Passive diffusion samplers have now been in use in the UK for the measurement of tritium at environmental levels in the form of HTO and HTO + HT for more than ten years. At the outset their main purpose was to identify the direction of a possible inadvertent release of tritium into the environment. More recently, however, there has been growing interest in their use as stand-alone devices for tritium determination at environmental levels. This has necessitated a more rigorous design to reduce the "draughts effect" in exposed locations and has also required modifications to enable the measurement of HTO + HT.

The paper describes the recent design improvements and the results of validation tests carried out against dynamic, discriminating (dry-bed) samplers.