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How much Tolerance do I use to Calibrate a 150kgs capacity Scale?

Dongzkie

Involved In Discussions
#1
I want to calibrate a 150kgs capacity with readability of 0.05kg
How much Tolerance Do I use?
Test weights available: ( in calibration certificate data )
nominal measured value uncertainty of measurement (+/-g) coverage factor (k)
20kg 19.999 0.0009 1.96
20kg 20.000 0.0009 1.96
20kg 19.997 0.0009 1.96
20kg 20.001 0.0009 1.96
20kg 20.000 0.0009
1.96
 
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BradM

Staff member
Admin
#2
Hi there! Welcome to the Cove!:bigwave: We are sure you dropped by.

That is a great first post, and it was helpful that you included the information about your available mass standards. :agree1:

I guess I would be more inclined to address your question based on what accuracy do you need. Do you know what your process requirements are for the scale? I would think that would be a great start.

Also, you might want to find out the specified tolerance given by the mfg.

Now.... there is no science or basis for my statement here... so please don't take it as a fact. But... how about starting with a +/-.5kg tolerance?

Would .5kg meet all your process requirements? The reason I suggest that is that I always try to take as much as I can get. :tg: I would rather have a balance (any instrument for that matter) that is calibrated, meets all the process requirements, and does not generate any calibration failures at a point so small that it does not give any significance to the process.

So.. if you are so inclined, maybe think about your process tolerance requirements, and see how liberal of a tolerance you can take for calibration.
 

Dongzkie

Involved In Discussions
#3
Thank you sir that was a very informative explanation which i learned from now. So I can decide the tolerance basing on our process? how do i support this during audit.
 
G

George Weiss

#4
The question being asked, reworded is, "to what specification should I test this scale?", given the listed information.
If you are calibrating this for a customer, then asking this customer for Manufacturer's specifications, a Specification sheet, or process requirements is a first course.
Assuming you can assign a specification to this scale I suggest the following:
The readability is +/-0.05kg, which is normal for a low accuracy digital scale.
The scale can be expected to be reasonably linear, and so the already offered
+/-0.5kg spec would a an absolute minimum.
Your readability will be your highest uncertainty in the calibration process.
There is a common absence in tolorance specifications in less accurate devices.
The scale is likely a floor scale for weighing shipping boxes/pallets and etc. 150.00kg/330lb max, resolution 0.05kg.
I am suggesting a nominal +/-1.0kg might be a specification more in line with this device.
A tolorance of specifically +/-0.5kg +/-0.5% of reading would be:
+/-0.60kg at 20kg (actual weight will be your 1ea. 20kg weight used)
+/-0.70kg at 40kg (actual weight will be your 2ea combined 20kg weights used)
+/-0.80kg at 60kg (actual weight will be your 3ea combined 20kg weights used)
+/-0.90kg at 80kg (actual weight will be your 4ea combined 20kg weights used)
+/-1.00kg at 100kg (actual weight will be your 5ea combined 20kg weights used)
150kg scale maximum
Using your 5ea. 20kg weights.
+/-0.0009kg k2 uncertainties of weights, +/-0.1kg k2 readout (this value can be debated)
This would give you a very safe TUR of 10:1 best / 6:1 worst (this value can be debated)
Since you will be assigning a specification limit to this device.
1. Select a specification value which the device will test and PASS hopefully always
2. Select a specification value at least twice your current data error(s)
3. Select a specification value which will be acceptable to the process it is used for. (better than 4 times test requirement)
 
Last edited by a moderator:

BradM

Staff member
Admin
#5
Thank you sir that was a very informative explanation which i learned from now. So I can decide the tolerance basing on our process? how do i support this during audit.
George has a good post here, to show the thought process that you should go through to determine a tolerance. He did a good job of coming up with

tolerance of specifically +/-0.5kg +/-0.5% of reading
:agree1:

That might be a good start for you.

You really should be writing up and documenting the process you are taking towards deriving your tolerance and such.

As far as the auditor, it all depends on the auditor, industry, etc. But in general, if you have documented the rationale for the tolerance, and it satisfies the process/customer requirements and is being done according to your calibration procedure and such, I don't think an auditor would have a huge problem.
:topic:

Determining calibration tolerances for me, is as much as an Art than a Science. While mfg. specifications are a good start, many times they are set too tight (for marketing purposes, in my opinion) and thus, you start having a bunch of failures.

Consider that you have a 500 gram balance, and the mfg. tolerance is .0005 grams. You find an error of .001 grams, so technically it's out of tolerance. Yet... you are weighing a product and the critical parameter is 75 grams +/- .01 grams. Did that error that was found have any impact on that parameter? No.

So in that instance, I might set the process tolerance to .005 grams, which the balance should be able to meet.

So, determining what the balance is going to be used for, can give you some guidance in determining what is an acceptable tolerance. It can also guide you if you have the wrong equipment for the process, which happens too. :yes:
 
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