Wet Film Hiding Power Meter

The Wet Film Hiding Power Meter is designed to measure the opacity of liquids either as a function of time or thickness. Its use can be applied to a wide range of material tests, for example:

• dispersion of pigments in paints
• growth of opacifying matter, such as mould, in agar
• carbon content of used engine oils

The sample chamber of the instrument consists of two horizontal parallel glass plates. The top plate consists of ordinary glass, whilst the bottom plate is opaque and divided into halves, one black, the other white.

The test material is sandwiched between the two plates and illuminated through the top plate by a light source from a 45° angle. By measuring the amount of light reflected at 0°, i.e. the normal, over the black and white sections it is possible to calculate the opacity of the material. For active materials, measurements may be made as a function of time, with a fixed gap (i.e. sample thickness) between the plates, as with culture growth.

For unchanging materials, measurements may be made as a function of thickness by squeezing the plates slowly together, as with paints.