Volume Measurements - Coping with Non-standard Conditions and Parasitic Effects

Volume measurements of large amounts of liquids are a matter of routine at many nuclear materials processing plants. One of the techniques used for these measurements is based on a pneumatic device called \"pneumercator\" or \"dip tubes system\". Although this technique is conceptually simple, its practical implementation requires the execution of a long chain of measurements of various kinds, which are often performed in different conditions and at different times. The desired result, i.e. \"the volume of the liquid\", is then obtained from an appropriate combination of the results of the partial measurements. Due to its practical complexity, the \"pneumatic\" technique is sensitive to a number of ambient and process parameters, as well as to the specific measurement procedures adopted by the operator. Deviation from standard process conditions and measurement procedures will definitely result in a biased volume. In the practice, unfortunately, full control of the process parameters and strict adherence to standard conditions and procedures are difficult to achieve, especially in large facilities or when working under the pressure of industrial production. The engineer in charge of the measurements shall therefore be prepared to recognize the factors which may affect the results obtained in less-than-ideal conditions, and to compensate the biases produced by them by means of calculations based on appropriate models. Models for bias propagation tend however to be rather complex, since: i) the measurement chain is very long and ii) different types of measurements are carried out with different periodicity, thus complicating the composition and propagation of errors. This paper discusses the problems related to the modelling of the biases generated by operating in nonstandard conditions. It also shows how corrections of biases can be made rather simply provided that the relevant calculations are based on a sound model. It is worth to mention that the Joint Research Centre has recently established a Laboratory at Ispra (Italy), where all the problems related to volume measurement will be investigated. This Laboratory, whose name is TAME, is also actively engaged in measurement modelling, both in standard and non-standard conditions.