Alliance Scale Blog

The Role of Uncertainty in the Industrial Weighing System

Apr 18, 2017 11:06:04 AM / by Ray Secour

Calibration Cannot Be Accurate without an Established Level of Uncertainty

Uncertainty is a core concept in the industrial weighing systems because it governs equipment accuracy. Upon the completing of calibration, you receive a certificate. This tells you first, that the weighing equipment is fit for purpose, and second, the uncertainty in the calibration. But, why is it so important to focus on the level of uncertainty of the reading? 

A Calibration Chain

industrial-weighing-system-blog.jpgWhen scales are calibrated a trained technician carefully weighs objects of known mass. Depending on the measuring range of the equipment he might use 1, 1,000 and 5,000 gram weights. As each is weighed the deviation in reading from the actual weight is noted. If necessary, the scale is adjusted to bring it back into an acceptable range. (If it can't be adjusted, it's time for new equipment!)

What gets forgotten is that those weights had to be weighed too. And of course, that was done with scales. Those scales also go through regular calibration, using weights whose mass is known to the closest approximation. However, it's impossible to know exactly what they weigh.

The scale has a finite resolution, perhaps down to three decimal places. Plus, it's probably not perfectly repeatable. Each time a mass is weighed the result may be slightly different, just because of how the scales work. As a result, those weights used for calibrating your scales themselves have some uncertainty.

The NIST Standard

All weighing equipment calibration involves comparing the indicated reading against the reading obtained by weighing something whose weight is known more accurately. Here in the US, your calibration certificate refers to traceability back to a National Institute of Standards and Technology (NIST) standard.

Since 1889, NIST has maintained what's called the “primary prototype national standard” weight, known as K20. This should weigh exactly 1 kilogram, and it's what all our calibration weights are traceable to.

Quantifying Uncertainty

A good question to ask is – how do we know K20 weighs 1 kilogram? The answer is that every few years it's compared to the International Prototype Kilogram that sits in a lab in Paris. When this was done in 1985 it was found that K20 is about 0.022 mg underweight.

Uncertainty in weight is inevitable. Calibration ensures you know what that uncertainty is. That's why it's so important to have your weighing equipment professionally calibrated.

Alliance Scale offers calibration, along with scale inspection and repair services. Our team of technicians provides exacting calibration service with military-inspired precision. All testing equipment can be tracked to NIST.

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