance" pages on the 'net and you'll find some interesting subtleties if you read them.

author: JBeale
author_rank: Frequent Contributor
timestamp: March 25, 2019, 02:44:13 am
content: Quote from: Alex Eisenhut on March 24, 2019, 07:24:19 amIndeed that looks like the same weight set that I have. I don't have a standard to compare so I can't speak to the accuracy, but at least they read within 1% of their nominal values according to the scale. The slippery plastic tweezers that are included with the set are awkward with the smaller weights and hopelessly inadequate to handle the larger ones, but you need some kind of tool to lower weights through the hole in the wind screen box on top of the scale. Dropping one of the larger weights on the pan probably won't be good for accuracy. Some larger steel tweezers (forceps?) with angled tips would do better, especially the kind with a grippy plastic coating around the tips.

author: helius
author_rank: Super Contributor
timestamp: March 25, 2019, 03:22:36 am
content: The forceps are included because you really shouldn't touch calibrated weights with your hands, the oils can corrode them enough to change their mass.I don't think those cheap weight sets are even F2 rated.

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2019, 01:26:49 am
content: I have seen old calibration sets which included ivory tweezers for handling the weights.

author: coppercone2
author_rank: Super Contributor
timestamp: March 26, 2019, 03:27:57 am
content: what material is best to handle those polished weights? like the expensive E or whatever grade ones. (well the small ones anyway, that you might not want to handle with glovesCeramic? Polished Steel? Ivory? Some kind of felt lining?

author: JBeale
author_rank: Frequent Contributor
timestamp: March 26, 2019, 07:32:54 am
content: I now use metal tweezers to handle my ultracheap weight set. I don't know how to really minimize mass transfer, but I think that's micrograms or below, not few-milligrams like this scale resolves.I did another set of measurements, this time I used two different "20g" weights A and B. The A unit was about 8 mg heavier than the B unit.I weighed A, back to zero, then B, then A&B together, then removed B to measure A again (shown as "A2"), then returned to zero. I did all this 10 times with results shown below.You can see there is some load cell nonlinearity as A&B together always measure slightly higher than the sum of their separate weights.On average it was 6 mg more, which as a fraction of the total weight is 0.015% so it's small but measurable. The scale does not offer multi-point calibration AFAIK, but if you have a reference mass of similar value, you can mostly calibrate this away.Code:200 g x 0.001g no-name ebay scale25-March-2019trialmassmassbothmassA2-A1nonlinearity#A1B1A&B A2 drift (A&B)-(A1+B1)120.02920.02140.05220.026-0.0030.002220.02720.02140.05520.030.0030.007320.02920.01940.05220.028-0.0010.004420.02920.0240.05220.028-0.0010.003520.02720.02240.05620.030.0030.007620.02920.0240.05720.030.0010.008720.02820.01840.05120.025-0.0030.005820.02720.02240.05720.030.0030.008920.02820.02140.05520.030.0020.0061020.02620.0240.05420.030.0040.008AVG20.027920.020440.054120.02870.00080.0058MIN20.02620.01840.05120.025-0.0030.002MAX20.02920.02240.05720.030.0040.008Range0.0030.0040.0060.0050.0070.006

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 26, 2019, 12:37:07 pm
content: I use a single 50 gram weight at work that was about $800. The tweezers that came with that are useless and would cause you to drop the weight. I handle it with Kimwipes and have considered making a plastic fork that would slip under the head. The tweezer problem seems to be traditional as my little Ohaus weight set at home came with similarly useless metal tweezers about 50 years ago.

author: ebastler
author_rank: Super Contributor
timestamp: March 26, 2019, 12:50:30 pm
content: Quote from: coppercone2 on March 24, 2019, 03:06:32 amHow would that help the precision? I should think that a lightweight and stiff internal mechanism (i.e. high internal mechanical resonance frequencies), combined with a heavy weighing table with dampened, soft suspension (to suppress any floor-coupled vibrations above a very low resonance frequency) would be desirable?

author: JBeale
author_rank: Frequent Contributor
timestamp: March 26, 2019, 03:46:02 pm
content: Typical vibration can be mostly low-pass filtered out but I think the concern is static deflection. I think the load cell wants a perfectly rigid base, and any flexure in the mounting adds an additional shear, torque and/or levelling change to the loadcell leading to a non-ideal response. The thinner the mounting surface is, the more it will flex under load. I don't know about plates or webbed structures but IIRC the deflection of a rectangular beam is proportional to the inverse cube of the thickness, so twice as thick = 8x less deflection.

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2019, 06:04:43 pm
content: Load cells are constructed with a geometry to make them insensitive to side loads and such. This is especially a problem for those intended for platform scales and they may be mechanically or electronically trimmed to provide the proper response under all conditions.This is easy enough to test for yourself; what is the reading with the same weight place at different and extreme points on the platform?

author: JBeale
author_rank: Frequent Contributor
timestamp: March 26, 2019, 08:13:32 pm
content: Quote from: David Hess on March 26, 2019, 06:04:43 pmMy "100g" test mass reads on average 100.098 at the front edge of the scale, 100.104 in the middle and 100.110 at the back edge running through the front-middle-back sequence four times, with a repeatability of 2 mg at each location. So the effect is small but it is above the noise level. When I did my previous experiments I tried to get the masses as close to center as possible.

author: SeanB
author_rank: Super Contributor
timestamp: March 26, 2019, 11:34:17 pm
content: Those eBay mass sets are reasonable, the one I got went through a verification traceable to the SI kilogram, and actually was well within the required specifications for accuracy. The Ohaus set is quite stable, and has been that way for decades as well, because they only get handled with plastic tweezers for the small masspieces (and those have a measured mass down into the nanogram range for the foil pieces, and the 1g is known to 4 decimals as 1.0000g, with an error of 80ug) and the larger masspieces I have a set of cotton gloves and cloths used to wipe them before use, and the gloves for handling them. They are stable as well, because they are handled with care, and not scraped or dropped. They were used for the certification as well, later date (previous week instead of 6 months for the ones on the van) and had been certified by the same national laboratory.For massmeters the cheap ones are really only 12 bit converters, as the 16 bit ADC's inside are too noisy at DC, and even massive oversampling is not going to make them any good, the reference drifts too much, and the internal IC temperature changes too much as well, they are just relying on digital clamping to show a stable zero, while internally it is drifting slowly.Only way to get better accuracy is to power it on for a few hours in a constant temperature environment, and then run your tests. Hard as most will power off after 10 minutes or so. You need a commercial or laboratory massmeter, as those in general will have selectable power behaviour, and in general most will stay on 24/7, so they are the most stable. As well they will have a better ADC inside, probably 18 bits, with much better references, and much better drift performance as well, as in service mode ( however you enable it, varies from manufacturer to manufacturer, but in most modern ones either a set of jumpers on the PCB or a key sequence, thankfully the days of bespoke dongles are seemingly over) you can often get the raw ADC value without zero compensation, so you can see the slow drift.But yes, for your once off application probably better to weigh out 10 times the mass, weigh out the mass of distilled water (more accurate than volumetric measures in most cases, especially without calibrated glassware) 10 times as much and dissolve, then discard the rest, or use it as fertiliser for the garden.

author: texaspyro
author_rank: Super Contributor
timestamp: March 27, 2019, 02:57:06 am
content: Quote from: coppercone2 on March 26, 2019, 03:27:57 amMine are made of polished Unicorn horn.

author: David Hess
author_rank: Super Contributor
timestamp: March 27, 2019, 04:08:58 am
content: Quote from: SeanB on March 26, 2019, 11:34:17 pmIf that kind of design is common, then current engineers need to study how it was done 30 years ago.Reference drift is irrelevant when you use do the conversion ratiometrically. Then a 7815 or 7805 will be fine because the drift and flicker noise cancel out during the measurement.Integrate the input over a whole number of 0.1 seconds to cancel out 50/60 Hz line frequency noise; forget about using a sampling converter unless it is part of an integrating converter and even that is a bad idea.The cheap ones use 12 bit converters because they are the most common high resolution converter available in microcontrollers. They are terrible for this application though with perhaps 10 bits of linearity at best.

author: nixxon
author_rank: Frequent Contributor
timestamp: May 08, 2019, 02:48:29 pm
content: I recall I watched a similar video by Applied Science on YouTube. I found it. It is a 2014 spin off of the 2011 video mentioned earlier in this thread.