Repeatability, Accuracy and Division size can be very confusing when trying to evaluate performance of an in-motion checkweigher. Unfortunately many in-motion checkweigher suppliers attempt to confuse the perspective purchaser rather than inform and educate.
For the sake of this article, let’s assume we have a 2 kg in motion checkweigher that has a division size of 1 gram and will use a test puck that weighs exactly 522.00 grams. Let’s examine each term:
Repeatability is the ability of checkweigher to show the same result over and over using the same test item (test puck).
Perfect repeatability would mean that every single in-motion (dynamic) weighment of the single test puck returned exactly the same result. Thus, if this particular test puck that weighed 522 grams were weighed 100 times, every single weighment would be exactly 522 grams. While this is possible, it’s somewhat unlikely depending on the division size, pieces per min, and speed of the belt.
Repeatability is often referred to as +/- X number of division. Perfect results is this example would be +/- 0 d (described below, see “scale division size”).
Accuracy is really not a proper term in the field of weighing, as it is not defined in NIST (National Institute of Standards and Technology, Handbook 44 (scale man’s bible)); However, accurate is defined there and is more of a pass fail determination rather than a method to measure performance.
Accurate as found in H44 is defined below:
Accurate – A piece of equipment is “accurate” when it’s performance or value – that is, its indications, its deliveries, its recorded representations, or its capacity or actual value, etc., as determined by tests made with suitable standards. If this criteria conforms to the standard within the applicable tolerances and other performance requirements, the equipment is deemed accurate.
Equipment that fails to conform is “inaccurate”.
A scale, in-motion checkweigher, automatic weighing system, conveyor scale, etc. is accurate if the device is able to meet a prescribed tolerance.
For instance, our example in-motion checkweigher above has a maintenance tolerance of +/- 2 divisions between 501-2000 divisions taken from the NIST handbook 44 automatic weighing systems section.
The checkweigher is accurate if weighments fall between 520 and 524 grams if the division size is 1 gram. The same scale would have to have all it’s weighments fall between 521.8 and 522.2 if the division size was 0.1 grams, to be considered accurate.
Since the test puck is so close to the tolerance level of 0-500d, we suggest using +/- 1 division tolerance to measure performance.
Scale division size, (commonly know as “d” in the weighing industry), is defined in NIST Handbook 44 (H44) as: Scale Division, value of (d).
The value of the scale division, expressed in units of mass, is the smallest subdivision of the scale for analog indication, or the difference between two consecutively indicated or printed values for digital indication or printing.
Since most modern weighing equipment is digital, it is really the amount that the checkweigher counts or displays by.
For example, our 2KG capacity checkweigher may have a division size of 1 gram. Thus, the division size is 2 thousandths of the check weigher’s capacity (2,000 divisions).
Unscrupulous manufactures and representative will often try to mislead a potential purchaser by trying to confuse the purchaser by claiming division size is accuracy. This is a lie! A deceptive supplier may configure a checkweigher that had perfect repeatability and be accurate and at 2,000 divisions instead at 20,000 divisions at 0.1 grams!
It’s not surprising that the repeatability turns bad in a hurry and the device will not be able to meet tolerances and is thus classified as inaccurate. Instead of reliable weighments that the purchaser can count upon to manage their business, the purchaser is left with endless garbled weighments and performance tests that are not only inaccurate but also very confusing.
Know your in-motion checkweigher, automatic weighing system, or conveyor scale terminology. Ask for repeatability specifications for a given division size just under the tolerance break points: 0-500d, 501-2000, 2001-4000, and 4001-up.
Never believe anyone who says accuracy is division size. Use NIST H44 tolerances as your guide to determine value and performance capabilities!