Digital scale?

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FatSebastian
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Re: Digital scale?

Post by FatSebastian »

Stan Pope wrote:I made some from 1X pine, then routed the sides and top to 1/16" thick to reduce the amount of shift needed to zero the scale.
Clever! Thanks!

(Although, because there's no shortage of various scales, I'd be inclined to just get a scale that seems better suited the purpose, so I may try to return the one I reviewed.)
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Stan Pope
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Re: Digital scale?

Post by Stan Pope »

FatSebastian wrote:(Although, because there's no shortage of various scales, I'd be inclined to just get a scale that seems better suited the purpose, so I may try to return the one I reviewed.)
Most scale pans will not support the car by its underbelly. Supporting by wheels is more dangerous to the car.

The interface I described eliminates the roll problem. If the interface is centered on the scale pan, then the car can have its CM centered on the scale pan as well, though many youngsters need to be coached to do it that way ... they seem to want to put the geometric center over the center of the scale pan!

Of course, the support need not be 6" long ... 1-1/2" X 1-1/2" is plenty big enough for most cars. Then instead of routing out the underside, you could use a 1-1/4" Forstner drill to do a very acceptable job of reducing the weight!
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Re: Digital scale?

Post by quadad »

I was real happy with the #93543 HF scales, and used one on each wheel at times. Their accuracy and repeatability, compared to even 3-beam scales, was amazing for the (often on sale) price. Our cars often had a row of 1/4" cubes in the very back so the cars usually stand up on the 'hind end' for easy measurements. Probably not the best method for kids to weigh them with though. We used these spare scales in the pit where people were modifying cars to get them through inspection and they were very appreciated.
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Re: Digital scale?

Post by Mr. Slick »

I use a 1" x 3" piece of foam wrapped with some duct tape to hold the cars.
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Stan Pope
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Re: Digital scale?

Post by Stan Pope »

Mr. Slick wrote:I use a 1" x 3" piece of foam wrapped with some duct tape to hold the cars.
That should work well, too, so long as all the tape's adhesive stays on the inside. Wouldn't want any sticking to a wheel!
Stan
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FatSebastian
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Re: Digital scale?

Post by FatSebastian »

quadad wrote:I was real happy with the #93543 HF scales, and used one on each wheel at times.
:nod: We prefer that scale too. It is also wide enough that you can fit both wheels of the axle line across the platform without needing a tray. These scales include an integrated cover that is tough and has instructions printed on the inside, which can be very convenient if you use such a device only seasonally.
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Re: Digital scale?

Post by rpcarpe »

Even simpler that Stan's... use a spare PWD block, zero out the scale (TARE function) and then ALL cars easily fit & balance on the plain wood block. I've used this for years, walk the kids through the process so they understand it's JUST weighing their car, works wonderfully.
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Stan Pope
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Re: Digital scale?

Post by Stan Pope »

rpcarpe wrote:Even simpler that Stan's... use a spare PWD block, zero out the scale (TARE function) and then ALL cars easily fit & balance on the plain wood block. I've used this for years, walk the kids through the process so they understand it's JUST weighing their car, works wonderfully.
The weight of the spare block + the 5-ounce car to be weighed must be within the scale's capacity! A scale with a 7 ounce upper limit requires a "spare block" lighter than 2 ounces!

Another concern, one that I've not tested, is my fear that the absolute scale error may be worse as the total weight increases. So, I've kept the spare block as light as possible.
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Re: Digital scale?

Post by rpcarpe »

Both my current HFT scales max out at ... over 10 oz... I think it's several pounds. So we're good. Have not seen any accuracy problems between car or car+block.
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Stan Pope
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Re: Digital scale?

Post by Stan Pope »

rpcarpe wrote:Both my current HFT scales max out at ... over 10 oz... I think it's several pounds. So we're good. Have not seen any accuracy problems between car or car+block.
Good! How did you test the accuracy issue?

I raised the issues because many measurement schemes have error specs expressed as a percentage of the scale load. Raising the load in that case by 4 or 5 ounces would almost double the absolute error! Whether that is the case with a specific scale ... dunno.

I think that I recall a popular brand of scale that topped out at 7 ounces ... I had a couple of those. Now, I'm using mainly the HFT scales with higher load limits... and that are inexpensive enough that I will accept an occasional failure due to "over enthusiastic use" from the youngsters.
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Re: Digital scale?

Post by rpcarpe »

Interesting that you'd ask Stan. My postal scale went down after being used at a Fishing Rodeo...
My HFT scale got dropped (me, this time) and needed calibration of 500g. When the coupon arrived for $7.99, bought new scale. Weighed out exactly 500g, turned on old scale and used the exact weights as 'calibration'. I think it's as good as we'll need for the local Packs. District has their own scales and appropriate calibration weights.

Have not worried much about the errors specs as % of total load. Just figured everyone was weighing on the same scale, so just as fair to everyone.
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Stan Pope
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Re: Digital scale?

Post by Stan Pope »

rpcarpe wrote: My HFT scale got dropped (me, this time) and needed calibration of 500g. When the coupon arrived for $7.99, bought new scale. Weighed out exactly 500g, turned on old scale and used the exact weights as 'calibration'.
The subject needs some study ... tolerances for a calibration weight are rather tighter than the scale tolerance that you are calibrating!

I go a bit overboard and prep 500.0 g calibration weights on a scale with 0.01 g accuracy. Similarly, since our rules specify weighing to 0.01 oz. (Fail if weight > 5.00 oz), reference weights are prepped on a scale capable of 0.001 oz accuracy. The reasoning is that the scale error and the calibration weight error may add together! Keeping the calibration weight tolerance an order of magnitude smaller than the target scale accuracy keeps the probability of scale error acceptable.
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Stan Pope
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Re: Digital scale?

Post by Stan Pope »

Stan Pope wrote:I go a bit overboard and prep ...
I've written about 200% of what I know about measurement theory in that post. It would be helpful if someone really trained in the theory chimed in and gave us the straight info on the topic. I think Randy L has the training ... maybe someone else, too.

Meanwhile, I'll add a few more percent beyond my training:

If my scale reads out to one decimal place and I pop a 5.00 oz reference on the scale pan and see 5.0 on the scale readout. I'm good to go, right?

Well, maybe so, maybe not.

If I pop a 5.04 oz reference on that scale it should still read 5.0, as it should if I use a 4.96 oz reference. If my calibration weight were off a hair, for instance 0.02 oz over or under, then the 5.04 reference might indicate 5.1 oz or the 4.96 oz reference might incidate 4.9 oz. These could allow a barely legal car to need adjustment because it read out 5.1 or induce a marginally underweight car to get an extra bit of weight.

While this is equally fair to all, it is really unfair to all in that raceday adjustments introduce risk, and your inspection process should avoid adding risk to the participants. Most of them have risk enough already!

Am I too fussy with all this? Well, maybe so, maybe not. I prefer to be safely in the "maybe not" category.

YMMV
Stan
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Re: Digital scale?

Post by rpcarpe »

Stan,
Thanks for pointing out the hazards... I haven't yet afforded a good 500g calibration weight. The good thing is that I can compare my scales with the District scales and their calibrated weight. So they will be close or at least we'll know the difference.

One other good/consistent thing is that many race teams weigh their cars on my scales as they build, so they're not surprised and it cuts down the risk.

It surprised me that Harbor Freight does NOT carry the needed calibration weight for their scales.
My wife started a new support group... Widows of the Pinewood Derby.
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