The 80/20 rule in Dry Film Thickness (DFT) measurement specifies acceptable tolerance limits: no more than 20% of individual DFT readings can fall below the specified minimum DFT, and no individual reading can be lower than 80% of the specified minimum DFT.
This rule is crucial for ensuring adequate corrosion protection and coating performance in various industries, particularly those involving protective coatings on steel structures. It essentially sets a lower bound on acceptable film thickness while allowing for some variation.
Breakdown of the 80/20 Rule:
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The "80%" Aspect: This means that any single measurement of the DFT must be at least 80% of the minimum thickness required by the project specification. For example, if the specification calls for a minimum DFT of 100 microns, no individual reading can be less than 80 microns.
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The "20%" Aspect: This refers to the number of readings that are allowed to fall below the specified minimum DFT, but above the 80% threshold. No more than 20% of all the readings taken in a given area or lot can be below the specified minimum. Using the same example above (100 microns specified DFT): if you take 10 readings, no more than 2 of those readings can be less than 100 microns, but they must still be at or above 80 microns.
Why is this Rule Important?
- Corrosion Protection: DFT directly influences the coating's ability to protect the substrate from corrosion. Insufficient thickness can lead to premature failure.
- Coating Performance: The specified DFT is often determined by the coating manufacturer to achieve optimal performance characteristics such as flexibility, impact resistance, and chemical resistance.
- Compliance: Adhering to the 80/20 rule ensures compliance with industry standards and project specifications.
Factors Affecting DFT Measurement:
Several factors can affect DFT measurements and must be considered:
- Surface Roughness: Rough surfaces can lead to inaccurate readings. Adjustments may be required, especially for DFTs below 30 microns.
- Measurement Technique: Proper calibration and use of DFT gauges are essential for accurate results.
- Calibration of Instruments: Equipment must be calibrated regularly.
- Application technique: Uneven application can lead to variations in DFT.
Example Scenario:
Let's say a project specification requires a minimum DFT of 125 microns for a protective coating on a steel bridge.
- 80% Threshold: 80% of 125 microns is 100 microns. No individual measurement should be below 100 microns.
- 20% Allowance: If you take 10 DFT readings in a specific area, no more than 2 of those readings can be below 125 microns, but they must still be at or above 100 microns.
- Non-Compliance: If 3 or more of those 10 readings are below 125 microns (but above 100), or any single reading is below 100 microns, the coating application in that area does not meet the 80/20 rule and requires corrective action (e.g., re-coating).
Conclusion
The 80/20 rule provides a practical and enforceable standard for assessing DFT compliance. Understanding and applying this rule correctly is critical for ensuring the long-term performance and protection offered by applied coatings.
