Dry Film Thickness / Standards
ISO 19840: Understanding Surface Profile Correction and DFT Measurement
Measuring dry film thickness on abrasive blast-cleaned steel is not the same as measuring a coating on a smooth surface. ISO 19840 provides a method for accounting for the influence of surface roughness when verifying coating thickness against the specified nominal dry film thickness.
Why surface profile affects DFT measurement
Dry film thickness is one of the most frequently measured characteristics of a protective coating system.
On smooth steel the principle is relatively straightforward. On abrasive blast-cleaned steel, however, the substrate contains peaks and valleys created during surface preparation.
This roughness influences the response of the coating thickness gauge.
When an instrument is adjusted using a smooth, flat steel surface and subsequently used on roughened steel, the displayed reading can be greater than the actual coating thickness above the peaks of the surface profile.
ISO 19840 provides a systematic approach for dealing with this effect.
What does ISO 19840 cover?
ISO 19840:2012 addresses the measurement of dry film thickness and its verification against nominal dry film thickness on rough surfaces.
It covers matters including:
Adjustment of the measuring instrument
Measurement on roughened surfaces
Inspection areas
Sampling
Correction for surface roughness
Expression of results
Acceptance and rejection
Reporting
The standard is applicable where the nominal dry film thickness is 40 μm or greater.
It is particularly relevant to protective coating systems applied to abrasive blast-cleaned structural steel.
What is the dry film thickness actually measuring?
For the purposes of ISO 19840, the dry film thickness represents the thickness of coating remaining above the peaks of the roughened surface after the coating has hardened.
This distinction is important.
A blast-cleaned surface is not flat. Coating material occupies both the valleys of the profile and the area above its peaks.
The instrument response is affected by this roughness, which is why simply taking the number displayed by a gauge adjusted on smooth steel can give a misleading interpretation.
What is the ISO 19840 correction value?
Where the instrument is adjusted using a smooth, flat steel surface, ISO 19840 uses a correction value to account for the effect of the roughened substrate.
In simple terms:
Individual DFT = Individual instrument reading − Correction value
The conventional correction values associated with the ISO surface profile grades are:
Surface Profile Grade | Correction Value |
|---|---|
Fine | 10 μm |
Medium | 25 μm |
Coarse | 40 μm |
These values are not additional coating thickness requirements.
They are allowances used to account for the influence of the roughened surface on the instrument reading when applying the relevant ISO 19840 procedure.
A practical example
Consider a blast-cleaned steel surface classified as having a Medium surface profile.
The corresponding correction value is 25 μm.
If the coating thickness gauge produces an individual reading of:
325 μm
the corrected individual dry film thickness would be:
325 μm − 25 μm = 300 μm
The value assessed as the individual DFT under this approach is therefore 300 μm, not 325 μm.
This is why inspectors need to understand how the instrument has been adjusted and how the substrate profile is being accounted for before interpreting the readings.
Correction value does not mean measured profile height
This is an important distinction.
The ISO 19840 correction values should not simply be treated as the measured peak-to-valley height of the substrate.
A surface described as Medium does not mean that its actual profile height is necessarily 25 μm.
The 25 μm is the correction value associated with the Medium profile grade for the ISO 19840 measurement procedure.
Surface profile assessment and DFT correction are related, but they are not the same measurement.
Confusing the two can lead to incorrect interpretation of coating thickness data.
Fine, Medium and Coarse surface profile
The profile grades used in this approach relate to the ISO 8503 system for characterising abrasive blast-cleaned steel.
ISO 8503-1 establishes ISO surface profile comparators, while ISO 8503-2 describes a visual and tactile comparator procedure for grading abrasive blast-cleaned steel surfaces.
Other parts of the ISO 8503 series address methods for determining surface profile using instruments.
The important point for the coating inspector is that the appropriate surface-profile classification needs to be established before the relevant ISO 19840 correction is applied.
Gauge adjustment matters
Correct DFT measurement begins before the first coating reading is taken.
ISO 19840 requires the accuracy of the measuring instrument to be verified using appropriate reference conditions.
The inspector should understand:
What type of instrument is being used
How the instrument has been adjusted
What substrate the adjustment was performed on
The expected coating thickness range
The substrate surface profile
Whether the ISO 19840 correction method is being used
Without this information, a collection of DFT readings may appear precise while still being incorrectly interpreted.
Why zeroing directly on blasted steel is different
A common source of confusion is adjusting or zeroing a coating thickness gauge directly on the prepared rough steel.
This is not the same procedure as adjusting the instrument on smooth steel and subsequently applying the ISO 19840 correction value.
ISO 19840 also contains an informative alternative approach involving adjustment using known thicknesses on a rough surface. Under that approach, the standard notes that a separate correction value is not used.
Therefore, inspectors should not mix measurement approaches.
The procedure being followed should be clearly established by the specification or agreed inspection method.
Why simply recording gauge readings can be misleading
Imagine two inspectors measuring the same coating.
One adjusts the gauge on smooth steel and records the displayed readings without applying the required roughness correction.
The other follows the appropriate ISO 19840 procedure and applies the relevant correction value.
Both may be using correctly functioning instruments, yet their reported DFT values can differ because the measurement data has been treated differently.
The issue is not necessarily the gauge.
It is the measurement methodology.
This is why an inspection report should provide enough information to establish how the reported coating thickness was determined.
Sampling is as important as individual readings
DFT inspection is not simply about finding one acceptable reading.
ISO 19840 incorporates the concept of inspection areas and sampling plans so that coating thickness is assessed systematically across the work.
Appropriate measurement distribution helps establish whether the coating system has been applied consistently rather than merely demonstrating acceptable thickness at selected convenient locations.
Particular attention may be required around:
Edges
Welds
Difficult geometries
Repairs
Areas of restricted access
Changes in application conditions
Different structural elements
The applicable specification and inspection plan should determine how the structure is divided and assessed.
Nominal DFT is not simply a target gauge reading
The nominal dry film thickness specified for a coating system should not automatically be treated as the raw number that must appear on the gauge.
Where ISO 19840 applies, the measurement needs to be interpreted using the required methodology.
This becomes particularly important where readings are close to acceptance limits.
A raw gauge reading may appear to exceed the specified nominal thickness, while the correctly determined DFT after accounting for surface roughness may be lower.
What about maximum DFT?
More coating is not automatically better.
Protective coating systems can also have maximum thickness requirements because excessive film build may adversely affect performance.
The applicable project specification and coating manufacturer’s requirements should therefore be considered when assessing both minimum and maximum coating thickness.
Inspectors should avoid treating DFT inspection solely as a search for areas that are too thin.
Reporting DFT correctly
A useful DFT inspection record should provide sufficient information to understand how the measurements were obtained and assessed.
Depending on the project requirements, relevant information can include:
Measuring instrument identification
Instrument adjustment/verification
Inspection area
Surface profile classification
Correction value used
Individual readings
Corrected individual DFT values
Mean DFT
Specified nominal DFT
Applicable maximum DFT where specified
Location of measurements
Acceptance status
Good reporting provides traceability rather than simply presenting a page of numbers.
DFT measurement as part of coating inspection
Dry film thickness is only one element of protective coating quality.
A coating can meet its specified DFT while other aspects of the work remain unacceptable.
DFT assessment should therefore sit alongside inspection of:
Surface preparation
Surface profile
Soluble contamination
Dust contamination
Environmental conditions
Mixing and application
Recoat intervals
Visual appearance
Coating defects
Cure
Adhesion where required
The purpose of independent inspection is not simply to collect thickness readings. It is to provide evidence that the complete coating process has been undertaken in accordance with the applicable requirements.
PRACTICAL INSPECTION NOTE
One of the easiest DFT mistakes to make on blast-cleaned steel is to accept the number displayed on the gauge without first establishing how the instrument was adjusted and how surface roughness is being accounted for. Under the conventional ISO 19840 correction approach: Fine = 10 μm Medium = 25 μm Coarse = 40 μm These are correction values, not the actual measured peak-to-valley surface profile. Always establish the measurement method before interpreting the result.
RELATED STANDARDS
ISO 19840:2012 — Measurement of, and acceptance criteria for, the thickness of dry films on rough surfaces ISO 8503-1:2012 — Specifications and definitions for ISO surface profile comparators ISO 8503-2:2012 — Comparator procedure for grading abrasive blast-cleaned steel surface profile
RELATED EXPERTISE
This technical content is provided as general guidance and does not replace the requirements of the applicable project specification, coating manufacturer or current edition of the referenced standard. Official standards should always be obtained from the relevant standards organisation.
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