A dry film thickness (DFT) gauge works by using either magnetic induction or eddy current principles to measure the thickness of a coating on a substrate. The specific method depends on whether the substrate is magnetic (e.g., steel) or non-magnetic (e.g., aluminum).
Here's a breakdown of the two primary methods:
1. Magnetic Induction (For Non-Magnetic Coatings on Magnetic Substrates)
- Principle: This method relies on the change in magnetic flux density caused by the presence of a non-magnetic coating between the gauge's probe and the magnetic substrate.
- How it Works:
- The gauge uses a permanent magnet and a Hall effect sensor (or similar device) to measure the magnetic field.
- The probe is placed on the coated surface.
- The magnetic field generated by the magnet interacts with the magnetic substrate.
- The presence of the non-magnetic coating (the dry film) increases the distance between the magnet and the substrate.
- This increased distance reduces the magnetic flux density measured by the sensor.
- The gauge then correlates this change in magnetic flux density to the thickness of the coating, using a pre-programmed calibration curve.
- Example: Measuring the thickness of paint on a steel car body.
2. Eddy Current (For Non-Conductive Coatings on Non-Magnetic Metal Substrates)
- Principle: This method uses electromagnetic induction to generate eddy currents in the conductive, non-magnetic substrate. The coating thickness affects the magnitude of these eddy currents.
- How it Works:
- The gauge uses a coil to generate a high-frequency alternating magnetic field.
- When the probe is placed on the coated surface, this magnetic field induces eddy currents in the conductive, non-magnetic substrate below.
- These eddy currents generate their own opposing magnetic field, which interacts with the coil in the gauge.
- The strength of the eddy currents (and the opposing magnetic field) is affected by the distance between the coil and the substrate, which is determined by the thickness of the non-conductive coating.
- The gauge measures the change in impedance (resistance and reactance) of the coil, which is related to the strength of the eddy currents.
- This change in impedance is then correlated to the thickness of the coating, using a pre-programmed calibration curve.
- Example: Measuring the thickness of paint on an aluminum panel.
General Gauge Operation:
Regardless of the method used, DFT gauges typically include:
- Probe: Contains the sensor and makes contact with the coated surface.
- Electronics: Process the signal from the sensor and calculate the coating thickness.
- Display: Shows the measured thickness value, usually in mils (thousandths of an inch) or micrometers.
- Calibration: Requires calibrating with known thicknesses to ensure accuracy. Gauges often come with calibration shims of known thicknesses.
Factors Affecting Accuracy:
Several factors can affect the accuracy of DFT gauge measurements:
- Surface Cleanliness: Dust, dirt, or other contaminants on the surface can interfere with the reading.
- Surface Roughness: A rough surface can cause variations in the measurement.
- Substrate Material: The gauge must be calibrated for the specific substrate material being measured.
- Edge Effects: Measurements taken near edges or corners may be less accurate.
- Gauge Calibration: Regular calibration is essential to maintain accuracy.
- Temperature: Extreme temperatures can affect the gauge's performance.
In summary, a dry film thickness gauge employs either magnetic induction or eddy current principles to accurately measure the thickness of coatings on various substrates, making it an essential tool for quality control in numerous industries.
