The standard "ISO 2808 Paints and varnishes - Determination of film thickness" published by the International Organization for Standardization (ISO) describes methods for measuring the thickness of coatings applied to a substrate. This standard includes methods for determining wet film thickness, dry film thickness and film thickness of uncured powder layers. For each method described, this standard provides an overview of the field of application, current standards and accuracy. Information on measuring film thickness on rough surfaces and measuring film thickness on wood substrates is included in the standard annexes (Annex B and Annex C).
For the purposes of this standard, in addition to the terms and definitions given in “ISO 4618 Paints and varnishes - Vocabulary”, the following definitions apply:
The annex to the standard provides an overview of the methods used to determine wet film thickness (Annex A).
In mechanical methods for determining wet film thickness, the measuring device is in principle placed on the surface of the coating substrate. The difference between the contact points and the coating surface touched by the top of the measuring device corresponds to the readable wet film thickness. The mechanical principle is suitable for all film-substrate combinations. The substrate must be flat in at least one direction in the area where the measurement is made. Curvature of the surface in a single plane is acceptable (e.g. the inner or outer surface of pipes).
Classification as destructive or non-destructive method depends on:
Since the possibility of pigment particles being trapped between the measuring device and the substrate cannot be excluded, all mechanical methods contain a systematic error: the displayed film thickness is smaller than the actual wet film thickness by at least the average diameter of the pigment particles. In the case of the wheel gauge, the wheel must be wetted by the coating material. Otherwise, this represents another source of systematic error that can lead to exaggerated readings and is a function of: the surface tension and rheological properties of the coating material, the material of the wheel gauge and the speed at which the wheel is rotated.
The main mechanical methods applied are:
Another method used to determine wet film thickness is the gravimetric method (Method 2 Mass difference). This method is applied in principle to a coating and the thickness is determined by dividing the mass of the coating by its density and the coated surface area. The wet film thickness is calculated in micrometers. The density of the applied liquid coating material can be determined in accordance with ISO 2811-1, ISO 2811-2, ISO 2811-3 or ISO 2811-4 standards. The gravimetric principle is generally applicable provided that the amount of highly volatile substances in the liquid coating material is low.
Another method used to determine wet film thickness is the photo thermal method (Method 3 Determination using thermal properties).
This standard defines the following methods for determining dry film thickness:
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