Weight and scales - Page 1
Forum section: electronics / general

author: BradC
author_rank: Super Contributor
timestamp: November 22, 2018, 12:43:32 pm
content: G'day all,I've been doing a bit of chemistry lately, and after a few titrations that "didn't go to plan", I decided to check my balance. It was not inconsiderably out of whack and needed adjusting. That solved that issue. Reading a random scan of the now lost manual indicates I should have been "calibrating" them quite a bit more regularly than once every 5 years. Oops.That led down the rabbit hole of "If I can adjust those, then how accurate are all my other scales and what do I need to calibrate them also?" and resulted in the acquisition of some additional calibration masses and the consequential learning about the different classes of mass and tolerance.That leads to my all-purpose scales. About 7 years ago, I sprung $35 delivered for a set of these to measure refrigerant :Cheap, cheery Chinese scales. Mine are rated to 35kg and a resolution of 1g. I use them for weighing fluids when servicing the cars (put the reciprocal of the relative density into the cost box and get a direct readout of fluid volume in the total summary), refrigerant when doing A/C work, pool chemicals and all sorts of fun stuff (even get the kids to stand on them from time to time).I'd always wondered how good you get for $35, but as nothing I was doing was particularly critical I never bothered to really check. As part of my acquisition of calibration masses, I picked up a 10kg +/- 500mg to check these, and I was quite astonished at how good they were after 7 years with the factory calibration.They were 6g low at 10kg. Re-calibrating them brought them back up to scratch and various linearity tests at a few different weights indicate they are as accurate as their readout resolution (1g) when compared with weights more accurate than that. Some digging indicates the uncertainty is supposed to be about +/- ~11.5g (1/3000 of FS). I'm pretty impressed. Of course I then used them to knock up some calibration masses for my bathroom scales, which only led to the disappointing result that *they* were actually close enough to mean that I am still a fat bastard and they haven't been lying to me. Can't win them all.This whole exercise was the result of working on standards to calibrate a pH meter. Now I have calibrated scales and a calibrated pH meter but I'm still fat.Yay. More stuff to calibrate and fuss over. My wife will be pleased. At least we know her kitchen scales are spot on.

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2018, 03:42:23 am
content: Good load cells are typically 0.02% to 0.05% linear with about the same hysteresis or 1 part in 5,000 to 2,000 so the specification of 1 part in 3,000 is in the right range. Things improve when active or digital calibration is used.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 23, 2018, 05:09:23 am
content: The most impressive set of scales I have used was in a college chemistry lab. They were so sensitive that the reading "counts down" several digits per second from the liquid evaporating.Quote from: BradC on November 22, 2018, 12:43:32 pmA lot of us can relate to being jealous of those who eat a lot and yet stay skinny.

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2018, 05:08:16 pm
content: Quote from: NiHaoMike on November 23, 2018, 05:09:23 amThat is routine for load cell based scales intended for things like weighing diamonds. If you press on the scale, then it will not return to zero until your finger print is cleaned off.But achieving that sort of resolution is easy compared to an equivalent integral non-linearity.

author: BradC
author_rank: Super Contributor
timestamp: November 24, 2018, 08:13:38 am
content: Quote from: David Hess on November 23, 2018, 05:08:16 pmI've been trying to figure out how best to test linearity, as I'd expect with a spec of 1:3000 and a resolution of 1:35000 I'd find some major fishhooks in there somewhere.Thus far the best I've been able to come up with is keep throwing combinations of calibrated test masses at it and measure as many data points as I can. At this point every test I've thrown at it has been +/- 1g, which I continue to be astonished by. I keep having to stop myself buying entire test mass sets for the sole purpose of trying to trip the scale up and get it to meet my (relatively) low expectations, but spending >$400 on a set of weights to demonstrate the inferiority of a $35 scale sounds insane to me when I read it back.I'm looking at "middle of the road" 1mg balances at the moment and trying to convince myself that I really don't need a $4k 0.1mg balance (I seriously don't need anything with that resolution. I also don't need to try and explain that one to the boss).If nothing else it has taken my mind off drifting resistors for a while. At least an electronic balance will track zero point drift!

author: SeanB
author_rank: Super Contributor
timestamp: November 24, 2018, 08:57:46 am
content: Just remember those test mass pieces need to be verified as well on a regular basis, and have a current certificate to be of any use, along with care in handling. I have a set, and more importantly the history of them as well, and when I bought a cheap Chinese set of masspieces used a sensitive balance to verify them as well. Had to adjust the new masspieces, as the one was over 0.3g low, bad on a 50g masspiece, so took off the top cap and added a bit of solder to the collection there to get it close to 50g. Then sent them off with the rest for certification. You have to remember that they will gradually change mass with time as they either pick up mass as they corrode, or lose mass as you polish them before use.OK, I can give you a test accurate to milligrams, and also have certified 5 milligram pieces in the set ( they did the lot, not the requested highest 5 mass units) plus have a 1.00000g masspiece as well. Also have the very old Oertling massmeter that will read that mass to that resolution, though it does need a 24 hour warm up to get the reading to be stable to that level.Plus it also had a Schaffner line filter in it that did the Schaffner signalling, so it was replaced with a pair of fusible resistors and new class X capacitors. Pointless having the filter when there are separate common mode chokes, line reactors and capacitors as well, and it feeds a toroidal transformer with interwinding shield and short circuit proof windings.

author: Domagoj T
author_rank: Frequent Contributor
timestamp: November 24, 2018, 06:37:58 pm
content: Quote from: BradC on November 24, 2018, 08:13:38 amMaybe this is a cardinal sin in metrology (I wouldn't know, I'm not into that stuff, so please don't crucify me just yet) but wouldn't you be able to use any weight in combination with you good ones to add additional data points? Who cares if you have a chipped and rusty 1 kg weight, it will be equally chipped and rusty throughout the measuring session (perhaps you can clean off the rust before you start, if nothing else, for sanitary reasons). If you are checking linearity of the scale, adding the chipped and rusty weight should always increase the reading by the same amount, so does it matter if its actual mass is different from advertised?

author: BradC
author_rank: Super Contributor
timestamp: November 26, 2018, 03:46:48 am
content: Quote from: SeanB on November 24, 2018, 08:57:46 amIndeed, which is why I don't have a set. I have just the pieces I need, but I bought the best I could justify for each element ensuring they have current calibration certificates. I also handle them with gloves. Given nothing I currently own has a resolution below 10mg it's not as critical, but I figure it's a good habit to get into.Quote from: Domagoj T on November 24, 2018, 06:37:58 pmI've kinda been doing that too. I figure as long as each individual element is known, and I have the relative uncertainty for the mass of each element I can just add both to get a total mass and total uncertainty of a larger combined mass. I'm not using them to adjust anything, but it seems like a logical way to combine masses and ensure the total uncertainty is known and accounted for. They are either calibration masses, or odd stable objects I've measured with a more accurate balance.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 26, 2018, 06:09:35 pm
content: For the level of accuracy I usually care about I just use water. It is easy to get clean water which has a well know density at a given temperature. Volume is easy at the 1 percent level, and with some care can be improved quite a bit over that. So now the big uncertainty is container weight. Which is dealt with using the tare function on most scales (or with simple technique on balances).The beauty of this approach is that any desire weight is available. If there is enough interest enough trials and statistics can tell you a lot about your test mass uncertainty and the measurement errors.Mass nuts will have to go to more extreme measures.