SSPC-PA 2 — DFT Measure- ment with Magnetic Gauges

16.4.1 SSPC-PA 2 — DFT Measure- ment with Magnetic Gauges

Various standards define methods of mea- suring DFT. SSPC-PA 2 and ASTM D 7091 both define similar methods to calibrate, verify, and adjust magnetic-type DFT gauges and to take DFT measurements of nonmagnetic coatings over ferrous magnetic metal surfaces. SSPC-PA 2 is the standard this course examines.

A project specification may, however, require other standards. Inspectors must be

Figure 16.13 SSPC-PA 2 Measurement Record

careful to use the specified standard. If the specification does not state a required stan-

Minimum Thickness

dard, then it is good practice to choose a The average of the spot measurements for

standard, such as SSPC-PA 2, whose guide- each 100 ft 2 (10m 2 ) area shall not be less lines are broadly accepted in the coatings

than the specified minimum thickness. No industry ( Figure 16.13 ).

single spot measurement in any 100 ft 2 Another option is for the inspector to sug-

(10m 2 ) area shall be less than 80% of the gest a standard to follow at the pre-job con-

specified minimum thickness (although a ference; this way all parties can have a say in

single gauge reading may under-run by a the decision.

greater amount). If the average of the spot measurements for a given 100 ft 2 (10m 2 )

SSPC-PA 2 requires five spot measurements area meets or exceeds the specified mini-

(a minimum of three readings per spot are mum thickness but one or more spot mea-

needed to account for surface irregularities surements are less than 80% of the specified

within the 1.5 in [4 cm] circle of the spot; minimum thickness, the operator may make

these readings are averaged to arrive at a sin- gle measurement) for each 100 ft 2 (10 m additional measurements to better define the 2 )

nonconforming area.

measured. Note that individual readings are not subjected to rules but are included in the

Maximum Thickness

average for a spot measurement. The average of the spot measurements for

The average of the five spot measurements each 100 ft 2 (10 m 2 ) area shall not be more (i.e., at least 15 individual readings) can be

than the specified maximum thickness. No no more than the specified maximum thick-

single spot measurement in any 100 ft 2 (10 ness and no less than the specified mini-

m 2 ) area shall be more than 120% of the mum thickness.

specified maximum thickness (although a single gauge reading may over-run by a

No single spot measurement can be less than greater amount). If the average of the spot

80% of the specified minimum thickness

16-12 Dry Film Thickness Measurement Instrumentation

measurements for a given 100 ft 2 (10 m 2 )

16.4.2 Other Standards

area meets or falls below the specified maxi- Another standard, often referenced in speci- mum thickness, but one or more spot mea-

fications, is ASTM D 7091, “Standard surement is more than 120% of the specified

Practice for Nondestructive Measurement

maximum thickness, the operator may make

of Dry Film Thickness of Nonmagnetic

additional measurements to better define the

Coatings Applied to Ferrous Metals and

nonconforming area. Consult manufactur-

Nonmagnetic, Nonconductive Coatings

ers’ literature to determine if higher maxi- Applied to Non-Ferrous Metals.” Like mum-thickness readings are allowable under

SSPC-PA 2, this standard describes the mea- specific circumstances.

surement of the thickness of nonmagnetic

Area Measured

coatings applied to ferrous metal substrates

For areas under 1,000 ft 2 (100 m 2 ), ran-

as well as nonmagnetic, nonconductive coat-

ings applied to nonferrous metals. Another m 2 ) areas. If the DFT in those areas com-

domly select and measure three 100 ft 2 (10

standard that may be specified is BS 3900, plies with the specified limits, proceed. If

“Methods of Test for Paints, part C5.” not, take more measurements to define the

Whatever standard method is used, it is nonconforming area, then begin again.

imperative to record the correct number of

measurements. The inspector may use a fig- first 1,000 ft 2 (100 m2) described above, and

For areas over 1,000 ft 2 (100 m 2 ), measure

ure similar to the following: provided DFT is OK for each additional 1,000 ft 2 (100 m 2 ), randomly select and

measure one 100 ft 2 (10 m 2 ) area.

All of these definitions and procedures may, according to the standard, be varied by agreement. SSPC–PA 2 states:

Other size areas or number of spot measure- ments may be specified in the procurement documents as appropriate for the size and shape of the structure to be measured.

As with all specified standards, take the time to become thoroughly familiar with this specification (located at the end of this chap- ter).

SSPC describes two methods of calibration adjustment and defines DFT gauges accord- ing to the two methods. Magnetic pull-off DFT gauges are termed “SSPC-PA 2, Type I,” while fixed-probe gauges are termed “SSPC-PA 2, Type II.”

Dry Film Thickness Measurement Instrumentation 16-13

It requires the following DFT verification measurements to be taken:

Surface measurements:

• One gauge reading per 50 ft 2 (5 m 2 ) of flat

surface area • One gauge reading at 6.5 to 10 ft (2 to 3m)

intervals, and as close as possible to tank boundaries, but not further than 0.6 in (15 mm), from edges of tank boundaries

Longitudinal and transverse stiffener mem- bers:

• One set of gauge readings taken at a 6½ to 10 ft (2 to 3m) run and not less than two sets between primary support members, as shown in Figure 16.15

• Three sets of gauge readings for each set of primary support members and two sets of gauge readings for each set of other members, as indicated by the arrows in the diagram

• For primary support members (girders and transverses), one set of gauge readings for a 6.5 to 10 ft (2 to 3m) run but not less than three sets

• Around openings, one gauge reading from each side of the opening

Figure 16.14 DFT Data Collection Chart

• Five sets of gauge readings per square inch (in 2 ) (square meter [m 2 ]) but not less

It is helpful, when nonconforming areas are

than three gauge readings taken at com-

found, to physically mark areas where the

plex areas (i.e., large brackets of primary

coating is either too thick or too thin. This

support members)

can be done by applying a contrasting color

Take additional spot checks to verify coating

of the same coating, but never make marks

thickness for any area considered necessary by

that could damage the coating. Ensure the

the coating inspector.

method to mark coating deficiencies is agreed upon during the pre-job conference.

International Maritime Organization (IMO) Resolution MSC.216(82), Annex 3, Dry Film Thickness Measurements

1. International Maritime Organiza- The new IMO 1 standard details a specific

tion (IMO), 4 Albert Embank- methodology to collect DFT measurements.

ment, London, SE1 7SR, United

16-14 Dry Film Thickness Measurement Instrumentation

Figure 16.15 Readings at Longitudinal and Transverse Stiffener Members

Dry Film Thickness Measurement Instrumentation 16-15