Specifications

METTLER TOLEDO Weigh Module Systems Handbook
(12/99)3-14
International Standards
Although NTEP certification is widely accepted in the United States, it is not a worldwide
standard. When selling products outside of the United States, you should understand
and follow the local standards. Some common standards include the Measurement
Canada standard that is used in Canada and the Organisation Internationale de
Métrologie Légale (OIML) standard adopted by the European Economic Community.
OIML is an independent international organization that develops standards for adoption
by individual countries. Its main task is harmonizing the regulations and metrological
controls applied by the national metrological services in the countries that are OIML
members. There are two main types of OIML publications:
• International Recommendations (OIML R) are model regulations that establish the
metrological requirements for scales, as well as requirements for specifying
methods and equipment used to check a scale’s conformity. OIML member
countries are responsible for implementing the recommendations.
• International Documents (OIML D) provide information to help improve the work of
the national metrological services.
A scale with NTEP certification does not automatically meet OIML standards. Several
European testing labs (such as NMi, BTS, and PTB) conduct performance tests to verify
whether the equipment meets OIML standards and is capable of performing its intended
functions. OIML has its own set of accuracy classes and acceptance tolerances.
Instruments are classified according to absolute and relative accuracy.
• Verification scale interval (e) represents absolute accuracy.
• Number of verification scale intervals (n = Max Capacity/e) represents relative
accuracy.
The accuracy classes for instruments and their symbols are listed below:
Accuracy Class Symbol
Special Accuracy I
High Accuracy II
Medium Accuracy III
Ordinary Accuracy IIII
Figure 3-8 shows OIML acceptance tolerances, and Figure 3-9 compares those with
NIST Handbook 44 tolerances. Again, the vertical axis represents the permissible error
and the horizontal axis represents the number of divisions that corresponds to the actual
weight on the scale. Note that OIML acceptance tolerances are identical to those in NIST
Handbook 44 from 0 to 4,000 divisions. At 4,000 divisions, the NIST acceptance
tolerance increases from ±1.5 divisions to ±2.5 divisions, while the OIML acceptance
tolerance remains at ±1.5 divisions up to 10,000 divisions.