Are all checkweighers accurate?

Checkweighers check product weight to ensure goods are within the specified limits. By removing any items outside the specs, they keep product quality and costs in check. But are all checkweighers accurate?

We define checkweigher accuracy as the standard deviation of the weighing (or “weighment” in checkweigher jargon) of one item weighed numerous times on the checkweigher. The “standard deviation” is the spread of the data from the average. Another way to look at it is that “accuracy” actually measures the checkweigher’s “uncertainty”. 

The same item weighed repeatedly will vary small amounts — just how much is the key. Accurate repeatability defines a good quality checkweigher, and it must do this in a very short time (as in, within micro seconds). 

Just how much the checkweigher varies has nothing to do with the actual weight of products on the line. As an example: a product’s nominal weight might be 150g with acceptable variations from 145g to 155g. The checkweigher, however, may only vary by up to 0.5g for different items measured on that line. 

Because checkweighers aren’t static, there are lots of dynamic variables affecting both the scale and the package on the checkweigher: there are fluctuations in temperature, air currents and humidity; debris and dust can fall on and around the scale; vibrations or “noise” are unwanted signals to the checkweigher’s scale (e.g.: a hopper, press, or another conveyor in contact with the checkweigher); electrical “noise” (electro-static discharge and radio frequency interference) from mobile phones or other machines; and cleaning methods (e.g. if something is too caustic). Products moving can also have an impact, so the scales don’t quite fully settle at single weight reading; a good analogy here is to imagine weighing yourself: step on the scales, see where the needle rests, then hop off and weigh yourself again by walking across the scales. The measurements will be different.. 

However, technology developments have made modern checkweighers very reliable and highly accurate. A checkweighing system can do in a flash what once took a QA officer some time to collect manually. 

 Not all checkweighers are the same though. As well as accuracy, other factors to consider are the machine’s build, robustness and application flexibility. You may spend more on a checkweigher with high accuracy, but the reality is it will be much more cost effective in the long run by reducing giveaway and rework costs. Lower-accuracy checkweighers might be cheaper up front, but could end up costing a lot more in those hidden costs. 

Testing accuracy 

You can test a checkweigher’s accuracy yourself by weighing a representative item on a set of scales you know to be highly accurate, then weighing the pack again on the checkweigher, multiple times. Calculate the average and the standard deviation (also called “sigma”). The checkweigher’s accuracy can be defined at +- 1, 2, or 3 standard deviations. The smaller the number the more accurate the machine. 

For example: if a 500g pack is weighed multiple times, and 95% of the time the weight shows between 499 and 501, then that checkweigher has 1g accuracy at 2 standard deviations from the norm. 

Finally, don’t shoot the messenger. Your checkweigher is just the tool, essentially reporting on processes further upstream in the process. To keep it doing its job properly, include it in your factory’s preventive maintenance (so that it can be calibrated) and cleaning programs. Also, make sure your staff are properly trained in how to use the checkweigher. 

For a checkweighing solution that is perfect for your needs, please contact us via email or call 1300 IQVISION (1300 478 474). We have years of experience customising applications from the simple to the complex. 

iQVision also has a host of information in our resource library, including case studies, whitepapers, FAQ’s, videos, our blog and brochures. They’re all free to download. 

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