The assessment of the cleanness of liquid aluminum alloys has traditionally been viewed as difficult, but has been tackled by a number of techniques, all of which have been comparatively expensive and complicated. In the past the presence of entrained defects known as bifilms in metals has not been realised because they are often only a few molecules thick, and so remain invisible, or at best, difficult to detect. The reduced pressure test (RPT) is sensitive to the size and number of entrained oxides which appear to be the major defects affecting metal quality in terms of workability, mechanical properties and corrosion. In addition, the test is a confirmation of the fact that such oxides are double, and thus conveniently called `bifilms'. The RPT is a simple, low cost, but fundamentally appropriate technique for the assessment of (a) the number and (b) the size of bifilms in a melt. This paper outlines an initial draft procedure. Future users will be welcome to suggest improvements.