Counting scale for electronics? - Page 1
Forum section: electronics / projects

author: george graves
author_rank: Super Contributor
timestamp: September 17, 2015, 09:45:29 am
content: Once in a while I need to count up parts - usually through hole stuff(as SMD stuff is easy to measure the parts per length, and then do some math)I bought a scale off of ebay (I know - get what you pay for!) - and it's accurate for counting small parts like 1/8watt resistors - it's spot on every time. The down side, is that it needs 25 units to set up the counting mode, regardless of weight. It's a fixed 25 parts. So you have to manually count out 25 parts, add them to the scale, and then you can count your parts - that's kinda annoying - I would think 10 or even 5 units would be better.Anyone know of a budget model that will do that?

author: KerryW
author_rank: Regular Contributor
timestamp: September 17, 2015, 01:02:35 pm
content: How about the one you have? (Hint - It can't REALLY count parts)Put 5 parts on the scale and let it think there are 25. Pile parts on it and divide the reported count by 5.

author: SeanB
author_rank: Super Contributor
timestamp: September 17, 2015, 05:12:23 pm
content: Zero it, then place 10 parts, and note the rerading. Then add single parts and check the mass increase is both 1/10 of the total, and that as you add a single part it goes up by the same exact 1/10 increment each time. You need the resolution to be such that you can tell the mass of 1 part with at least 2 digits of accuracy ( ie if it actually is 1.2g or 1.1 or 1.3g) so that the math is correct.That you need to place 25 means that 25 parts is a single measuring digit, so it is not suitable. You want something with 0.5g resolution or better ( but they get a lot more expensive as you go down or want a wider range) that has around 4000 counts in the max resolution, so it will be 0.5g resolution and a max mass of 15kg as a standard off the shelf counting scale. Minimum for a single part in the 1g range, though it is better to have one that reads 0.2g and 10kg, though now you no longer are using a cheap 16 bit converter and oversampling like crazy to get to 14 bits of usable resolution, but have to step up to a proper 18 bit ADC and do the same again.At work I have a nice Oertling force balance unit, which will read down to the milligram level, though it has a single slope converter on it made from discrete components, to get the resolution, stability (it drifts around 50 counts from cold if not doing zero tracking) and the full scale range of 1.5kg it can handle. Really nice, and at a replacement price of around $10k for a similar unit I will not be replacing it fast. I use it as a sanity check for the small mass pieces, as it will show up the mass of the 5 milligram mass piece, and agrees with the 1g masspiece as being 1.0000g. Those all have a traceable calibration certificate, and more importantly do not drift much, as I only clean them before they go for recertification, using a soft cloth and alcohol.

author: KJDS
author_rank: Super Contributor
timestamp: September 17, 2015, 06:16:18 pm
content: If you want something good, then Mettler Toledo are my preferred scales. Not cheap, even second hand.

author: Circlotron
author_rank: Super Contributor
timestamp: September 18, 2015, 10:07:38 am
content: Quote from: SeanB on September 17, 2015, 05:12:23 pmI was surprised to find that with precision weights you have to take into account barometric pressure. It never occurred to me that if for example you immersed a block of lead in water, even though it doesn't float it will still weigh less because of the volume of water that it displaces. Same with precision weights in air.

author: SeanB
author_rank: Super Contributor
timestamp: September 19, 2015, 02:15:47 pm
content: That is why precision balances are calibrated on site, as moving them even from floor to floor can be visible as an error. Impressive with the Mettler Toledo units is that the most precision units have a warning to close the measurement chamber door to ensure that the IR radiation emitted from your hand does not affect the force balance pan. Air currents yes, but that the light pressure can be measured on a 10cm pan.

author: HighVoltage
author_rank: Super Contributor
timestamp: March 27, 2016, 11:34:37 am
content: Quote from: Circlotron on September 18, 2015, 10:07:38 amI know its an older thread but it seems to fit....I was just working with precision scales down to 1 mg or 0.001 Gramm precision.And I was wondering what the influence of barometric pressure compared to vacuum would be.I needed to measure 1 kg exactly to 1 mg and lets assume it is steel with a spcific density of 7853 kg/m^31 kg Mass is then 1 * 1000 / 7853 = 0.12733987 liter (Volume)1 m^3 air is 1.3 kg, so 1 liter is equivalent to 0.0013 kg0.127 * 1.3 / 1000 = 0.0001655 kg = 0.1655 GrammSo, essentially my 1 kg steel mass would show 0.1655 g more in vacuumThat is 165 ppm differenceIt seems small, but still significant for precision measurement.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 27, 2016, 11:48:28 am
content: Closing the door and not having your hand directly above with precision balances is no about radiation pressure - this is way to small. The more troublesome things are thermally driven convection flow in air and electrostatic charges. Also fluctuations in air pressure (e.g. someone closing a door in the building or wind gusts) can be a problem, as there is air flow in or out of the balance.There is a small effect of pressure on the air density, but this is still rather small as it's only a small fraction of the 165 ppm noted above (though it gets more with lower density samples).

author: HighVoltage
author_rank: Super Contributor
timestamp: March 27, 2016, 12:21:14 pm
content: Quote from: Kleinstein on March 27, 2016, 11:48:28 amI agree totally. A while back, I did a test on a 0.1 mg Mettler scale and placed an incandescent 100 W light bulb just above the scale and it absolutely had no influence at all. So, one might need to have a micro gram scale and a large enough surface to make the IR radiation pressure visible. Any air movement will be orders of magnitude larger that the radiation influence.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 27, 2016, 03:31:15 pm
content: Radiation pressure from sunlight would be about 100µg on a 10cmx10cm perfect reflector. This is waaaay higher than the energy output of your hand.

author: SeanB
author_rank: Super Contributor
timestamp: March 27, 2016, 03:48:50 pm
content: Mettler Toledo make massmeters that measure down to the nanogram region. Those come with a dedicated granite block mount that you have to place on a firm concrete footing. Generally if you ask them for a price then you don't need it. I think the factory celebrates every time they sell one.

author: dom0
author_rank: Super Contributor
timestamp: March 27, 2016, 04:04:39 pm
content: I saw one of those at a chem lab. The lab director told me that they only used it at night and even then preferably during the weekend, because the vibrations of equipment in the building and the traffic outside contribute errors to their readings with it.

author: SeanB
author_rank: Super Contributor
timestamp: March 27, 2016, 04:14:55 pm
content: the SANS lab near me has 3 of them in the metrology lab. This is a massive walk in safe built into a hillside, where these 3 sit as centre pieces on a separate floating slab in the middle of the floor. They built the rest of the building around them. Next to them is another safe with 1kg of platinum iridium alloy in it, secondary reference kilogram.

author: HighVoltage
author_rank: Super Contributor
timestamp: March 27, 2016, 04:30:34 pm
content: Probably one of these in the picturePrecision of 0.1 µg

author: vk6zgo
author_rank: Super Contributor
timestamp: March 28, 2016, 02:47:08 am
content: Get a grip,guys!The OP just wants to count resistors.In my Storeman days,many,many years ago,we just used a small spring balance,& did the Maths in our heads.Of course,as an Electronics Store doing a stocktake,we didn't really care much if a drawer had 200 or 202 resistors in it,as even then,they were the cheapest parts commonly sold.Valves (vacuum tubes) were counted individually,as they were much more expensive devices.

author: TheDirty
author_rank: Frequent Contributor
timestamp: March 28, 2016, 03:14:31 am
content: Quote from: vk6zgo on March 28, 2016, 02:47:08 amOP's question was answered 6 months ago.The necro thread second question was environmental factors and a scales accuracy. Not my thing, but interesting discussion. I learned stuff.

author: ChunkyPastaSauce
author_rank: Unknown
timestamp: March 28, 2016, 03:38:25 am
content: Quote from: george graves on September 17, 2015, 09:45:29 amOne of these reasons for doing this is to account for variations in the parts you are weighing. If you grab 1 component out of a bucket of 100 (with a normal distribution of weight), will that reference part represent a true mean weight? If you are off with the weight of the reference component by a small amount, then that accumulates with the number of components you try to weigh. So even if you had a scale with incredible specs, you'd still have to weigh a group of components to find the mean unless you are sure the reference component is an true representation of the weight........I think or something like that....

author: george graves
author_rank: Super Contributor
timestamp: March 30, 2016, 10:50:00 am
content: Yea. That how it works. But when the accuracy of the scale is way beyond one part, and it still makes you count out 25, that's annoying.

author: Circlotron
author_rank: Super Contributor
timestamp: March 31, 2016, 11:10:01 pm
content: Quote from: ChunkyPastaSauce on March 28, 2016, 03:38:25 amOr you could just dump the entire quantity to be measured in the scales, take a reading, then count them all and divide the total weight by the total you counted. Then discard the total weight you measured. This will ensure that any error in the scales has no effect on the quantity you counted. Voila! Error-free measurement.

author: epiflow
author_rank: Contributor
timestamp: March 31, 2016, 11:55:21 pm
content: Quote from: george graves on September 17, 2015, 09:45:29 amPlease define budget model. What would you like to pay for a scale that can do what you want?If you want to count small resistors that weigh next to nothing (.01g), AND want to be able to place less parts on the scale to tare, then you need to get a scale that is more sensitive to weight changes. It will most likely have a smaller total span. I.E. Get a 10lb. scale as opposed to a 50lb. or even a 25lb. Fairly accurate scales with this capability will cost you between $1K-$2K brand new.

author: lincoln
author_rank: Regular Contributor
timestamp: April 01, 2016, 10:44:33 pm
content: Dose it have a serial / usb port? If it is a dedicated counting scale it should have database integration. The way it work the last time I work at a company that carried inventory was the scale was connected to the PC and database through a serial port. When you kitted parts you would scan the bar code and the applications would dump the part weight to the scale, basically a one time setup.