Scale accuracy and how scales work

[FatSebastian]

Starting a new topic about an interesting question from here:

I really don't care what a car weighs, I just care if it is over the limit.

The participant wants to know how much, so they can add or subtract a little or alot. But for the ultimate purpose of inspection, it is a pass fail situation.

Interesting question is, does temperature and humidity effect the scales acurracy to measure or just the starting point. So, is it like a motorcycle speedometer, accurate at low speed, and lying outright at highway speeds?

That is why we don't use spring scales, right?

[gpraceman]

Temp can have an affect on digital scales. Take a scale that was stored in a cold vehicle or garage, power it up before it acclimates to room temperature and you will likely notice that the reading drifts. So, you should make sure the scale has warmed up before using it for check-in.

[whodathunkit]

Randy,

Digital scales will also read inaccurately with low power batteries
or if used on an un-stable horzontal surface.

The display value may also be affected on some scales..
By electromanetic disturbances from outher electronic equipment that operate near by.
Such as raidos,microwaves, cell phones ect.

[FatSebastian]

That is why we don't use spring scales, right?

Perhaps spring scales are inherently less accurate, but as you say, absolute accuracy may be less important than repeatability at the established limit.

Aside from the accuracy question, the most noticeable issue that we have found with mechanical spring scales is dial parallax. This means that different readings are perceived by looking at the dial hand from different angles. Therefore, the builder (standing) and inspector (sitting) can look at the same scale at the same time and perceive two different readings, which is confusing. It is particularly noticeable with a 1-lb. / 500g diet scale where the indicator moves vertically (like the one on the right in the image); readings can change by up to 1/2 ounce if the viewer is sitting instead of standing.



Also, is a weight limit reached when the hand just touches the 5-ounce mark, is centered over the mark, or passes the mark? Etc. A digital display does well to take away the subjectivity of an analog display.

[whodathunkit]

FS, Do you have any good info about useing a triple beam scale?

[FatSebastian]

Do you have any good info about using a triple beam scale?

Not really. (I might have used a beam balance in a university physics chemistry lab decades ago; that would be the limit of my first-hand experience.)

My impression has been that beam balances seem less practical for pinewood derbies because they are not very portable and are relatively expensive. Also, my understanding is that wear of the delicate pivot points (cone, knife-edge, bearings, etc.) can degrade the accuracy of a mistreated or well-used balance, which would be expensive and tricky to repair. Because of this impression, we have had little motivation to research their use (but if I was looking for information from hobbyists I might start with fora and web sites of outdoorsmen who have historically used beam balances to make their own ammunition).

I would also imagine that a balance-type scale would take more time to operate as an inspection device unless it was used as a go/no-go indicator (where the scale is pre-set to balance against 5 ounces). In that case, a failure indicated by the visible motion of a beam balance might be a neat alternative to the "black box" readings coming out of a cheap digital scale.

[Speedster]

Scale Accuracy.
I have 2 scales I use at our Pack weigh-in, a Triton T2 from Max V and a Pocket scale from Hodges. The Triton uses a 200 gram weight for calibration and the Hodges a 100 gram weight. I also have a 141.75 gram weight (5 oz.). Both scales need to weigh the 3 weights accurately and they do. These 3 weights are lined up in front of the scales with Dr. Jobe's book, "The Physics of the Pinewood Derby" next to the weights and facing the scouts. Intimidating? Perhaps, but not meant to be. It is set up that way to give the people the feeling we truly care about fairness and accuracy. At our Pack, and I'm sure many, many other Packs, we care what the cars weigh especially if they are a lot over weight or under weight. Those scouts need help and we are fortunate to have people and equipment that can provide that help. We have a simple chart that shows the weight of a wheel, an axle, and a bare uncut block. We have those items next to the chart so the scout can pick up a wheel, or 3 wheels and 1 axle, and get the feeling what he needs to add or subtract from his car. This works well for us and the parents have always been appreciative.

[Shawn Stebleton]

I remember reading somewhere that postal scales round up if "just over" the ounce. So if a well-calibrated 2-decimal precision scale registers 5.01oz for a particular car, the postal scale registers 5.1oz for the same car. This is due to postage being charged per ounce or fraction thereof. A 1-decimal scale would register 5.0oz for the same car.

I don't know what the actual precision is for the postal scale, to know what the actual weight would need to be to trigger moving up to the next 0.1oz. If it is 0.001oz, then a car weighing 5.001oz would register as 5.1oz on the postal scale, 5.00oz on the 2-decimal scale, and 5.0oz on the 1-decimal scale.

If one of these postal scales was used as a tune-up area scale, then there wouldn't be any issues with weight at the "official" scale. Hmmmm.

[FatSebastian]

I remember reading somewhere that postal scales round up if "just over" the ounce.

I have read this before too, not just as it relates to postal scales, but as a characteristic of digital-scale technology in general. However, the source was not authoritative enough for me to preserve the reference.

I don't know what the actual precision is for the postal scale, to know what the actual weight would need to be to trigger moving up to the next 0.1oz.

Postal scales that I have used are precise to 0.1 ounce. For someone with a set of finely incremented calibration weights, it might be an interesting after-hours experiment at the local post-office lobby to see if the "just over" round-up hypothesis holds.