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Anonymous Poster #1

Scale Accuracies

08/25/2016 10:39 PM

I have 2 weight scales (one electronic strain gauge scale and one a spring scale) each rated at accurate to +/- 2% of FS weight and both have the same FS range. I am measuring an unknown weight within the ranges of the scales. The values agree within the 2% difference. Is the best way to estimate the value of the unknown weight just to use the average of the 2 values ?

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#1

Re: Scale Accuracies

08/25/2016 10:51 PM

No the best way is to check the calibration of each scale with test weights....

https://www.grainger.com/category/calibration-weights/scales-and-scale-accessories/material-handling/ecatalog/N-loy

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#2

Re: Scale Accuracies

08/25/2016 11:01 PM

NO! Use one or the other!

If you need a more accurate measurement, get a better scale.

A man with two watches never really knows what time it is.

A man with two scales never really knows how much it weighs.

Worry about something important, like your next job.

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#4
In reply to #2

Re: Scale Accuracies

08/26/2016 12:02 AM

I've heard that countless times, but have never seen the reason.

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#5
In reply to #4

Re: Scale Accuracies

08/26/2016 12:07 AM

Do you mean this?

"Worry about something important, like your next job."

Or, what?

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#6
In reply to #5

Re: Scale Accuracies

08/26/2016 12:46 AM

No. I'm talking about the (alleged) rationale for picking one measurement and ignoring the other. Maybe there is a reason for that, but I have never seen it. However, I have seen lots of dumb cliches like how a stopped clock is right twice a day.

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#17
In reply to #6

Re: Scale Accuracies

08/26/2016 7:15 PM

Why not enlighten the forum as to why when two scales are both within the specified tolerance range, both should be used.

As many have said, if more precision is needed, get a better weighing apparatus.

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#18
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Re: Scale Accuracies

08/26/2016 11:12 PM

See post 11, for instance. Also, in simple terms, more information is usually better.

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#19
In reply to #18

Re: Scale Accuracies

08/27/2016 11:45 AM

Post #11 jumps to a completely erroneous conclusion, based on an assumption that has no factual support anywhere in the OP!

More information merely adds unnecessary work to arrive at an unneeded, overly complex answer that is not needed.

The precision of each scale is within the limits required, so weighing twice is a waste of time. Either scale gives an answer that meets the required weight tolerance.

For the third time, if a more precise weight is required, get a more precise weighing device!

All the more reason to ban anonymous posters!!!!!!!!!!!!

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#3

Re: Scale Accuracies

08/25/2016 11:31 PM

This looks like a homework question.

Use either scale to get an estimated weight. If you want something super accurate use a calibrated scale, there is absolutely no guarantee the accuracies of the two different scales will average to the actual weight (your more likely to add further inaccuracies into the measurement).

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#10
In reply to #3

Re: Scale Accuracies

08/26/2016 9:32 AM

This looks like a homework question.

Yup, schools starting up again!

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#7

Re: Scale Accuracies

08/26/2016 6:38 AM

If you can't re-calibrate them, use the one which was most recently calibrated (check the label!).

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#8

Re: Scale Accuracies

08/26/2016 7:00 AM

Why not put a known weight (or maybe several) on each and see what the actual difference is. Then you could calculate the actual weight of your "unknown".

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#9

Re: Scale Accuracies

08/26/2016 8:33 AM

For those from a weights and measures background, there could be a whole discussion based on where in the measuring range the "estimate" is. (Based on linearity of springs versus strain gauges)

For the statisticians, they would argue to take the mean. (Based on central limit theorem derivations.)

For those selling goods,they would want you to use the higher of the two readings, while a purchaser would want you to use the lowest.

A truckdriver would want the lowest if facing a fine for overloading, or the highest if being paid for "tonnage" delivered.

Depending on background and experience (as you have seen) others would argue for the most recently claibrated, or to perform a confirmation experiment yourself.

In reality, like so many questions we see here,you have not provided sufficient details for a single and agreed determination of the "BEST" way.

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#11

Re: Scale Accuracies

08/26/2016 10:16 AM

I assume you are taking a statistics class.

If you assume that the population of all scales produce a reading that is a bell shaped curve about the true weight, then the best estimate of the mean of this population (true weight) is the mean of your sample of two scales.

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#12

Re: Scale Accuracies

08/26/2016 10:17 AM

So, you have two scales...

Both scales accurate within 2%...

Two results, both within tolerance... Q.E.D.

Sounds as if all is well. Do you need to know mass within lower tolerance? If yes, another instrument may be in order.

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#13

Re: Scale Accuracies

08/26/2016 10:56 AM

If the unknown weight measured falls within the upper half or 3/4 range of the FS....that will be a safe guesstimate or you!

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#14

Re: Scale Accuracies

08/26/2016 12:11 PM

SE has it correct, #1

We check/certify our scales weekly.

If both are with in 2% accuracy, that's is not what's the problem.

If you're trying to get the closest accuracies, then get a more accurate scale, because that sounds like what you need.

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#15

Re: Scale Accuracies

08/26/2016 1:23 PM

If your answer is within 1 part in 50, and the scales are accurate to 1 part in 50, then it doesn't really matter, does it, Mildred?

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#16

Re: Scale Accuracies

08/26/2016 1:51 PM

Repeat the weighings a number of times (not less than 4 times) with each scale and compare the averages and standard deviations of each scale and get back with us.

By the way, you cannot buy more accuracy by weighing the mass over and over and over, although you will have a more precise value of the central tendency of the data.

If what you really need is more precision, spend the money on a better measuring instrument.

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