Calibration of Scales for Weigh Counting

R

red53angel

#1
First, I would like to say that I have gotten a lot of my ISO questions answered by searching this forum. It is a wonderful resource.
However, I have not found a concise answer to my current question.
My company makes very small parts which are weigh counted to fill orders. Depending on the size of the parts, we count out anywhere from 25 to 150 pieces and weigh them. Once that weight is determined, we enter the count on the scales and then continue adding parts until the desired quantity is achieved.
The argument against calibration is that it does not matter if the scales are accurate, meaning that if it weighed, say 100 pieces at 25g or 25kg would be irrelevant. We do not care what they actually weigh, we are just obtaining a count.
We are working towards ISO 9001 certification, if that makes a difference.
Please let me know your opinion as well as your argument for your opinion.
 
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L

lrowe

#2
You may run into a problem if the uncalibrated scale has a non linear error in it's measurement, that is if the weight error percent is greater when the load is heavier. When you set up the counting function, the scale just assumes the weight divided by the number of pieces you enter is a good average weight per piece. If the scale has some weight measurement error that was not know because of no calibration being run, well that no loner will provide an accurate count. If you depend on this count to fulfill a customer requirement, in this case the number of pieces required, you are obligated to make sure the reading is reliable.

Hope this helps

Larry
 

WCHorn

Rubber, Too Glamorous?
Trusted Information Resource
#3
We use scales for small-parts counting too. We use a two station set up, where a five-pound load cell is used for the sampling, and a separate-but-connected 50-pound platform scale is used to count the parts. We calibrate both because they are separate load cells; an error on either one would affect the count. Do you use a similar set up or is the same scale used for both sampling and counting? If so, I recommend calibrating both.

Regarding ISO, you are required to calibrate devices used to determine product conformance. If you take a wider view of product conformance to include the quantity shipped as specified by the order, then the scale must be calibrated. That calibration need not verify the accuracy of the scale to absolute weights, but the accuracy of the count. You need to demonstrate the accuracy of the count, perhaps by periodic checks of count accuracy using the full range of the scale. You need not worry about the absolute weights, unless of course you use the scale for that purpose. If you’re not going to use it for absolute weights, I’d put a permanent notice to that effect on the scale.

I’ve seen this advice many times on the Cove and I’ll echo it here: when in doubt, calibrate. You have two customers you want to satisfy, the user and the end customer. I’ve found that users have more faith in their counting scales if they are periodically checked for accuracy. Accurate counts help customer satisfaction also.
 
D

Daniel Walker - 2011

#4
The above responses have hit the nail on the head.... kudos. You definately need accredited calibrations done on the scales. (Accredited) Calibration service providers do more than just put a weight on an write down a number. You should be getting a fairly comprenensive test which follows nationally recognized or validated procedures. This serves as a preventative maintenance routine which prolongs the life and accuracy of the scale by identifying potential problems before they do harm to the process.

:2cents:
 
J

jfgunn

#5
The responses thus far have been great. It is true that a counting scale that only has one platform will often count accurately even if the scale is not calibrated. This is true if the error is linear. Most errors found on scales are linear. Note that i said "most" and not all, therefore you shold have them calibrated.

If you have a dual base system where the sample is made on a small scale and the parts are counted on the larger scale, it is imperative to have both calibrated.

Counting Scales are like cash registers. Parts are money and you would would want your money counting machine to be accurate.

Typically the cost of scale calibration is relatively small when you look at the possilibty of losing inventory. Note that you can provide your customer extra parts all day long and they probably won't call you. If you short them parts once, they will call.

Be sure to use a large enough sample size to get a true part average. If the parts are likely to vary a larger sample would be appropriate. Of course as you increase your sample size, you likelyhood of counting the sample incorrectly increases. Counting scales are pretty smart. Lets say you want to fill boxes to 100 parts. If you tell the scale that the sample has 25 parts but you only count out 24 parts, the boxes will get 96 parts every time.

Hope this helps a bit.
 
#6
The argument against calibration is that it does not matter if the scales are accurate, meaning that if it weighed, say 100 pieces at 25g or 25kg would be irrelevant. We do not care what they actually weigh, we are just obtaining a count.
We are working towards ISO 9001 certification, if that makes a difference.
Please let me know your opinion as well as your argument for your opinion.

You've had some great answers here! As you proceed towards registration, I hope that the person/people arguing against calibration of these scales are not responsible for process controls or for aspects of the calibration system!

You may wish to take some 'competency adjusting' action with them! It would be very unfortunate if their 'opinion' of the need for calibration 'came out' during an audit! And, in some ways, a few answers here, while a great place to start with the rationalization of such calibrations, it won't change a person's mind - if they were already thinking like that!
 
M

merrick65

#7
I didn't see this mentioned ablove; but I may have missed it. The only part of a counting scale that is legal for trade is the actual weighing; the counting function is not legal for trade. Also look for the accuracy of the counting function you will not find it the accuracy; the accuracy is for the weighing. It's very hard to get a manufacturer to say what the counting accuarcy is (in my opinion). Also the old saying you get what you pay for is correct for this. Do not use a cheap counting scale; spend a some more cash and buy a nice one. I have always had luck using a PC820 from weigh-tronix (I do not work for them). I would also use a small capacity counting scale in tandem with the larger scale as mentioned above. I am a certified scale technician through the state of Illinois.
 
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R

red53angel

#8
Thank you, thank you. Armed with good information I will be able to make a logical, sound argument for calibration. I was sure it needed to be calibrated, but they were wearing me down and making me second guess myself.
We are using a single scale. We have 4 at different locations. (Setra 500, 1000, 2000, and 5000)
The procedure in place when I started here in September requires that the Setra 5000 not be used to fill orders because it is inaccurate. Actually states that in the procedure!! That will be my next battle when I tell them they have to get it fixed or get rid of it.
Thanks again for all your help.
Mary
 

rmf180

Involved In Discussions
#10
As always, great advice offered from the group!:applause:

As an additional resource, I am attaching a MS Excel worksheet which will tell you if your scale is capable of measuring the parts you are attempting to measure. When dealing with small parts (light), one must use an appropriate size scale. Many times small parts weigh less than the scales error which results in inaccurate counts.
 

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