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ANALYSIS


• Can be calibrated using single or two points, on smooth, rough and special substrates,


• Can provide pre-defined calibration routines to meet ISO, SSPC, and other Standards, and


• Can store up to 40,000 readings in 999 batches.


Fig 1 The principle of a modern gauge


Francis explains the principle of a modern gauge: “An electromagnetic induction gauge has two small coils within the probe. A small AC voltage is fed to one coil, which induces a voltage in a second coil. The magnitude of this voltage depends on the distance the probe is from the steel substrate, in practice the coating thickness. This voltage is amplified and displayed as thickness units.” (see figure 1) The standards and procedures that are still used for thickness testing, however,


were developed when the older manual gauges were all that was available. Francis points out that there are also different ways to adjust gauges, different inspections plans and different criteria for dealing with results. Each of the standards has their own approaches to these issues.


THREE STANDARDS The main standards used for DFT testing are: • The US SSPC-PA 2 (Measurement of dry coating thickness with magnetic gauges)


• ISO 19840 (Paints and varnishes, Corrosion protection of steel structures by protective paint systems, Measurement of, and acceptance criteria for, the thickness of dry films on rough surfaces)


• And (although not a standard in the accepted sense), the International Maritime Organisation (IMO) resolution on Performance Standards for Protective Coatings (PSPC) which provides for coating quality for ships, especially of ballast tanks.


Fig 2


Unfortunately the terminology used in the different documents is inconsistent. Even the term ‘spot measurement’ has different definitions.


DAILY ADJUSTMENT A paint inspector would usually adjust his gauge at the start of work, using supplied shims of known thickness to verify upper and lower readings after first establishing a correct ‘zero’ reading on bare metal. But Francis points out that the requirements are significantly different. A ‘spot measurement’ under the SSPC requires three gauge readings to be taken in a 40mm circle and averaged to give a figure, whereas the ISO requires just one reading. The IMO’s PSPC refers to the SSPC standard without specifically requiring three readings, and in practice one reading is usually thought acceptable. Setting up the gauge is also covered in detail by the ISO, but the SSPC only requires that ‘manufacturer’s instructions’ are followed. Francis continues: “The requirement for three readings to be averaged in SSPC


Fig 3


is a carry-over from the older permanent magnet types where gravity and other factors can affect instrument readings. With modern electronic instruments, a single reading is sufficient, and this requirement seems unnecessary and makes analysing results complex, even with modern statistical instruments. Furthermore, it means the effective number of readings an inspector can obtain in a given work period is one- third (or less if calculations have to be done) of the number that can be done using the method in the other standards where all readings are of equal significance.”


MAGNETIC BASE READING After gauge verification a further step is usually required to determine the magnetic base reading (MBR). ISO 19840 requires ten “DFT” measurements to be taken on the blast cleaned surface and averaged to give the MBR. The MBR is subtracted from the actual DFT readings obtained to give the ‘true’ dry film thickness, with an adjustment figure for surface profile. Besides referring simply to ‘manufacturers instructions’ the SSPC Standard (also


Fig 4


referred to by the IMO’s resolution) allows gauge verification by placing shims of a known thickness over a blasted surface and adjusting the gauge to show the thickness of the shim. Francis points out that, though attractive, this method has weaknesses and could give false readings through distortion of thin shims bending between peaks of the surface profile, through thick shims which simply rest on high peaks of the surface profile, and through the impossibility of getting the same readings from different ‘representative’ spots (see figure 2). It is also impractical


26 PCE APRIL-JUNE 2013


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