A measurement point has value only if someone, three years from now, will be able to find it exactly where it was. The grid is the tool that makes that position repeatable.
- ✓ Coded points that stay the same in every campaign
- ✓ Matrix of rows and columns reproducing the component
- ✓ Immediate point-by-point comparison between successive inspections
- ✓ Documented coverage even where thickness has not decreased
In corrosion monitoring the object of the inspection is not the component as a whole, but a list of positions chosen at the planning stage: the coded measurement points, referred to in current practice as CMLs, that is the stably identified positions at which the measurement is repeated campaign after campaign. The choice of those positions follows the expected behaviour of the component: bends, welds, low areas where liquid stagnates, stretches subject to erosion.
For this structure to be useful it must remain identical over time. If in the next campaign the points are redefined, renumbered or taken roughly where they were, the comparison between the two series loses meaning: a difference in thickness becomes indistinguishable from a shift of the probe. Repeatability of position is, in this type of inspection, as important as the accuracy of the reading.
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The grid is the form this structure takes on the instrument. Unlike the sequential batch, which lists the readings in the order they are taken, the grid arranges them in a matrix of rows and columns that reproduces the actual layout of the points on the component. The operator sees on the display the position they are at, and at the end of the campaign the matrix can be read as a map of the condition of the wall.
How to build a grid
| Element | Criterion |
|---|---|
| Choice of points | Critical zones of the component indicated by the inspection plan |
| Coding | Stable identifier, kept in all subsequent campaigns |
| Grid pitch | Fine where degradation is localised, wider where it is diffuse |
| Physical reference | Marking on the component that allows the position to be found again |
| Order of travel | Defined once and repeated, to make the series comparable |
From the grid to the thickness map
A completed grid is not just a list of numbers: it is the representation of the wall at the points that have been inspected. On the computer, the data management software organises the matrix by project and returns statistics and charts, making visible where the remaining thickness is lowest and how degradation is distributed over the surface.
The density of the points also depends on the speed of the instrument. With the scan mode of the CMX series, which acquires up to 250 readings per second, an area can be swept looking for the local minimum before recording the value of the point; on the MX1-DL and MX2-DL models scanning works at 200 Hz, on the ZX6-DL at 100 Hz.
Recommended instruments
| Model | Use |
|---|---|
| Dakota CMX10-DL | Extensive corrosion monitoring campaigns, scan at 250 readings/sec |
| Dakota CMX3-DL | Corrosion with A-Scan and structured storage of points |
| Dakota MX2-DL | A-Scan and B-Scan with 200 Hz scanning |
| DakMaster | Organisation of grids by project, statistics and charts |
Frequently asked questions
The sequential batch stores the readings one after another, in the order of acquisition. The grid arranges them in a matrix of rows and columns that reproduces the layout of the points on the component, and allows the result to be read as a map.
Yes, but it is advisable to add them at the end, keeping the existing points and their coding unchanged. Renumbering points already measured makes previous campaigns non-comparable.
It depends on the type of degradation expected. Where thickness loss is localised a finer pitch is needed so as not to miss the minimum; where degradation is diffuse and uniform a wider pitch already describes the trend correctly.
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Further information and contacts
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Contact for general information : info@roder.it
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