What is the true meaning of a “scale division” (d) versus a “verification scale division” (e)

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Introduction

If you are a state weights and measures official, scale manufacturer, or any other stakeholder dealing with scales used for commercial transactions, then this article may be of interest to you.

In 1984, an initiative was taken by the National Conference on Weights and Measures (NCWM) to harmonize the scale code in section 2.20, Scales, of the NIST Handbook 44 (NIST HB 44) with OIML R 76, the international standard on non-automatic weighing instruments used in legal metrology applications. Although many aspects of OIML R 76 were adopted in NIST HB 44, the exact same wording was not incorporated directly which led to unintended deviations, including the application of “scale division” (d), and the “verification scale division” (e). Furthermore, the current text in NIST HB 44 section 2.20 contains some inconsistencies and contradictions. This has led to confusion by field inspectors and has created non-uniformity in application of NIST HB 44 to scales where the “scale division” (d) differs from the “verification scale division” (e).

There is a current item (SCL-23.3, Verification Scale Division e: Multiple Sections …) on the agenda of the Specifications and Tolerance (S&T) Committee of the National Conference on Weights and Measures (NCWM) to amend the scale code, Section 2.20 of NIST HB 44 for the correct implementation of “scale division” (d) and the “verification scale division” (e). This item has prompted much discussion (and some confusion) recently among the weights and measures community.

What are the exact implications of this proposal? What do the “scale division” (d), and “verification scale division” (e) actually represent? And what is wrong with the current language in section 2.20 of NIST HB 44? How did we get here?

Let’s begin by first understanding the differences between “scale division” (d) and the “verification scale division” (e).

What are the differences between the “scale division” (d) and the “verification scale division” (e) ?

The indication on a scale has a certain resolution. This is the incremental size between two consecutive indications on a digital indication or two consecutive marks on an analog indication. This resolution is in the scale requirements called “scale division” and is denoted with the letter (d). But a scale used for commercial transactions also has a “verification scale division”, denoted in NIST HB 44 with the letter (e), which is specified by the manufacturer and is basically a measure for the accuracy of the instrument. The “verification scale division” is a value that is used to determine the tolerance of the scale.

The tolerance of a scale is expressed in e. For example, the maintenance tolerance at the lower end of the measuring range is equal to 1 e.  For a scale with a “verification scale division” of e = 10 g, the tolerance is therefore 10 g at the start of the weighing range. For most scales in the field, the “verification scale division” (e) is the same as the “scale division” (d) which is probably the biggest reason why there is confusion about the meanings of “scale division” and “verification scale division”.

Although the “scale division” and “verification scale division” are entirely different concepts for the verification of scales, they are related. Except for weight classifiers, on all scales of classes III, IIIL and IIII, the resolution of the indication “scale division” (d), must be the same as the “verification scale division” (e). Only on class I or class II scales and dynamic monorail scales, the “scale division” (d) may be smaller than the “verification scale division” (e). In case of a digital scale, the display contains an additional digit. This digit differs in size, color, shape or in any other distinctive way indicating that this extra digit does not contribute to the accuracy of the scale.

When the “scale division” is the same as the “verification scale division”, only the “scale division” (d) is mentioned on the name plate of the scale (or near the display). If the “scale division” (d) deviates from the “verification scale division” (e), then both values must be mentioned.

What are the issues with the current regulation in NIST HB 44?

During the prior effort in 1984 to harmonize NIST HB 44 with OIML R 76, some unintended deviations between the two codes were introduced. The most significant deviation with respect to the “scale division” and “verification scale division” was introduced in Table 6, Maintenance Tolerances, of NIST HB 44, section 2.20 in which scales are to be based on the tolerance on the “scale division” (d), whereas for OIML R 76, it is based on the tolerance on the “verification scale division” (e). This is an unintended deviation because the definition of “verification scale division” in NIST HB  44 clearly states that the value of e is used to determine the tolerance of the scale.

Besides the issue with the tolerances defined in Table 6, the scale code in NIST HB 44 contains several other inconsistencies and contradictions regarding the use of the “scale division” (d) and “verification scale division”(e). To avoid confusion and maintain uniformity in the application of the scales code, it is important that these inconsistencies and contradictions are corrected.

But what are the consequences of this unintended deviation?

The vast majority of scales used for commercial transactions are class III and class IIIL scales. With these scales the “scale division” (d) is required to be the same as the “verification scale division” (e) and should not pose an issue in field applications since they have the same value. However, the problem may occur with class I and class II scales (and dynamic monorail weighing systems) where the “scale division” (d) is smaller than the “verification scale division” (e).

Table 6, Maintenance Tolerances, of NIST HB 44, Section 2.20 is basing the tolerance on the “scale division” (d) (the resolution of the indication) instead of the “verification scale division” (e). Because d is now smaller than e, the tolerance for a class II scale with an auxiliary digit is also smaller than the tolerance of the same scale without the auxiliary digit. This could be as small as one-tenth of the tolerance based on e as it was originally intended. It is very likely that these scales with a differentiated scale division (d < e) will not pass inspection when these tighter tolerances are applied.

Alternatively, states might deviate from the tolerances in Table 6, Maintenance Tolerances, and apply tolerances based on the “verification scale division” (e). If this occurs, these scales will not be held to tighter tolerances, which affects the accuracy requirements and hinders harmonization among the states. Besides the applied tolerances in Table 6, Maintenance Tolerances, there are other inconsistencies and deviations between NIST HB 44 and OIML R 76 with respect to the use of “scale division” (d) and the “verification scale division” (e). One being the inconsistent language within the Scale Code, which has caused an incorrect understanding among some stakeholders on the meaning and correct application of the “scale division” (d) and the “verification scale division” (e).

What commercial applications are affected?

The only instruments that may have a “scale division” (d) smaller than the “verification scale division” (e) are class I and class II scales and dynamic monorail weighing systems.  Table 7a, Typical Class or Type of Device for Weighing Applications, in user requirement UR.1.1, General, of NIST HB 44, section 2.20 provides an overview of the typical type of device or weighing application for each accuracy class. According to this table class I and II scales are typically used in laboratories, applications for weighing of precious metals and gems and in applications to test grain.

Another possible application for inclusion in Table 7a is the commercial transaction of cannabis products*. It can be expected that states will require a class II scale in these transactions due to the high value and cost of these type of commodities.

NCWM Verification Scale Division Task Group

In 2020, the NCWM Verification Scale Division Task Group was formed to identify all instances within the section 2.20 of NIST HB 44 where the code deviates from OIML R 76, or where inconsistencies exist with respect to the “scale division” (d) and the “verification scale division” (e). The task group’s goal is to provide a proposal for reparation of these instances. This work has resulted in the submittal of a proposal to the NCWM S&T Committee and is currently on the S&T agenda under item SCL-23.3 as stated above.

Need more information?

For more information regarding “scale division” (d) and the “verification scale division” (e), and how they were intended to be implemented in the scales code in section 2.20 of NIST HB 44, please visit the in the NIST OWM’s FAQs or contact Jan Konijnenburg (jan.konijnenburg [at] nist.gov (jan[dot]konijnenburg[at]nist[dot]gov)).

* In contrast to hemp, marijuana remains a Schedule I substance under the Controlled Substances Act. NIST does not have a policy role related to the production, sale, distribution, or use of cannabis (including hemp and marijuana). NIST participates in the National Conference on Weights and Measures (NCWM) as part of NIST’s statutory mission to promote uniformity in state laws, regulations, and testing procedures.