[en] An instrument is described which is capable of the direct measurement of Tritium surface contamination. Consideration of the fundamental physics involved shows that air absorption of the low energy (18.6 keV) β emission makes it necessary to ensure that the detector is 0.02 in (0.5 mm) +- 0.005 in (0.125 mm) from the surface over the whole detector area to achieve an acceptable measurement error (+-25%) and that outside this region the error is potentially infinite. Practical experience of direct measurements of the working surfaces suggests that swipes of smooth bench tops can be in error by more than a factor of 1000. The magnitude of the errors shown in these observations gives cause for concern for the radiological hazard of so called fixed contamination where Tritium is concerned because of the absorption through the skin and inhalation of volatile fractions