Author Topic: mg scale - interesting effect  (Read 5213 times)

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Offline JBealeTopic starter

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mg scale - interesting effect
« on: March 23, 2019, 09:32:42 pm »
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.
 

Offline T3sl4co1l

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Re: mg scale - interesting effect
« Reply #1 on: March 23, 2019, 10:43:58 pm »
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? :-DD

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Offline Kleinstein

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Re: mg scale - interesting effect
« Reply #2 on: March 23, 2019, 10:50:58 pm »
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.
 

Offline helius

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Re: mg scale - interesting effect
« Reply #3 on: March 23, 2019, 11:36:20 pm »
Do you think the EM susceptibility could be improved by coating the glass shields with a conductive layer (say ITO or the like)?
 

Offline Alex Eisenhut

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Re: mg scale - interesting effect
« Reply #4 on: March 24, 2019, 12:26:08 am »
They used to make mechanical balances that could resolve milligrams
http://collection.sciencemuseum.org.uk/objects/co13992/torsion-balance-to-weigh-up-to-500-mg-balance
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Offline ChristofferB

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Re: mg scale - interesting effect
« Reply #5 on: March 24, 2019, 12:44:48 am »
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.
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Offline Mr. Scram

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Re: mg scale - interesting effect
« Reply #6 on: March 24, 2019, 12:50:06 am »
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? :-DD

Tim
To be fair, not catching fire is a good test if it's going to be the only one.
 

Offline coppercone2

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Re: mg scale - interesting effect
« Reply #7 on: March 24, 2019, 01:07:05 am »
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.

I 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.
 

Offline Mr. Scram

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Re: mg scale - interesting effect
« Reply #8 on: March 24, 2019, 01:26:41 am »
I 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.
If everything's vibrating, you may want to lay off the coffee.

 

Offline coppercone2

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Re: mg scale - interesting effect
« Reply #9 on: March 24, 2019, 01:36:24 am »
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.
« Last Edit: March 24, 2019, 01:42:12 am by coppercone2 »
 

Offline JBealeTopic starter

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Re: mg scale - interesting effect
« Reply #10 on: March 24, 2019, 01:42:37 am »
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 :-)

 

Offline coppercone2

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Re: mg scale - interesting effect
« Reply #11 on: March 24, 2019, 01:43:41 am »
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 looks very anemic 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=20160821201325

The 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.
« Last Edit: March 24, 2019, 01:58:06 am by coppercone2 »
 

Offline SenseofScale

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Re: mg scale - interesting effect
« Reply #12 on: March 24, 2019, 01:55:58 am »
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.

« Last Edit: March 24, 2019, 01:59:47 am by SenseofScale »
 
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Offline ChristofferB

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Re: mg scale - interesting effect
« Reply #13 on: March 24, 2019, 02:27:54 am »
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.

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Offline JBealeTopic starter

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Re: mg scale - interesting effect
« Reply #14 on: March 24, 2019, 02:50:46 am »
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!
« Last Edit: March 24, 2019, 03:07:50 am by JBeale »
 

Offline coppercone2

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Re: mg scale - interesting effect
« Reply #15 on: March 24, 2019, 03:06:32 am »
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.
« Last Edit: March 24, 2019, 03:13:18 am by coppercone2 »
 

Offline JBealeTopic starter

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Re: mg scale - interesting effect
« Reply #16 on: March 24, 2019, 03:27:19 am »
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: [Select]
#   Weight A  Weight B
------------------------
1   100.108   100.123
2   100.105   100.118
3   100.104   100.118
4   100.106   100.115
5   100.105   100.114
6   100.106   100.121
« Last Edit: March 24, 2019, 03:32:16 am by JBeale »
 

Offline coppercone2

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Re: mg scale - interesting effect
« Reply #17 on: March 24, 2019, 03:28:32 am »
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
« Last Edit: March 24, 2019, 03:34:04 am by coppercone2 »
 

Offline JBealeTopic starter

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Re: mg scale - interesting effect
« Reply #18 on: March 24, 2019, 03:39:32 am »
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.
« Last Edit: March 24, 2019, 04:08:54 am by JBeale »
 

Offline David Hess

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Re: mg scale - interesting effect
« Reply #19 on: March 24, 2019, 04:26:36 am »
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.

Lack 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.

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.

I 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.

A well designed one will be insensitive to AC, or well enough filtered to not mind ambient noise.

It 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.
 

Offline Alex Eisenhut

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Offline Conrad Hoffman

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Re: mg scale - interesting effect
« Reply #21 on: March 24, 2019, 02:06:10 pm »
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.
 

Offline JBealeTopic starter

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Re: mg scale - interesting effect
« Reply #22 on: March 25, 2019, 02:44:13 am »
Maybe you can buy one of these just for fun Ebay Weight Test Set
Indeed 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.
« Last Edit: March 25, 2019, 02:46:22 am by JBeale »
 

Offline helius

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Re: mg scale - interesting effect
« Reply #23 on: March 25, 2019, 03:22:36 am »
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.
 

Offline David Hess

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Re: mg scale - interesting effect
« Reply #24 on: March 26, 2019, 01:26:49 am »
I have seen old calibration sets which included ivory tweezers for handling the weights.
 


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