What Does Dry Film Thickness Gauge, Magnetic Mean?
Dry film thickness gauge, magnetic type is an instrument used for the non-destructive measurement of dry film thickness of a nonmagnetic coating on a magnetic substrate like iron and steel. The thickness is determined using two types of magnetic gauges:
- Type 1 or the magnetic pull-off gauge which determines dry film thickness based on the force needed to pull the magnet away from the coated surface.
- Type 2 or the magnetic / electromagnetic induction gauge which determines dry film thickness by the changes in the magnetic field as the instrument comes in contact with the non-magnetic coating over a magnetic substrate.
Types I and 2 refer to the gauge’s designation in the SSPC-PA 2 (Society for Protective Coatings Paint Application Standard No. 2) document for the “Measurement of Dry Coating Thickness with Magnetic Gauges.”
Corrosionpedia Explains Dry Film Thickness Gauge, Magnetic
The two types of dry film thickness gauges, magnetic type that are commonly used to non-destructively measure dry film thickness are the magnetic pull-off and the magnetic induction instruments. Both measuring tools include magnets used as follows:
- Type 1 or the magnetic pull-off gauge consists of a permanent magnet, a calibrated spring and a graduated scale. To measure the coating thickness, the probe of the magnetic gauge is placed on the coated surface and pulled off. The instrument displays the force required to pull off the gauge off from the coated surface.
The force increases and decreases proportional to the coating's thickness, which represents the distance separating the magnet from the magnetic substrate. The attractive force between the magnet and the magnetic surface increases as the coating's thickness decreases; the force decreases as the coating's thickness increases and keeps the gauge's magnet farther from the magnetic substrate.
Pull off magnetic gauges are usually either the pencil-type or the roll-back dial type.
- Type 2 or the magnetic / electromagnetic induction gauge measures the thickness of non-magnetic coatings over magnetic substrates based on the changes in the magnetic field as the probe is placed on the substrate or on the non-magnetic coating.
The change in magnetic field varies with the distance between the substrate and the probe. The distance depends on the coating's thickness. Most magnetic / electromagnetic induction gauges have constant pressure probes for more consistent readings and are less dependent on the operator's technique.
It is important to always follow the manufacturer’s operating instructions and calibration schedule for accurate results.