Materials Specification and Analytical Measurement Precision

Paper presented as a poster at the Royal Photographic Society Conference on Photographic Gelatin, Cambridge, 15-19 September 1997.

Abstract

Chemical manufacturing processes are often sensitive to changes in conditions and minor components, for which there may be limited data and no simple relationships. The product specialist has the dilemma of basing raw materials and process specifications either on current practice or intuitive experience, without adequate knowledge that their attainment will ensure the product’s fitness for use.

However, a natural reaction is to demand increasingly tighter specifications and measures of the process capabilities involved. A judgement has to be made of the acceptable level of variability in the measuring process, in relation to the overall variability of the factor to be controlled. There is empirical evidence to suggest that there are limits on the precision to which analytical measurements can be taken and the application of process capability indices may be inappropriate when applied to chemical specifications.

To avoid the false security of over-tight specification limits, which cannot be meaningfully monitored, it is suggested that the specification process should start from the best practical estimate of the test capability for the measurement of the parameter or component being considered. From this, the raw material or process parameter specifications can be calculated, taking account of the balance of risk between the supplier and user.

Departure from specification would then have a basis of veracity and be less likely the subject of the random combination of chance effects. It would highlight areas in which there was a need for a real breakthrough in analytical technique or more robust product design.

Notes

Paper presented as a poster at the Royal Photographic Society Conference on Photographic Gelatin, Cambridge, 15-19 September 1997.