Determining Weighing / Counting Scale Tolerance - No manufacturer specification

[etdiyas]

weighing/counting scale tolerance

What would the tolerance to be used in the absence of manufacturer's specification. I'd like to ask for any publication stating the applicable computation for tolerance. I've read something about 0.5 of scale division and i was confused because how can my digital counting scale 300g capacity with readability/resolution of 0.001 can read 0.0005g tolerance if i have to follow 0.5(0.001)- 1/2 of scale division. And is it possible to use 2 times of scale division as a basis for my tolerance because i'm working on weighing cal procedure because I'm anticipating QC staff in asking where and what is the basis of my computation of tolerance. thank's in advance

 

[Marc]

I haven't seen anything written. Maybe one of the others can help.

Are the scales calibrated? Is uncertainty stated?

 

[etdiyas]

the scales are calibrated but the uncertainty is not stated.
how about the NIST Handbook 44 and 105-1, some people i know says that these could help, where can i purchase these references thus it cost much?

 

[tomvehoski]

There was a freeware uncertainty calculator I had about a year ago that gave a good example on calculating uncertainty for a weighing scale. I can't seem to find it right now, but I believe there is a reference to it somewhere in the archives here.

The 1/2 of the least significant digit is only one part of the uncertainty for a scale. For example, if a scale has a resolution of 1 gram, a reading of 100 grams could be 99.5 or 100.4, so there is a rounding error.

Other components include the uncertainty of the calibration of the weight standard you are using (this should be stated on a calibration certificate for the standards), buoyancy due to air, operator methods, and others.

It's been awhile since I worked on a scale project, but I will see if I have something in my files I can upload.

Tom

 

[M Greenaway]

Another question on weigh count scales.

Do weigh count scales need to be calibrated against known standards of weight ?

The reason I ask is that we use our weigh count scales by hand counting a certain quantity of product, entering that count into the scales, and then tipping in product up to a certain count. As we sell product in a counted quantity and not by weight do we need calibrate these scales against standards of weight. After all what we are really doing is using the scales as a comparitor between the known sample size and the quantity of parts tipped into the scales.

What is important however is the linearity of the scales, but couldnt I determine linearity using the product itself ?

Any thoughts....

 

[David Mullins]

MG:
Yes, known mass', over the range of the scales (or the range used by you - as you don't need to calibrate a range if you don't use it). One mass doesn't do it, because of linearity. You could calibrate a point (1 calibrated mass), and then use that to check the range using your product, e.g. if a 5kg mass used, and that equated to 100 component parts, then you could check 10kg accuracy with 200 component parts, and so on - this doesn't calibrate the range (given component part weight variation) but it provides documented levels of confidence that you can deem adequate for your process.


National testing authorities usually dictate tolerance levels of lab measuring equipment. In Oz we have Australian Standards that also provide tolerance levels based on capacity, graduation, etc. At the end of the day, not enough information on your process and how this is being applied.

My understanding of the half scale division thing, is that you have a reading error which equates to half the smallest graduation, so if you are using a ruler with 1 millimetre graduations to measure something, the READING ERROR is 0.5 mm + 0.5 mm (you are measuring at each end of the ruler after all) = 1mm.
This is not to be confused with tolerances!

That's mythoughts anyway.

 

[M Greenaway]

David

My point was that I do not need to know a relationship to a known standard (or standards) of mass as I am counting parts, not weighing them.

 

[Ryan Wilde]

MG:

What you are proposing is a 1:1 calibration. Are there any variations to the weight of your parts? Of course there is, and let's say it is 0.1% variation. I could be off by a count per thousand units. From what I've seen, that is actually very good, as things such as fasteners tend to vary by around 0.8-1% of weight per item, and that is just what I've observed. If your weight variation of your part is sufficiently small, and the allowable error of your count is sufficiently large, then using your parts as "standards" is just fine, and you should write a procedure and use that method.

If I were your customer, however, I would be very uneasy with the method, and I would ask for your proof that it is adequate. You are, in fact, doubling the possible error of the count, whereas calibration against calibrated standard weights does not.

Ryan

 

[M Greenaway]

Thanks Ryan

But why calibrate agianst a known weight when we do not count against a known weight ?

We do not say that, for example, 100 pieces weighs 100 grams. so if we weigh out 1 kg we must have 1000 pieces. What we do is a manual count of a sample of say 100 pieces, we put the counted sample onto the scales and then set the scales count to 100. Then we tip in the product up to the required quantity.

To my mind any relationship to true weights is totally irrelevant.

 

[Ryan Wilde]

etdiyas said:

the scales are calibrated but the uncertainty is not stated.
how about the NIST Handbook 44 and 105-1, some people i know says that these could help, where can i purchase these references thus it cost much?

NIST Handbook 105-1 concerns standard weights, and does you no good for scales. NIST Handbook 44 Section 2 has quite a bit of information, but it is used for legal metrology in the USA (such as scales used to verify the weight of train cars, etc.), and it would not help you all that much. Both Handbooks, as well as many others, are available online at
http://www.nist.gov , and they are free to download (at least in the USA they are, give it a try, see if it works from there).

That said, the tolerance of your scale, if it is made by a reputable manufacturer, will probably be around 2-3 counts, with a repeatability of <1 count (using the RSS of 10 measurements). But, if you are making measurements that are ±200 counts, then you can calibrate it to ensure a 10:1 or 4:1, or whatever your quality system deems acceptable, regardless of the manufacturer's tolerance.

"2 times of scale division" may, or may not, be adequate. If the scale was not designed to meet that criteria, and you are trying to use it at a tighter specification than it was designed, you will have some fairly rigorous proof to accomplish to show that your scale does meet that criteria throughout its calibration cycle.

The 0.5 division that you've read about is "zero accuracy", which is simply that a beam balance must zero within 0.5 division. It does not apply to digital scales.

The best bet is always to find the manufacturer's specification. In the absence of manufacturer's specification, you will have to research what tolerance is required of the unit, and calibrate to that tolerance.

Ryan