Using Wet Film Wheels
An accurate solution to measuring wet film thickness is using wet film wheels. Whilst wet film combs have teeth to determine the coating thickness; a wet film wheel consists of three connected discs; two outer concentric wheels and one eccentric wheel in the centre.
The wheel, which should be held by its central spindle, using either a finger and thumb or the wet film wheel handle, is placed on to the coating (wet paint) evenly with the maximum scale reading positioned closest to the substrate. It is then rolled along the substrate, either through half a rotation (180°), or one complete rotation (360°), depending on the standard being used, then removed from the surface and inspected.
When the central wheel reaches the thickness of the coating, it touches the paint and is “coated”. To determine the wet film thickness, the first point of contact of the paint on the central wheel is located, which is closest to the wheel’s maximum value, and the thickness is read from the scale on the outside of the wheel.
When placing the wheel into a wet film, it should never be placed with the zero closest to the substrate; as the coating film tension will pull the paint up the wheel, resulting in an inaccurate reading. This is why the maximum reading on the scale should always be closest to the substrate.
The Elcometer 3230 Wet Film Wheels are available in a wide range of scales - from the 0-25 microns thin coatings wheel; to the thick coatings wheel which measures thicknesses up to 1mm. For even more accurate results, Elcometer’s wet film wheel handle can be used. The handle is attached to the wheel, giving more stability and improving access to hard-to-reach surfaces.
In addition to the standard wheels, Elcometer also offers a number of wet film wheels specifically designed for slippery or fast moving substrates, for example, when testing on the production line in the coil coating process, instead of the user moving the wheel across the substrate, the substrate is steadily moving past them and the wheel is placed on the wet film and removed after either 180 or 360 degrees, depending on the standard being tested to. This is why the Elcometer 3230 Coil Coating Wet Film Wheels have knurled outer wheels, which help grip the substrate to minimise skipping.
Calculating Dry Film Thickness
Once the wet film thickness has been recorded it can be used to predict the dry film thickness after the coating has cured. When paint is applied to a surface, the volatile components evaporate, leaving the non-volatile solids on the surface. The ratio of volatile to non-volatile components, known as the volume solids content, can be found on the coating’s data sheet. The components are often written as a ratio or as a percentage.
For example, if the ratio is 50:50, or 50%, then a wet film thickness of 100 microns will mean that the dry film thickness is 50 microns, once the volatile components have evaporated.
Take a look at Elcometer’s full range of Wet Film Wheels here.
For a step-by-step tutorial video on how to use Wet Film Wheels, click here.