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 tendifferent results or will they get the same result all ten times. In the scale industry we call this “repeatability.”
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).
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
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.
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.
So you have a checkweigher, conveyor scale, monorail, deck scale, or bench scale, but is your scale accurate statically?
Lets first define accurate, since we are talking about a scale here. We’ll use the definition that “NIST” gives us. (NIST is the National Institute for Standards and Technology). This is the Government agency that makes the rules for legal for trade scales).
NIST defines accurate as: A piece of equipment is “accurate” when its 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-conforms to the standard within the acceptable tolerances and other performance requirements.Equipment that fails to conform is “inaccurate.”
To simplify this, a scale need only be within tolerance to be considered accurate,therefore your scale can be off a small amount and still be considered accurate.
How do you determine the tolerance for your scale so you can determine if your scale is accurate?
Table 7a below describes the different classes of scales. Legal for Trade scales will have the class marked on a tag somewhere on the scale. This is important because there are different tolerances for each class of scale.
Typical Class or Type of Device for Weighing Operations
Weighing Applications or Scale Type
|Precision laboratory weighing|
|Laboratory weighing, precious metals and gem weighing, grain test scales|
|All commercial weighing not otherwise specified, grain test scales, retail precious metals and semi-precious gem weighing, animal scales, postal scales, scales used to determine laundry charges, and vehicle onboard weighing systems|
|Vehicle, axle load, livestock, railway track scales, crane, hopper (other than grain hopper) scales, and vehicle onboard weighing systems|
|Wheel-load weighers and portable axle-load weighers used for used for highway weight enforcement|
|NOTE: A scale with a higher accuracy class than that specified as “typical” may be used.(Amended 1985, 1986, 1987, 1988, 1992, and 1995)|
Table 6 below describes the tolerances that a scale is allowed. We are going to look at a class III scale, the 1 , 2 , 3 , and 5 that you see are the number of divisions that a scale can be off and still be within tolerance or “Accurate”.
Looking across the chart for a class III scale we see that between 0 – 500 divisions a scale can be off by one division, and that between 501 – 2,001 divisions you are allowed two divisions.
(All values in this table are in scale divisions)
Tolerance in scale divisions
0 – 50,000
0 – 5,000
0 – 500
0 – 50
50,001 – 200,000
5,001 – 20,000
501 – 2,000
51 – 200
|200,001+20,001+2,001- 4,000201- 400||4,001+401+|
|Add 1d for each additional 500d or fraction thereof|
What is division?
Let’s use a 5,000 pound deck scale for an example.
Most 5,000 lb deck scales will weigh in 1 lb increments, a division is essentially 1 increment.
In other words a division is the number that the scale counts by.
On a 5000 lb deck scale, from 0 to 500 divisions (0 lb to 500 lb) you can place a 250 lb test weight on the scale and have the scale read anywhere from 249 lbs to 251 lbs and still be accurate.
If you test with a 1000 lb weight, then you’re to the 501 – 2,000 column. Now you can be off by 2 divisions and still be accurate (998 lbs – 1002 lbs).
Now, use a bench scale for an example.
The bench scale has a capacity of 100 lbs and weighs in increments of .02 lbs.
The division size is now .02
To determine the weight ranges to use for the table:
.02 x 500 = 10 lbs Between 0 lbs and 10 lbs the scale is allowed 1 division of error.
.02 x 2000 = 40 lbs Between 10.02 lbs and 40 lbs the scale is allowed 2 divisions of error.
There are other factors that need to be considered when determining if the scale is accurate:
- Does it return to zero?
- Does the scale repeat?
- How do the corners test?
The purpose of this article, however, is to show that scales can have a small amount of error in them and still be accurate.
Recommended Minimum Load
Value of scale division
(d or e*)
Recommended minimum test load
(d or e*)
Equal to or greater than 0.001 g
0.001 g to 0.05 g, inclusive
Equal to or greater than 0.1 g
|*For class I and II devices equipped with auxiliary reading means (i.e., a rider, a vernier, or a least significant decimal differentiated by size, shape or color), the value of the verification scale division ”e” is the value of the scale division immediately preceding the auxiliary means. For Class III and IIII devices the value of “e” is specified by the manufacturer as marked on the device; “e” must be less than or equal to “d”** A minimum load of 10 d is recommended for a weight classifier marked in accordance with a statement identifying its use for special applications.(Amended 1990)|
A scale is suitable for its intended use if the minimum load which will be weighed on it is 20 measurement divisions or more.
A scale whose smallest division is 0.05 lb is suitable for minimum loads of 1 lb. or more.
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!
NTEP stands for National Type Evaluation Program.
It is administered by the National Conference of Weights and Measures and is endorsed by most states as being the benchmark to allow a device to be used as a legal for trade point of sale weighing device.
To get an NTEP approval (also known as a Certificate of Conformance (CoC) you must submit an application, conduct laboratory testing and in most cases field testing to verify accuracy and performance of the automatic weighing system (in-motion checkweigher, conveyor scale, etc).
There are weighment accuracy tolerances that must be met.
Testing of performance for these tolerances occur at temperature variations from 14º F to 104º F (environmental chamber), often in the field locations to prove industry usage, and permanence testing where they run up to 100,000 weighments across the scale between testing sessions to verify the device’s ability to remain accurate.
It is a VERY expensive commitment by the manufacturer of the device and in itself is a testament to the seriousness and commitment of the manufacturer to a given product design and weighing system application of that product.
Simply put: it often separates the ‘wheat from the chaf’ in the weighing industry giving the perspective buyer of an automatic weighing system the added assurance that the manufacturer who has a CoC on a device knows his stuff and is not only capable of building a high quality product, but has demonstrated it to his peers.
What is an automatic weighing system?
At its most basic, it weighs products automatically without an operator to physically place product on a scale, record the weight, and then remove it.
Think of a conveyor line moving product.
Do you want a scale in that line weighing each product?
Or do you want to physically remove each product, place it on the scale, record the weight, and then replace the product back on the conveyor to weight 100% of the product? Probably not. Labor simply costs too much these days.
Automatic weighing systems basically fall into two categories, checkweighers and weigh labelers.
Checkweighers simply check the weight and often reject out of weight spec product, while a weigh labeler will do the same, but also will record the weight on a label or database of each individual product for use in the sales transaction.
A weigh labeler will handle various types of products including variable length, width, height, and weight (this is tougher to do), while a checkweigher is basically “tuned” to run one product (this is easier to do).
In NTEP testing, the difference is subtle but very distinct.
A weigh labeler must be calibrated, sealed (so you cannot change calibration), and then tested completely including temperature, field, and permanence, while a checkweigher may be calibrated before every test.
You could say ‘well, just get a checkweigher approval because it is easier to pass,’ and you would be right!
But hold on just a minute, if you got a checkweigher approval you would only be able to sell your automatic weighing system to check weigh, not to record variable weights on labels or databases as a bases for individual product sale.
It is also worthy to note that NTEP approval is not required of checkweighers while NTEP approval IS required of weigh labelers in most states.
Think of it this way, a checkweigher (with or without NTEP approval) is used to verify net weight labeling on something like cans of soup where every can is supposed to weigh the same net amount and the product more often than not is the same physical size.
The soup manufacturer is required to meet the National Institute of Standards and Technology’s Handbook 133 (Checking the Net Contents of Packaged Goods) requirements.
Basically, for the soup manufacturer, the regulatory people say “meet the H133 standards including Maximum Allowable Variation and we don’t care how you do it, just do it.”
The soup manufacturer will often install a checkweigher to assist them in making sure they meet the H133 standards.
The tray pack manufacturers are faced a more direct regulation requirement.
Each package weight may be different and all of them must be accurate.
A weigh labeler would be used for variable priced trays that could easily vary by several pounds in weight and physically be very different in terms of size and shape of product (chicken, beef, seafood, vegetables, etc.).
The labeler must still be accurate for all weighments and variations of products being chaotically run without calibrating (tuning) the scale to a specific product.
As with anything, there are limits to the variability, but the key idea is you can feed that chaotic (various) product to a weigh labeler with peace of mind.
The neat thing is, if you go through the extra tight and stringent work of NTEP-testing your automatic weighing system as a weigh labeler and add a simple easy-to-pass Maximum Allowable Variation test, you also get the checkweigher NTEP approval. This allows for listing both on the Certificate of Conformance (CoC).
Well this is a no brainer right?
Well not really, because if you submitted for NTEP approval only as a checkweigher you would be allowed to calibrate before each test and would not have to seal your calibration for the duration of the NTEP testing like you have to do on a weigh labeler.
THAT MAKES IT A WHOLE LOT TOUGHER TO GET A WEIGH LABELER APPROVAL THAN A CHECKWEIGHER APPROVAL.
Basically, when you look at automatic weighing systems, if there is no NTEP approval, BUYER BEWARE!!!!
You are considering purchasing a unit that has not been evaluated for performance, thus you may not even be capable of meeting any kind of tolerance at all, let alone being able to maintain its performance without constant “tuning.”
If the unit has a NTEP approval but does not specifically call out class III weigh labeler, you could be looking at a device that has proven it is capable of being “tuned” to weigh correctly for a given single product for a given temperature for a given period of time.
However, if you buy an NTEP approved weigh labeler, it IS a given that you are purchasing a unit that has been proven to be capable of accurately weighing a VARIETY of product, at a VARIETY of temperatures, for a long period of time without having to mess with calibration of the scale system to make sure it is working properly.