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

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

Are the scales calibrated? Is uncertainty stated?

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?

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

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....

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.

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.

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

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.

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

M Greenaway said:

Thanks Ryan

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

Herein lies a problem. A calibration versus a known weight is not to prove a count, it is to prove the linearity and repeatability of the scale. The NOMINAL weight of the piece must also be known to properly set the scale (for minimum uncertainty).

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.

Remember, you are counting, but the scale is weighing and using a bit of math to convert to a count. You are, in fact, taking the possible per piece variation and making it the accuracy of the scale. Here is an example:

Number of pieces: 100
Possible variation of weight per piece: 1%

You manually count out 100 pieces from Lot A, and set the scale using the product, which may be 1% below the nominal weight of 100 pieces.

You now count out 100 pieces of Lot B using the scale, and Lot B had a bit less slag in the steel, and weighs 1% more per piece than nominal. You will count 100 pieces, but in actuality, you will have 98 pieces, due to the weight variation. We have not even used any scale linearity error, and we have 2% error. If you now count 5000 pieces of Lot B, you would measure 5000, but actually have 4900 pieces, and that would be with perfect scale linearity. This may be acceptable to your process, it may not.

Example 2:

You calibrate the scale using standard weights. You set the scale using the nominal per piece weight as your reference (the nominal weight of each piece would have to be known). Lot A would measure 100, but actually contain 101, and Lot B would measure 100, but actually contain 99. Your error is halved.

No measurement will ever be perfect, but calibration against known weights reduces the error contribution of the scale itself, leaving the major component of counting error as that of the weight of the piece itself.

Ryan

Ryan

I can appreciate that linearity and repeatability are vitally important for weigh count scales, but I cannot see how accuracy to known weights matters in this application.

Also it would be practically impossible to pick at random 100 pieces all of the same weight. Our sample size would in all probability be representative of the range of weights of the pieces would it not ?

Therefore by taking a count of say 100 pieces and setting our scale to 100 pieces we are in effect taking the actual mean weight of these 100 pieces. What this actual weight is is totally irrelevent, if it were given by the scales as 100 grammes then 1000 pieces would weigh (according to the scales - if linear) 100 kg. Also if the scales weighed the pieces at 1 gramme then 1000 pieces would weigh 1 kg. Whether or not the pieces were actually 100 kg or 1 kg doesnt matter in the slightest. All that is important is the ratio of the weight of the sample to the weight of the total count required.

I talked to my former boss where we used scales to count hundreds and thousands of small brackets made of titanium, stainless and carbon steel and aluminum. Every six months the scales are calibrated by an outside source. Did not dig into the method the lab uses.
1. Weigh the container empty
2. The scale has a function called "Tare" which zeros the machine with the container on it.
3. Put in one piece-"Tare" again.
4. Count 9 more pieces and put them into the container. It must read 10.
5. Add pieces until it reads the amount you want to ship. Great for things like washers. I have counted another 10 and see 20 on the readout. Now I feel better.

For the layman, such as myself, I would only feel comfortable with trusting the count when I periodically calibrate the machine to insure that the conversion mechanism is functioning properly. Whether it be against a "standard" weight suggested by the manufacturer of the scale, or using, for example, 10 or 100 "standard (certified)" weights to duplicate the way we use it.

JMHO Calibrate it! :p :ko: :smokin:

MG
as this thread is under 17025, I think a few of us are assuming a Lab environment.
If you want to weigh against a counted number of parts, go right ahead.
Have you tried holding 100 parts counted in your left hand and then picking up about the same size handful in your right? This would probably prove just as, if not more, accurate. (If you have 2 free hands of course)

Thanks Dave

I think your sarcasm actually hits the nail right on the head. I could perform the same function using a simple balance - calibration is not required !

M Greenaway said:

Ryan

I can appreciate that linearity and repeatability are vitally important for weigh count scales, but I cannot see how accuracy to known weights matters in this application.

Here we agree, the actual accuracy is all but moot, BUT calibrated weights are the only method to prove linearity.

Also it would be practically impossible to pick at random 100 pieces all of the same weight. Our sample size would in all probability be representative of the range of weights of the pieces would it not ?

That depends on a few factors, basically. Is your sample taking equal amounts of pieces from separate lots? Does it take from different lots of raw material? I've been doing metrology just long enough to know that the density of metals can vary significantly from batch to batch (Our Class 1 standard weights, which are very controlled throughout their manufacturing process, vary from the nominal density of stainless steel by 0.8%, and I have to compensate for it when calibrating scales due to the difference in bouyancy).

Therefore by taking a count of say 100 pieces and setting our scale to 100 pieces we are in effect taking the actual mean weight of these 100 pieces. What this actual weight is is totally irrelevent, if it were given by the scales as 100 grammes then 1000 pieces would weigh (according to the scales - if linear) 100 kg. Also if the scales weighed the pieces at 1 gramme then 1000 pieces would weigh 1 kg. Whether or not the pieces were actually 100 kg or 1 kg doesnt matter in the slightest. All that is important is the ratio of the weight of the sample to the weight of the total count required.

Again, the actual weight is not an issue, you are correct. The mean weight, as I said, would have to be proved, which I would highly suggest. I also agree that the ratio is what your scale computes. I would also caution that any error in the 100 piece tare is now multiplied by 10 at 1000 pieces. For the ratio to be most accurate, the tare would be on a larger sample than that which is to be used, to minimize variation from piece to piece. A sample of 1000 would provide a much closer mean than a sample of 100.

Another problem is that as the scale approaches 0 parts, it must become increasingly more accurate (as in % of reading, not % of span). Your "calibration" method would actually be most accurate at the higher readings (larger sample), and become less accurate at lower readings, as possible variance from mean would increase with a smaller sample. I do not see how the scale linearity can be proven by any means other than calibrated weights, as it also proves the capability of the ratio of the unit. How would you prove the linearity of the scale?

There is one more point on this that should be made. In the USA, if the counting scale is being used as the final method to count parts that are to be sold to customers, then it is being used for "legal trade", and by law MUST be calibrated with traceable standards.

This is fun!

Ryan

M Greenaway said:

Thanks Dave

I think your sarcasm actually hits the nail right on the head. I could perform the same function using a simple balance - calibration is not required !

You still have to prove that your device reads a true balance. That's done with calibrated weights. Then you can proceed with confidence that your see saw method works.
:rolleyes: :ko: :smokin:

energy said:

You still have to prove that your device reads a true balance. That's done with calibrated weights. Then you can proceed with confidence that your see saw method works.
:rolleyes: :ko: :smokin:

I hate when my knife edges dull, or my agate gets scratched, and fulcrum friction makes my balance all whacky. :frust:

Ryan Wilde said:

I hate when my knife edges dull, or my agate gets scratched, and fulcrum friction makes my balance all whacky. :frust:

Ryan.

I was once in a fishing contest for the largest Fluke. They had a device they made out of what looked like coat hanger material. You hung the two fish in question on opposite ends of this thing. It would tip towards the heavier fish. I noticed the loop formed to represent the fulcrum was a little closer to one end than the other. The prize was $200. The winner was chosen between two fish that were identical, by eye. I mentioned that the device didn't look even. I was told that weight was weight and mind my own business. I said "O.K. Switch them". "What are you, a trouble maker?". :vfunny: My personal safety became paramount and I beat a hasty retreat, lest I end up as Lobster Trap bait. At the least I could have ended up in the hospital of the Police Station. To this day, I feel it was rigged because we were from out of town and the locals knew which end to use if it got close. You also had to watch out that they didn't insert a lead weight inside the poor fish. :vfunny: "Calibrate it" would have been another language there. "Calibrate this, Buddy!":ko: :smokin:

Thanks for the responses guys.

It had clarified my understanding of the calibration requirments of this type of equipment, and I have found some good pointers on the web.

I certainly agree that linearilty is vital, and can only be proven against calibrated weights.

Factors that I need consider in our application is calibration of linearity over the actual range we use, on the cal certificate the first weight is at 400g, our samples can weigh as little as 30g ! Also it appears that resolution can be a big problem, as the scale rounds up or down you get an error, which is multiplied by your final count.

Thanks again, lots to think about.

Did the medication kick in or what?

Sorry Dave, dont know what the f@ck you are on about !

M Greenaway said:

Factors that I need consider in our application is calibration of linearity over the actual range we use, on the cal certificate the first weight is at 400g, our samples can weigh as little as 30g ! Also it appears that resolution can be a big problem, as the scale rounds up or down you get an error, which is multiplied by your final count.

Thanks again, lots to think about.

This is a giant point that I'd like to make, as a provider of calibrations to companies such as yours. Tell your provider what you need, such as the fact that you need a deviation from their standard procedure, etc. Next time you get the scale calibrated, insist that their first test weight be only 30g, and if you have specific weights that you use it at, specify those as well, in writing. Any accredited lab will have to follow your instructions, as 17025 requires us to do so, and it is just good customer relations to provide what the customer wants.

As a provider, this is the invaluable information that we like to have, and if we need to, we will write a specific procedure for your application. I make it a point when I calibrate an oven to ask the user at which temperatures he/she uses the oven. This temperature is invariably never one of the procedure test points, so I add it. It seems I may need to ask the same when calibrating scales...

Ryan

energy,

My idea of fishing is to go up the street to our friendly Portuguese fish market. :vfunny: They use a scale calibrated by the State of New York and there is never any dispute. Besides, I'd need a translator to dispute their findings...

Ryan:bigwave:

You must first understand how counting scales work. A reference sample is applied to the platform. This ref. sample is entered via the key pad of the scale (lets say 10 pieces) and divide by the weight of the reference sample on the platform. This results in the average piece weight. The more sample used, the more accurate the average piece weight. The scale can be out of calibration but still count correctly as long as it is linear. Another thought, a counting scale weighing resolution is normally around 1:10,000 0r 1:5000 in the weighing mode. When you enter the count mode you are using the internal resolution that can be 1: 500,000 or better.

Calibrate counting scales as you would a weighing scale. Ref. to NIST Handbook 44 table 4 and table 6.