TP913 Coating Thickness Check

8/17/2019 TP913 Coating Thickness Check

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Technical Services GroupProcedure

The Measurement of Dry Film Thickness of Coatings on Structural Steelwork(Abrasive Blast Cleaned

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The purpose o this procedure is to descri"e the e#uip$ent and techni#ues used to $easure thedry il$ thic%ness o coatings over a"rasive "last cleaned su"strates.

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This procedure applies to steel su"strates& 'hich have "een a"rasive "last cleaned inaccordance 'ith (S 1)2*.4& to at least class Sa 2+& as descri"ed in (S 1)2*.,.

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The presence o a te-tured or proiled surace results in a gauge reading& 'hen coated& 'hich isso$e'hat higher than the actual thic%ness& $eaning that the thic%ness o paint applied $ay "esigniicantly less than that speciied. t is i$portant that the applied il$ ade#uately cover anyrogue pea%s produced "y the "last cleaning process& other'ise pre$ature rust spotting 'ill occur.

n this procedure& the gauge is /eroed and cali"rated on a s$ooth reerence surace& and thenthe readings ta%en on a reshly "lasted su"strate to esta"lish a correction or the error induced inthe gauge "y the proiled surace. The error reading is added to the target thic%ness& there"yensuring that the thic%ness speciied is achieved& over and a"ove the average proile height.

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(S1)2*.4& Metal inishing Preparation and pre3treat$ent o suraces ("rasive "last cleaning

(S1)2*.,& Metal inishing Preparation and pre3treat$ent o suraces Pictorial suracepreparation standards or painting steel suraces

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Dry il$ Thic%ness 3 the thic%ness o the coating $easured& a"ove the pea%s o the suraceproile o the su"strate.

Reerence Su"strate 3 a s$ooth steel surace used to set the instru$ent /ero and or co$paringresults o"tained ro$ the test piece.

Secondary Standards 3 standard shi$s that are reerenced to a pri$ary standard& or cali"ratedor thic%ness& 'hich are used in the ield.

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).1 (pparatus

PR5T6D 78P6S (R6 95785TR86D D879M65TSile ocation: ;:


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• 6ddy current type thic%ness gauge.

• Secondary standards o appropriate thic%ness.

).2 nstru$ent 7ali"ration

7hec% the operation o the instru$ent in accordance 'ith the $anual& ensuring that it is operatingsatisactorily& and that "attery level is suicient to co$plete the testing session. The instru$entshall "e /eroed and cali"rated on a s$ooth steel plate& in accordance 'ith the $anuacturer=sinstructions. >here instru$ent cali"ration has "een sho'n to "e su"?ect to drit over ti$e& re3cali"ration shall "e carried out 'ith suicient re#uency to ensure accuracy and precision.

).@ Thic%ness Measure$ent

)[email protected] Proile 6ect

The gauge shall irst "e /eroed and cali"rated on a s$ooth steel plate. (ppro-i$ately 20 readingsshall then "e ta%en over a "road area o reshly "lasted steel& and a median value deter$ined.Readings shall only "e ta%en over s$ooth lat non3pitted su"strate. >here relevant& this e-erciseshall "e repeated "y the 7ontractor and the results co$pared. The results ro$ the 2 e-ercisesshall "e co$pared& and a consensus reached and the value recorded.

This value shall "e used or all su"se#uent 'or% on the sa$e pro?ect& unless changes are $ade

to the "lasting $aterials or e#uip$ent& in 'hich case the testing shall "e repeated as a"ove anda ne' error value agreed upon.

Note: This is not a measure of profile height but a measure of the error induced in the gauge as a result of surface profile.

)[email protected] Statistical Sa$pling

Suicient thic%ness readings shall "e ta%en to esta"lish the conor$ity o the 'or% 'ith thespeciication re#uire$ents. (s a $ini$u$& 1 single point reading shall "e ta%en or each andevery @ lineal $etres o girder& on each acet. This $eans that or a 10 $etre 3"ea$& a $ini$u$o 2A readings 'ill "e ta%en over the 'hole "ea$. (dditional readings $ay "e ta%en in high ris%areas such as the "otto$ lange o an 3"ea$& or the underside o a "o- girder.

>here high variation in il$ thic%ness is o"served& 1 single point reading shall "e ta%en or eachand every 1 lineal $etre o girder& on each acet. n this case& each lange and 'e" acet shall "erecorded as a separate "atch to ena"le isolation o non3conor$ing areas.

( Sa$pling plan shall "e prepared "y Technical Services Group& ield Services Section or eachpro?ect.).@.@ Speciied Thic%ness Range

The speciication 'ill nor$ally provide a $ini$u$ thic%ness re#uire$ent only. The $anuacturer=sspeciied $a-i$u$ shall "e used as the "asis or calculating the top end o the thic%ness range.

This igure 'ill usually "e "ased on the highest thic%ness 'hich 'ill not result in sagging&"listering& gassing& crac%ing or other il$ deects.



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The $ini$u$ and $a-i$u$ re#uire$ents or the irst coat in a syste$ shall "e corrected or uncertainty o $easure$ent& esta"lished in accordance 'ith the S8 Guide to 9ncertainty o Measure$ent& and proile error deter$ined as in )[email protected] a"ove& to provide a 'or%ing range. (ne-a$ple is sho'n "elo':

Speciied $ini$u$ or pri$er !0B$9ncertainty C312E 3)B$7orrected $ini$u$ 44B$Proile 6rror F20B$Absolute minimum 1/3m

The speciied $a-i$u$ is calculated ro$ the $anuacturer=s speciied value& as sho'n "elo':

Speciied $a-i$u$ or pri$er 100B$9ncertainty CF12E F12B$7orrected $a-i$u$ 112B$Proile 6rror F20B$Absolute ma4imum #$+3m

(ll li$its shall "e rounded to the nearest !B$ 'hich $eans that in this e-a$ple all readings $ust"e "et'een )!B$ and 1@0B$ in order to co$ply 'ith the speciication re#uire$ents.

>here a inish coat is applied& and the $ini$u$ speciied or the inish coat 'as 1!0B$& then the

target $ini$u$ or the syste$ 'ould "e as ollo's:

Speciied $ini$u$ or inish coat 1!0B$9ncertainty C3AE 312B$7orrected $ini$u$ 1@AB$Pri$er $ini$u$ F)4B$Absolute minimum +&+3m

Si$ilarly& i the $a-i$u$ speciied or the inish coat 'as 2!0B$& then the target $a-i$u$ or the syste$ 'ould "e as ollo's:

Speciied $a-i$u$ or inish coat 2!0B$

9ncertainty CFAE F20B$7orrected $a-i$u$ 2*0B$Pri$er $a-i$u$ F1@2B$Absolute minimum /&+3m

(ll readings $ust thereore "e "et'een 200B$ and 400B$ in order to co$ply 'ith thespeciication re#uire$ents.

( speciic dry il$ thic%ness range shall "e provided or each pro?ect "y Technical Services Groupield Services Section.

)[email protected] Single Point Readings

>here a single point reading on a large lat surace& such as a 'e"& is ound to "e "elo' thespeciied $ini$u$& or a"ove the speciied $a-i$u$& a urther 4 readings shall "e ta%en on the



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10E Magnetic properties o pig$ents 3 The $agnetic eect o certain pig$ents and e-tenders&such as $icaceous iron o-ide CM8E and red iron o-ide& can aect the accuracy o$easure$ents o"tained 'ith so$e instru$ents.

11E Te$perature eects 3 6-tre$es o te$perature $ay inluence "oth the readings o"tainedro$ the instru$ent and the deor$ation properties o the coating "eing $easured.

).! 7oating Syste$

The pri$er is the $ost critical co$ponent in an anti3corrosive coating syste$. t $ust providecorrosion protection to the su"strate& adhesion to the su"strate and adhesion to su"se#uent

coats. t is thereore critical that the applied thic%ness "e suicient to ensure corrosion protectionand adhesion to the su"strate "ut not so high as to co$pro$ise cure and integrity o the il$.

Dierent pri$ers have characteristics 'hich $ust "e ta%en into account 'hen assessing theconor$ance o a thic%ness result 'ith a speciication. or e-a$ple& solvent3"orne inorganic /incsilicates peror$ "est at lo' il$ thic%nesses& and are diicult to recoat 'ith the$selves. t $ay "epreera"le to accept a lo'er Cnon3co$plyingE il$ thic%ness rather than atte$pt to "uild thic%nessand co$pro$ise peror$ance.

The $a-i$u$ coating thic%ness reco$$ended "y a $anuacturer is oten "ased on theresistance to sagging o a coating. >hilst relevant on vertical suraces& and less so on theunderside o langes& it has no relevance to the top o langes. t is thereore i$portant that the

location o a non3conor$ing reading "e considered 'hen re$edial action is proposed.

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The esti$ated uncertainty o $easure$ent or a single coat using this procedure is 12. Theesti$ated uncertainty o $easure$ent or a 23coat syste$ using this procedure is A. Theesti$ated uncertainty o $easure$ent or a @3coat syste$ using this procedure is A.

9ncertainty o Measure$ent deter$ined "y and or DPT Technical Services Group& ieldServices Section.6% D)C'M*-TAT,)-

il$ thic%ness records $ay ta%e the or$ o hand 'ritten 'or%sheets or "oo%s or a print out ro$the instru$ent $e$ory and shall include an esti$ate or the uncertainty o $easure$ent.


TP913 Coating Thickness Check

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