How to measure DFT per SSPC-PA 2
SSPC-PA 2 is the procedure most steel coating specifications cite for measuring dry film thickness with electronic gages. It does not tell you how thick the coating should be - the specification does - but it defines how measurements are taken, averaged, and judged.
The structure below reflects the standard as commonly applied. Always verify the exact clauses and Table 1 tolerances in the edition your project specification cites.
Three layers of measurement
PA 2 works in three layers. A gage reading is a single instrument reading. A spot measurement is the average of at least three gage readings taken within a small circle, nominally 4 cm (1.5 in) across. An area measurement is the average of five spot measurements taken over each roughly 10 square meters (100 square feet) of coated surface.
Averaging is the point. Single readings on blasted steel scatter because the probe sits on peaks and valleys of the profile; the spot average smooths that scatter into a number that can be judged fairly against the specification.
- Gage reading: one instrument reading
- Spot: average of at least 3 readings in a 4 cm circle
- Area: average of 5 spots per roughly 100 sq ft
Restriction levels
The specification states a minimum and maximum DFT. A coating thickness restriction level, 1 through 5, then sets how far individual spot measurements may deviate from that range. Level 3 is the default when the specification is silent: spot measurements may run from 80 percent of the specified minimum to 120 percent of the specified maximum, while the area average must stay inside the specified range.
Level 1 is the most restrictive - no tolerance beyond the specified range for any reading, spot, or area. Levels 4 and 5 progressively relax the spot maximum. CoatSpec models these levels and judges each spot and the area average automatically; state the level your specification requires and verify against its Table 1.
Field workflow
Verify gage accuracy before use (see the calibration guide), record the instrument and its calibration status, then work the surface systematically: pick spot locations representative of the area, take at least three readings per spot, and log every reading against its spot and location.
Log low and high spots as findings rather than deleting them. A nonconforming spot triggers additional measurement in the surrounding area under the standard's procedures, and the record of where thickness ran out of range is exactly what the contractor needs to correct the work.
Frequently asked questions
How many DFT readings does SSPC-PA 2 require?
At least three gage readings per spot, five spots per roughly 100 square feet (10 square meters) of coated surface. That is a minimum of 15 readings per area, more when a spot is discarded or nonconforming areas need additional measurement.
What is the default restriction level in SSPC-PA 2?
Level 3, as commonly applied: spot measurements from 80 percent of the specified minimum to 120 percent of the specified maximum, with the area average inside the specified range. Verify the level and tolerances in the edition your project cites.
Do single gage readings have to be within the specified range?
At most restriction levels individual gage readings are unrestricted - only the spot averages and area averages are judged. Level 1 is the exception, restricting individual readings to the specified range.