• Part 2 of ISO 14253 (published) is the so called "Poor mans version of GUM" and describes a cost benefit and practical estimation method for industrial use (the PUMA-method).

• Part 3 of ISO 14253 (under preparation) describes a procedure for how two parties can reach to agreement on disputed measurement uncertainty statements.

However, in many cases it is realized that disagreements on the measurement value from two different parties cannot always be explained by the presence of measurement uncertainty only.

Therefore, ISO/TC 213 has defined another type of uncertainty, namely "specification uncertainty".

It is the responsibility of ISO/TC 213 to develop unambiguous standards stating clear rules and to develop adequate tools for drawing indication and specification of all the necessary conditions that might affect any measurement result, e.g. filter type, stylus tip diameter, and mathematical calculation algorithms.

An ambiguous drawing indication and/or incomplete specification will lead to specification uncertainty. Consequently, it is also the designer’s/draughtsman’s responsibility to use the GPS-system correctly according to the rules stated in the ISO standards.

The combination of the measurement uncertainty and the specification uncertainty is called "conformance uncertainty" and should be taken into consideration when a given specification on a workpiece is to be verified.

ISO/TC 213 has introduced a third type of uncertainty called "correlation uncertainty". This uncertainty is to be used for the description of how well the actual Geometrical Product Specifications match to the actual function of the part.

It is the intention of ISO/TC 213 to enrich the GPS-language to allow expression of requirements relating to a wide range of workpiece functions.

It is the designer’s responsibility to choose and use the special GPS relating best possible to the specific function of the part.

The correlation uncertainty is not to be taken into consideration when a given specification on a workpiece is to be verified, but may serve to explain why a part is functioning although some specifications on a workpiece are not met or vice versa.

The combination of the measurement uncertainty, the specification uncertainty and the correlation uncertainty is called "total uncertainty".

Some people in industry believe that the uncertainty management is very time consuming and not cost- and time beneficial, but purely academic stuff meant only for highly educated people at e.g. universities and accredited laboratories. This couldn’t be more wrong and is just the opposite of the intentions laid down in the 14253-series.

Uncertainty management is a powerful tool to communicate magnitudes of "risk" on making decision based on verification results. The intention is only to use time on uncertainty estimations where it is of importance for the function of the part and where it is cost-beneficial for the company.

The fundamental ideas concerning the uncertainty management as described above have great impact on the standards prepared by ISO/TC 213. Especially the Measurement equipment standards will in future be completely different from those we know today.

Many terms will be defined in order to make everything more clear and unambiguous. An example of this is the new ISO 10360 part 1 (soon to be published) which is a very useful vocabulary for coordinate measuring machines (CMM).

There will be a clear distinction between "design characteristics" and "metrological characteristics".

The purpose of the measuring equipment standards shall not be to restrict the design of a measurement instrument and hereby limit the development of measurement technology, unless it has influence on the measurement result. Only the uppermost important design characteristics, i.e. for purposes of interchangeability, may be subject to the standardization, e.g. the geometry, size and tolerances of the clamping and mounting systems.

However, in the case for the metrological characteristics the situation is quite the opposite. A metrological characteristic is a characteristic of measurement equipment that may influence the results of measurement (a quantity that can be a potential uncertainty contributor). In order to be able to compare the performance of measurement instruments from different manufacturers it is very important that all the necessary metrological characteristics are well-defined. It is unnecessary - and can be of disadvantage for the user - to standardise the limitations of the characteristics, i.e., the maximum permissible errors (MPE’s). Every manufacturer should be free to state "the quality" of the measurement equipment in terms of his MPE-values, and the user/buyer shall be able to compare and classify the measurement equipments from different manufacturers for his own purpose.

Furthermore, it makes no sense to standardise what a calibration of specific measurement equipment shall contain. It can be costly, time-consuming, and meaningless to calibrate metrological characteristics of measurement equipment if it has no influence when the equipment is in use. It should be up to the user to decide, based on the use of the equipment and on the uncertainty budgets, which metrological characteristics are to be calibrated.

which are very seldom specified e.g. on the drawing of the workpiece. If it isn’t specified, either on the drawing or in standards, the specification is ambiguous and therefore has a large (specification) uncertainty.

If everything is specified, the specification uncertainty is eliminated, but this doesn’t assure that the intended function of the part is described properly. If the function isn’t characterized by the specifications in accordance with the reality, the specifications will correlate badly to the intended function and therefore result in a correlation uncertainty.

Diameters on workpieces are used for the purpose of a large variety of functions, e.g. rollers, fit without clearance, fit with clearance, hydrodynamic bearings, and roller bearings. The workpiece-material, which can be either rigid, flexible, or can be deformed permanently, will usually also have an impact on the function. Today there are very few possibilities in the current GPS-standards to indicate which type of function the feature shall fulfil or to indicate a more detailed specification, e.g. which algorithm is to be used for the calculation of the diameter. Consequently, in many cases a correlation uncertainty, which can be quite significant, may also be present.

The intention of ISO 14405, "Geometrical Product Specifications (GPS) — Geometrical tolerancing — Linear size" (currently under preparation) is to give the designer a better tool with more possibilities to indicate the type of size that relates the best to the intended function of a workpiece. The type of size can be indicated on the drawing with the use of a symbol, see figure 1.

The symbol shall be indicated on the drawing after the specification (the nominal value and the tolerance) for the feature of size in a circle, a so-called modifier, see figure 2.

Figure 2 - Indication of least square size with a modifier.

Furthermore, the standard defines the default definition of size (the detailed specification for the feature of size when no modifiers are indicated after the feature of size). The advantage of a default definition is that it reduces the specification uncertainty and simplifies the drawing. It may be the case that the default definition results in a correlation uncertainty. Therefore the default definition shall only be used when the feature has no important function. If the feature has an important function, it is the designers responsibility to indicate the appropriate modifier in accordance with the intended function of the workpiece, but in any case the specification is more precise.

For the purpose of obtaining more precise specifications in the new improved GPS-system, ISO/TC 213 has developed the concept of operators. An operator in this sense is not the person who manoeuvres the measurement instrument but as defined in ISO 17450 part 2:

"operator"
ordered set of operations

and:

"operation"
specific tool required to obtain features or values of characteristics, their nominal value and their limit(s)

The idea is to build a system where the specification procedure is parallel to the verification procedure, see figure 3, where every single operation that may influence the verification result shall be specified either by a special detailed specification or by the use of defaults (a standardised specification of the operators to be used when no detailed information is given on the drawing).

When all the necessary specification operations are present and known, there will be no specification uncertainty. If the verification operators are identical to the specification operators, the measurement uncertainty will be relatively small depending, of course, on the practical circumstances. The use of an actual verification operator that deviates from the actual specification operator will normally result in an increased measurement uncertainty.

The skin model, which is a model of the real world (a model of the workpiece including form errors), is intended to be of help for the designer to understand the necessity for the specification of all the needed operators in order to achieve the value as unique as possible for the actual characteristic.

Figure 3 - Operators.

Goal # 1: To reduce the correlation uncertainty by developing the necessary functional related "tools" which makes it possible for the designer to express exactly what he needs.

Goal # 2: To reduce the specification uncertainty by:

a) identifying and defining operations that have influence on a characteristic

b) developing clear and unambiguous rules

c) stating default rules for the operations

Goal # 3: To enrich the improved GPS-language and still strive to keep it as simple as possible.

Guide to the expression of uncertainty in measurement (GUM). BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML, 1st edition, 1995

ISO/TS 14253-2:1999, Geometrical product specifications (GPS)  — Inspection by measurement of workpieces and measuring equipment  — Part 2 : Guide to the estimation of uncertainty in GPS measurement, in calibration of measuring equipment and in product verification

ISO/DTS 14253-3:— ¹⁾, Geometrical product Specification (GPS) — Inspection by measurement of workpieces and measuring instruments - Part 3: Procedures for evaluating the integrity of uncertainty of measurement values

ISO 14660-1:1999, Geometrical product specifications (GPS) — Geometrical features — Part 1: General terms and definitions

ISO 14660-2:1999, Geometrical Product Specifications (GPS) — Geometric features — Part 2: Extracted median line of a cylinder and a cone; Extracted median surface; Local size of an extracted feature

ISO/TS 17450-1:— ¹⁾, Geometrical product specifications (GPS) — General concepts — Part 1: Model for geometric specification and verification

ISO/DTS 17450-2:— ¹⁾, Geometrical product specifications (GPS) — General concepts — Part 2: Operators and uncertainties

ISO/CD 14978:— ¹⁾, Geometrical product specifications (GPS) — General concepts and requirement for GPS measurement equipment

ISO/TC 213 N355 Annex 1, Next generation of the Geometrical Product Specifications (GPS) language – The vision for an improved engineering tool