How Accurate is Your Scale?

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Mar 24, 2014

Being in the scale business we are often asked by customers how accurate are our scales? This can be a difficult question to answer because “accuracy” means different things to different people.

Most people that ask about accuracy want to know if a 100 lb scale can measure down to .01lbs or down to .001 lbs. In the scale industry we call this “division” size.

Some people want to know if they put something on a scale ten different times will they get ten different results or will they get the same result all ten times. In the scale industry we call this “repeatability.”

In-Motion Checkweigher with Alarm Light

In-Motion Checkweigher with Alarm Light and Reject Mechanism

Let’s take a closer look at “division” sizes. Usually static scales are limited to 5,000 divisions. What this means is that a scale that has a capacity of 5,000 lbs would be limited to a minimum division size of 1 lb, while a scale that has a capacity of 500 lbs would be limited to a minimum division size of 0.1 lbs. In some cases, it is acceptable to have a larger division size, for instance, a scale with a capacity of 500 lbs may have a division size of 0.2 lbs, however it is generally unacceptable to go the other way and have a scale with a capacity of 500 lbs with a division size of 0.05 lbs. (Scales are available that allow more than 5,000 divisions, but they become very expensive, tend to be delicate in nature  and are generally only used in labs or for precious metals and gems).

conveyor-scales

Some scales are used to weigh products as they travel across a conveyor belt,  this is referred to as in-motion weighing. Scales that are used for in-motion weighing that need to be legal for trade are generally in the range of 2000 divisions or less. Remember the 500-lb scale we talked about earlier that could have a division size of 0.1lbs? Well, if we do the math (500 divided by 2000 equals 0.25).

You might think that we could have a division size of 0.25, but the regulatory agencies have determined that this is not an acceptable division size. There for we have to round that number up to 0.5. If we need to have a legal for trade in-motion conveyor scale that we need to have a division size of 0.01 lbs, the capacity of the scale would be limited to 20 lbs.

conveyor scale gallery

Now that we understand more about division size, let’s talk about repeatability. We can use our scale with a 20 lb capacity that has a division size of 0.01 lbs as an example. In order for this scale to pass an inspection, the scale should be tested at several different weights. At weights between 0 lbs and 5.00 lbs the scale is only allowed to be off by 0.01 lbs and at weight between 5.01 lbs and 20.00 lbs, the weight can only be off by 0.02 lbs to technically be “accurate.”

In summary, selecting the proper division size is key to achieving the accuracy that you are looking for when selecting a new scale.

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