In industrial maintenance inspections, metal surfaces are almost always covered with paint, epoxy or galvanising that protects them from external corrosion. Removing these coatings to take a thickness reading is expensive and, above all, destroys the protective coating. Echo-Echo mode — known commercially as ThruPaint™ across the Dakota NDT range — solves the problem by measuring the metal thickness while ignoring the coating.
The physics behind Echo-Echo
The standard measurement mode (Pulse-Echo, PE) derives the thickness from the time between the pulse emitted by the probe and the first returning echo. On a coated surface the first echo comes from the top face of the paint, so the reading wrongly includes the thickness of the coating as well.
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Echo-Echo (EE) mode works differently: the instrument measures the time between two consecutive echoes from the back wall of the material. Since both echoes travelled through the same coating in the same way, the coating is mathematically eliminated from the calculation. The result is the bare metal thickness, independent of the surface coating.
The 2 mm coating limit
According to the official Dakota datasheet, Echo-Echo ThruPaint™ mode can ignore coatings up to 2 mm (80 mils). Beyond that limit the attenuation of the coating becomes too great and the multiple echoes are no longer clean enough for a reliable calculation. Typical coatings compatible with ThruPaint™ mode:
- Acrylic, polyurethane and epoxy industrial paints
- Hot dip galvanising
- Protective enamels
- Marine antifouling coatings
- Primers and multi-layer finishes up to the combined 2 mm limit
Dakota instruments with Echo-Echo ThruPaint™
Echo-Echo ThruPaint™ mode is available on the following models, according to the official datasheets:
- CX series: CX4, CX6-DL, CX8-DL (not on the CX2, which is Pulse-Echo only)
- MX series: MX1-DL and MX2-DL
- CMX series: CMX1-DL, CMX2-DL, CMX3-DL, CMX10-DL (these have even more advanced modes such as PETP, CT, PECT)
- ZX series: ZX6-DL (on the ZX1 and ZX3, Pulse-Echo only)
- FX series: every flaw detector includes EE mode
For an overview of the CX series instruments, see our CX2, CX4, CX8-DL comparison article.
Advanced modes beyond ThruPaint™
On the CMX series, Dakota offers more advanced modes for specific measurement applications on coated parts:
- PE (Pulse-Echo): standard mode, measures the total (paint + metal)
- EE (Echo-Echo / ThruPaint™): ignores the coating, measures the metal only
- PETP (Pulse-Echo Temperature/Comp Mode): automatically compensates for temperature variations during the measurement
- CT (Coating Only Mode): measures the thickness of the coating only
- PECT (Pulse-Echo Coating Mode): measures coating thickness and metal thickness at the same time in a single reading
PECT mode is particularly useful when both pieces of information (coating thickness + metal thickness) matter, for example in combined inspections of protected corrosion conditions.
Typical applications
- Inspection of storage tank shells to API 653 with external weather-protection coatings
- Pipeline testing to API 570 with epoxy anti-corrosion coatings
- Inspection of ship hulls (below the waterline) with antifouling coatings
- Galvanised structural steelwork: pylons, lighting masts, supports
- Port infrastructure with multi-layer coatings
- Petrochemical industry: heat exchangers with external paint insulation
Operational and economic benefits
- Time saved: no need for preliminary blasting or pickling
- Protection preserved: the intact coating goes on protecting the metal from corrosion
- Fully non-destructive inspections: nothing is done that leaves a mark on the surface
- Operator safety: no working around abrasive dust or chemical solvents
- Productivity: large-scale inspections become possible with no significant preparation time
To go further into applying Echo-Echo and ThruPaint™ modes to Your inspection campaigns, contact our technical department.