BS EN ISO 2808:2019 pdf – Paints and varnishes – Determination of film thickness (ISO 2808:2019).
4.4 Photothermal method 4.4.1 Principle The film thickness is determined from the difference between the time a thermal wave is radiated towards the coating and the time the re-emitted wave (either heat or ultrasonic) is detected (see Figure 4). Irrespective of the type of excitation involved or the method of detection, all photothermal methods use the same principle: the periodic or pulsed introduction of energy in the form of heat into a specimen and the subsequent detection of the local temperature increase. The time difference measured is compared against values obtained with the instrument for films of known thickness under fixed conditions (excitation energy, pulse length, excitation frequency, etc.) (see 22.214.171.124).
4.4.4 Method 3 — Determination using thermal properties 126.96.36.199 Instrument and reference materials (reference specimens) 188.8.131.52.1 Measurement system There are various methods for producing thermal waves in a coating material and for detecting the thermal effects induced at the heated location in the specimen (see EN 15042-2  ). Thermal radiation sources (e.g. laser sources, light-emitting diodes, incandescent light sources) are mainly used as the excitation system for coatings. The following detection methods are used: — detection of the re-emitted thermal radiation (photothermal radiometry); — detection of the change in refractive index (in the heated air above the measurement area); — pyroelectric detection (heat flow measurement). 184.108.40.206.2 Reference material Reference materials with different absorption properties and a range of film thicknesses are required for calibration purposes (see, for example, EN 15042-2  ). 220.127.116.11 Calibration Calibrate the measurement system with the reference materials (see 18.104.22.168.2) for each film-substrate combination (in particular for each coating material). 22.214.171.124 Procedure Operate the apparatus and measure the film thickness in accordance with the manufacturer’s instructions. 5 Determination of dry-film thickness 5.1 General Annex A gives an overview of the methods used for determination of dry-film thickness. 5.2 Mechanical methods 5.2.1 Principle A micrometre or dial gauge (method 4A, see 5.2.4) is used to measure the film thickness as the difference between the total thickness (substrate + film) and the substrate thickness. There are two ways of determining the film thickness: a) Measurements are made before and after coating removal (destructive). The total thickness is first measured in a defined measurement area and then, after the coating has been removed in the measurement area, the substrate thickness is measured.
The film thickness is calculated from the difference between the two readings. A depth gauge (method 4B, see 5.2.5) or a profilometer (method 4C, see 5.2.6) enables the film thickness to be determined directly as the difference in height between the surface of the film and the surface of the exposed substrate. NOTE Only the “coating removal” variant is possible with a depth gauge or profilometer (methods 4B and 4C). 5.2.2 Field of application The mechanical principle is basically suitable for all film-substrate combinations. Where mechanical gauging is used, the substrate and coating shall be constituted so as to prevent the reading from being falsified as a result of the measuring tip creating an indentation. The micrometre or dial gauge (method 4A) is also suitable for measuring the film thickness of cylindrical specimens of circular cross-section (e.g. wires, pipes). The profilometer (method 4C) is recognized as the referee method in the event of a dispute. 5.2.3 General In the “coating application” variant, a template with labelled holes is used to ensure that the determination of the substrate thickness and total thickness is carried out at exactly the same points. NOTE 1 The “coating application” variant is preferred in the case of plastic substrates since in most cases the substrate cannot be exposed without causing damage. In the “coating removal” variant, the measurement areas shall be circled and labelled. The coating shall be carefully and completely removed in the measurement areas without damaging the substrate mechanically or chemically. The substrate may be partly masked using adhesive tape before coating in order to obtain well-defined steps from one layer to the next. In the case of the depth gauge and profilometer (methods 4B and 4C), the coating which is not removed in the region of the measurement area shall remain undamaged. In the case of the profilometer (method 4C), the shoulder between the substrate and the film surface shall be sufficiently well defined.