Specifications

METTLER TOLEDO Weigh Module Systems Handbook
(12/99)3-16
The biggest difference between NIST and OIML, besides the units used (English and S.I.
respectively), is the creep rate specification. Creep is the change in a weight reading
when a weight is left on a scale over a period of time. NIST specifications allow a creep
rate of 0.5 division for test loads of 0 to 500 divisions, 1.0 division for test loads of 500
to 2,000 divisions, 1.5 divisions for test loads of 2,000 to 4,000 divisions, and 2.5
divisions for test loads of 4,000 to 10,000 divisions when the load is applied for one
hour. OIML standards allow a creep rate of 0.5 division for test loads equal to the scale
capacity when the load is applied for 30 minutes. As you can see, for most capacities
OIML standards are more stringent, allowing a smaller error over a shorter time period.
To meet OIML standards, a scale must satisfy all requirements and perform within the
calibration tolerance limits.
Under EC Weights and Measures regulations, there is a difference between the concepts
of a "test certificate" and an "approval." Approval is given only for entire scales (not for
indicators or load cells alone). There are two types of approval:
• EC Type Approval for a self-contained complete scale.
• EC "Umbrella" Approval for a modular scale, made up of components (indicators,
load cells, junction boxes, printers, etc.). Each component must have an EC Test
Certificate, which must be listed on the umbrella approval.
Once an umbrella approval has been given, additional EC Test Certified components can
be added to it later. The approval covers scale systems made up of various
combinations of certified components. It also allows you to have one component
approved while other components are still being developed.