In theory, shock tubes provide a pressure change with a very fast rise time and calculable amplitude. This pressure step could provide the basis for the calibration of pressure transducers used in highly dynamic applications. However, conventional metal shock tubes can be expensive, unwieldy and difficult to modify. We describe the development of a 1.4 MPa (maximum pressure) shock tube made from unplasticized polyvinyl chloride pressure tubing which provides a low-cost, light and easily modifiable basis for establishing a method for determining the dynamic characteristics of pressure sensors.

Platinum–iridium (Pt/Ir) kilogram mass prototypes are known to gain contamination from the environment in which they are stored. The current method of cleaning these mass prototypes is called nettoyage–lavage and involves the physical rubbing of a kilogram with a chamois leather cloth soaked in a solvent followed by removal of any solvent residue using a jet of steam water. The manual nature of the technique means the effectiveness of the cleaning process is reliant on the human operative. An alternative cleaning method involving exposure to ultraviolet light and ozone (UV/O3) has been tested on Pt/Ir foils and kilogram mass prototypes. The changes to the surface of the Pt/Ir foils as a result of this process have been quantified using x-ray photoelectron spectroscopy and have shown a clear reduction in the quantity of carbonaceous contamination. Variation of the UV intensity, ozone concentration and exposure duration enabled the optimum cleaning conditions to be established. The UV/O3 cleaning method was then used to clean two Pt/Ir kilogram mass prototypes and gravimetric weighing of the kilograms before and after cleaning gave the amount of contamination removed. These gravimetric weighing results demonstrated that UV/O3 cleaning was as effective as the nettoyage–lavage process.

To determine a new value of the Avogadro constant with a relative combined standard uncertainty of 2 × 10 −8 , the mass determination of a 1 kg 28 Si sphere is crucial and should be determined with the highest level of accuracy. In the next two years the laboratories involved in the International Avogadro Project should be able to determine the mass of a 1 kg silicon sphere under vacuum with a combined standard uncertainty within 5 µg. To obtain such a target it is essential to gain experience and to promote cooperation in mass measurement on silicon spheres among the laboratories involved. For this purpose an international comparison has been performed to evaluate the weighing procedure and to reveal the difficulties encountered in the mass determination of a silicon sphere.This particular comparison, which is the first international mass comparison under vacuum conditions, has demonstrated that the reference value for the mass of a 1 kg silicon sphere can be determined traceable to the International Prototype with a standard uncertainty of 4.0 µg.

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