Most laboratory and commercial applications require scales that provide a certain level of accuracy or higher. According to the NIST Handbook 44:
“Equipment shall be suitable for the service in which it is used with respect to elements of its design, including but not limited to, its capacity, number of scale divisions, value of the scale division or verification scale division, minimum capacity, and computing capability.”
As part of these requirements, the handbook outlines scale classes and what applications a particular class may be used for.
In this post, we explain the differences between the various classes and provide some examples.
The Different Classes of Scales
Accuracy classes help categorize scales in a couple of ways:
- They determine which applications the scale will be suitable for.
- They establish the tolerances that will be applied when the scale is tested.
To be assigned a particular accuracy class, a scale must meet specifications. They are grouped into classes based on two factors:
- The number of scale divisions (n): This is the scale’s capacity divided by its readability (the smallest number it can display). For example, for a scale with a 500 g capacity and a readability of 0.01, n is equal to 500 divided by 0.01, which is 50,000.
- The verification scale interval (e): This is the lowest interval that may be used to allocate weight-based pricing for a given scale in a commercial setting.
The table below defines the main classes as per the above specifications, and shows the applications that these classes are typically used for. Note that the list of applications is not exhaustive. This information is from pages 40 and 58 of the NIST Handbook 44.
|
Class |
Verification scale interval (e) (in SI units) |
Minimum number of scale divisions |
Maximum number of scale divisions |
Applications |
|
I |
≥1 mg |
50,000 |
High-precision weighing in laboratories. |
|
|
II |
1 to 50 mg |
100 |
100,000 |
Laboratory weighing for products including precious gems and metals, grain, and medical cannabis. |
|
≥100 mg |
5,000 |
100,000 |
||
|
III |
0.1 to 2 g |
100 |
10,000 |
Commercial weighing for products such as precious metals, semi-precious gems. Also used to weigh animals, laundry, mail, and vehicles (vehicle scales with capacity ≤ 30,000 lb). |
|
≥5 g |
500 |
10,000 |
||
|
IIIL |
≥2 kg |
2,000 |
10,000 |
Commercial weighing for heavier loads, for example, livestock weighing systems, axle-load scales, and higher-capacity vehicle scales. |
|
IIII |
≥5 g |
100 |
1,200 |
Highway weight enforcement, for example, portable axle-load scales and wheel load scales. |
Note that not all scales are assigned a class. Unmarked scales can still be used for some of the above applications, but in many cases, there will be an upper limit on the value of the scale division. For example, for retail food scales with a capacity less than or equal to 50 pounds, the maximum scale division allowed is one ounce. For animal scales, the maximum scale division is one pound. A full table showing these limits can be found on page 58 of the handbook.
Examples of Scale Classes
Below are a few examples of some Class I, II, and III scales:
Class I
Most analytical balances and microbalances are designated Class I, including these ones:
Left to right: An AGN Pro Analytical Balance and a XA 4Y Microbalance.
Class II
Precision and high-capacity laboratory balances tend to be assigned Class II. Here are some examples:
Left to right: An AG Pro Precision Balance, and an Cubis® II High Capacity Balance.
Class III
An Ohaus Scout SJX6201N/E and a Torbal DRX-3 Mechanical Torsion Balance.
