Weight Check Tolerance of in-line Scale on new pallet conveyor system

S

Swagg - 2009

Not sure how to ask this question.

The process: We have installed a new conveyor system in our warehouse and there is an in-line scale. As each carton crosses the scale the weight is recorded and totaled for each pallet. (Their are 40 lanes that the carton may be diverted down, each lane builds a pallet for shipping)

We then perform a pallet weight check. (System sums carton weights from in-line scale and compares to actual weight entered by operator per pallet)

Currently the variance for the weight check is 1/2 % of total pallet weight (example: pallets weighs 500lbs (acceptable variance 2.5 lbs)

I don't know where the variance settings came from. Now for my real question, how would I go about finding out what the variance should be?
 

ScottK

Not out of the crisis
Leader
Super Moderator
Re: Weight Check Tolerance

I think that would be a function of the accuracy of the check scale and the pallet scale.

You're scale vendor should be able to help you with that.
 

Hershal

Metrologist-Auditor
Trusted Information Resource
You know, you have two potentials.....practical and esoteric.....

Practical: You should have your scales calibrated, but if you do, you should absolutely DEMAND that the calibration provider provide the specific uncertainty.....

Understand, the uncertainty here is not to - necessarily - establish an ABSOLUTE pass/fail.....it is to understand the error involved with a given measurement.....no matter what that measurement is.....

Now, you have calibration, including local acceleration of gravity, eccentricity, barometric pressure.....and other Type B uncertainties.....

Esoteric: Most of this is BS, taking uncertainty way beyond reality.....

Let's get real.....some of the real, and almost none of the esoteric, should be considered.....

Basically, a measurement in this context is non-commital and based on the original question posed......
 
S

Swagg - 2009

Let me give a little more detail, I'm not sure I made myself clear. Below is the table the system uses to determine if a pallet passes the weight check.

Loading Weight Acceptable Variance
0 – 17 lbs 5%
17.1 – 32.9 3%
33.0 – 81.0 2%
81.01 – 199.0 2%
199.01 – 499.0 1%
499.01 – 1,201 ½ %
1,201.01 – 10,000 1%

No one here knows how the Acceptable Variances were determined and we want to find a formula to validate the settings. We are currently getting several false failures (pallets fails weight check, then broken down & verified, all items are accounted for). One thought is to make small adjustments of the acceptable variance until we have no more failures, however we would like a more statistical foundation to our settings.
 

ScottK

Not out of the crisis
Leader
Super Moderator
Let me give a little more detail, I'm not sure I made myself clear. Below is the table the system uses to determine if a pallet passes the weight check.

Loading Weight Acceptable Variance
0 – 17 lbs 5%
17.1 – 32.9 3%
33.0 – 81.0 2%
81.01 – 199.0 2%
199.01 – 499.0 1%
499.01 – 1,201 ½ %
1,201.01 – 10,000 1%

No one here knows how the Acceptable Variances were determined and we want to find a formula to validate the settings. We are currently getting several false failures (pallets fails weight check, then broken down & verified, all items are accounted for). One thought is to make small adjustments of the acceptable variance until we have no more failures, however we would like a more statistical foundation to our settings.

I'm still going to guess that the variance is based on the accuracy of the load cell(s) in the scales.

Then when you do a verification of a full pallet you could get a situation similar to what I've seen in mechanical drawing, like tolerance stacking.
The variances, while the measurement is in spec, add up (or take away) to put the final measurement out of spec.

How far out is the pallet measurement? Could it be a function of the pallet weight? Or is that measured and zeroed before stacking on it.
 
S

Swagg - 2009

The pallet is weighed (tare weight)prior to stacking cartons and the tare weight is added to the system when the pallet is closed prior to weight check.

We have three known variables:

1)In-Line scale variation (0.1)
2)Pallet weight check scale (1.0)
3)Pallet weight (Not sure of variation)

We have started to re-weigh the pallets on the failed weight checks to see what the difference is between what was written on the pallet and the new actual empty pallet weight . We have found some variation (< 2% of the pallets failures empty pallet weight was different) We are now weighing the empty pallet within minutes now of the stacking process, prior we weighed them days in advance.
 
C

crendfrey

Re: Weight Check Tolerance

I think that would be a function of the accuracy of the check scale and the pallet scale.

You're scale vendor should be able to help you with that.

High speed check weighing can be tricky. You have to be careful of making demands on the machine that it is not capable of performing. If you give me the make and model number, capacity and resolution of the scales, I may be able to look them up for you to see what the manufacturer's specifications are.
 

Tim Folkerts

Trusted Information Resource
Let me give a little more detail, I'm not sure I made myself clear. Below is the table the system uses to determine if a pallet passes the weight check.

Loading Weight Acceptable Variance
0 – 17 lbs 5%
17.1 – 32.9 3%
33.0 – 81.0 2%
81.01 – 199.0 2%
199.01 – 499.0 1%
499.01 – 1,201 ½ %
1,201.01 – 10,000 1%

If I had to guess, I think these are based on the specs of the scale.
I would further guess the scale is quoted as good to something like +/- 0.5% +/- 0.5 lb.

At 17 lb, this would be +/- 17*0.005 +/- 0.5 = +/- 0.585

This happens to be just under 3.5%. So above 17 lb, the error is no more than (about) 3%.

At 33 lb, the error is has dropped to about 2.1%, so above 33 lb, 2% is a reasonalble estimate.

At 81 lb, the error has dropped to about 1.1%, so (if hy hypothesis is correct!) the error could have been dropped to 1%

At 200 lb, the error has dropped to about 0.75% (if hy hypothesis is correct!) the error could have been dropped to 0.75%

At 500 lb, the error is down to about 0.6%, so 0.5% as listed in your table) is back closer in line with my hypothesis.

At 1200 lb, my guess is that the specs for the scale say the error increases to 1% +/- 0.5 lb, so the % error was bumped back up!


This is all conjecture, of course! Check the manual for this scale (if you still have it) and see if they give specs something like I conjectured. The numbers also don't match exactly with my hypothesis, so I may be totally off-base, of perhaps the person who made the table rounded strangely, or ...


Tim F
 
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