When to Use Profile Depth？

Diver's pressure exposure over the time of a dive

A dive profile is a description of a diver's pressure exposure over time. It may be as simple as just a depth and time pair, as in: "sixty for twenty," (a bottom time of 20 minutes at a depth of 60 feet) or as complex as a second by second graphical representation of depth and time recorded by a personal dive computer. Several common types of dive profile are specifically named, and these may be characteristic of the purpose of the dive. For example, a working dive at a limited location will often follow a constant depth (square) profile, and a recreational dive is likely to follow a multilevel profile, as the divers start deep and work their way up a reef to get the most out of the available breathing gas. The names are usually descriptive of the graphic appearance.

The intended dive profile is useful as a planning tool as an indication of the risks of decompression sickness and oxygen toxicity for the exposure, to calculate a decompression schedule for the dive, and also for estimating the volume of open-circuit breathing gas needed for a planned dive, as these depend in part upon the depth and duration of the dive. A dive profile diagram is conventionally drawn with elapsed time running from left to right and depth increasing down the page.

Many personal dive computers record the instantaneous depth at small time increments during the dive. This data can sometimes be displayed directly on the dive computer or more often downloaded to a personal computer, tablet, or smartphone and displayed in graphic form as a dive profile.

Concept

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The profile of a dive is the variation of depth, measured as ambient pressure, over time during that dive. The actual location of the diver at any time is generally not considered, as the dive profile is a tool for dive planning and decompression status calculation. Other data may be added to the depth graph, such as partial pressures of the breathing gas constituents, calculated estimates of accumulated gas concentrations in the theoretical tissue, gas consumption rates and cumulative gas consumption. These additional values are available when the dive computer uses them to estimate decompression status, to provide the diver with a recommend decompression schedule for the exposure of the actual dive.

Types of dive profile

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Some types of dive profile have been named. An analysis of dive profiles logged by dive computers by the Divers Alert Network used categorization rules which were based on the fraction of the dive time spent in four depth zones: descent, bottom, multilevel, and decompression. The descent zone was defined as the part of the dive between the surface and first reaching 85% of the maximum depth. The bottom zone is the part of the dive deeper than 85% of maximum depth. The multilevel zone is ascent from 85% to 25% of maximum depth, and the decompression zone is less than 25% of maximum depth. A square dive profile was defined as having more than 40% of the total dive time in the bottom zone and not more than 30% in the multilevel and decompression zones. A multilevel was defined as having at least 40% of the total dive time in the multilevel zone. All other dives are considered to be intermediate.

Square profile

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Or constant (bottom) depth profile. The diver descends directly to maximum depth, spends most of the dive at maximum depth and then ascends directly at a safe rate, with any required decompression. The sides of the "square" are not truly vertical due to the need for a slow descent to avoid barotrauma and a slow ascent rate to avoid decompression sickness.[1] The term has also been used more loosely, for example DAN's definition of more than 40% of total dive time in the bottom zone which is within 15% of maximum depth.

This type of profile is common for dives at sites where there is a flat sea-bed or where the diver remains at the same place throughout the dive to work. It is the most demanding profile for decompression for a given maximum depth and time because inert gas absorption continues at maximum rate for most of the dive. Decompression tables are drawn up based on the assumption that the diver may follow a square profile and be working while at the bottom, which is common practice for professional divers.[2]

Multi-level diving

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Multi-level diving, in the broader sense, is diving where the activity other than descent, direct ascent, and decompression, takes place in more than one depth range, where a depth range can be arbitrarily defined for convenience, and usually follows the depth graduations of the decompression tables in use. Most recreational diving is multi-level by this description. In the narrower sense, it implies that decompression is calculated based on the time spent in each of more than one depth range. Decompression calculations using dive tables for multi-level dives were moderately common practice for advanced recreational diving before dive computers were widely used.[3]

Where the dive site and underwater topography permit, divers often prefer to descend initially to maximum depth and slowly ascend throughout the dive. A slow ascent, and therefore slow pressure reduction, is a good decompression practice. Multi-level decompression calculation takes this into account and does not burden the diver with decompression obligation for all the time not spent at maximum depth, so the decompression schedule will be less conservative than for a square profile for the same maximum depth. Stepped multi-level decompression calculation uses local maximum depth for each sector of the dive, which is more conservative than real time calculations following instantaneous depth profile, but more conservative than for square profiles.

A practice developed of calculating decompression during the dive, using tables printed on a plastic card, to remain within no-decompression limits for multi-level dives. Although this procedure had very little controlled experimental verification, it did appear to be reasonably safe in the field. This may be attributable to the relative conservatism of the tables used.[3]

Dive computers, unlike decompression tables, measure actual depth and time at short intervals during the dive and calculate the exact gas loading and decompression indicated by the decompression model, so their decompression calculations are inherently multi-level at a fine resolution.[4]

Repet-Up profile

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A commercial diving term for a multi-level dive in which each recorded change of depth is to a shallower depth range. This profile type is used to maximise dive time while limiting decompression time when using decompression tables, but could also use decompression software. At each change of depth range limit the nominal residual inert gas loading is recalculated for the dive to that point by the supervisor, and a new effective dive time established based on the most recent depth limit. The procedure has been shown to be acceptably safe, and is economically advantageous.[5]

Hang-off profile

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A hang-off is a procedure used in commercial bounce diving to reduce unnecessary inert gas accumulation during idle periods when the diver is waiting for surface support activity to be completed before the diver's underwater work can continue. During a hang-off the diver ascends to a shallower depth, usually 30 feet (9.1 m), at or below the first decompression stop depth, where ingassing is effectively stopped, and decompression obligation is put on hold, then descends back to the working depth to continue with the job. By its nature, this profile does not apply to recreational diving, but could be used in any surface oriented professional diving application.[5]

Repetitive diving

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At the surface the remaining excess of absorbed inert gases from the dive are eliminated as time passes. When completely "desaturated" the levels of those gases in the diver's body have returned to those normal at atmospheric pressure. The interval to complete desaturation varies depending upon factors such as the depth and duration of the dive, the altitude of the dive, the gas mixtures breathed on the dive, and the decompression strategy used. The maximum interval until desaturation is considered to have occurred depends on the decompression algorithm in use. On the BSAC 88 dive table it is deemed to take 16 hours.[6] The US Navy tables revision 5 considered the diver unsaturated in 12 hours for normal exposure, and the Buhlmann tables allow 24 hours for the slowest tissues to fully desaturate after a long dive.

Repetitive diving occurs when two dives are separated by a short surface interval, during which the diver has not completely outgassed from the first dive. The gas loading from the first dive must then be taken into account when determining no stop times and decompression requirements for the second dive.[7][8] Multiple decompressions per day over multiple days can increase the risk of decompression sickness because of the buildup of asymptomatic bubbles, which reduce the rate of off-gassing and are not accounted for in most decompression algorithms.[9]

Reverse profile

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Reverse profiles occur when a repeat dive is deeper than the earlier dive. The term is also sometimes used to refer to a single dive profile where the depth generally increases during the bottom phase of the dive until the start of the ascent. Many recreational diver training agencies discouraged or even prohibited reverse profiles for reasons that were not clearly expressed, until the American Academy of Underwater Sciences workshop on reverse dive profiles concluded there was no evidence to support prohibiting reverse dive profiles. [10]

Findings:

• Historically neither the U.S. Navy nor the commercial sector have prohibited reverse dive profiles.
• Reverse dive profiles are being performed in recreational, scientific, commercial, and military diving.
• The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any definite diving experience that indicates an increased risk of DCS.
• No convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.

Michael A. Lang and Charles E. Lehner, Co-Chairs of the Reverse Dive Profiles Workshop, October 29-30.[10]

Forward profile

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This term is occasionally used to indicate that a repetitive dive is shallower than the previous dive. This sequence has practical advantages of longer no-stop bottom time, or shorter decompression time than reverse profile dives, but although for several years some of the recreational diver certification agencies promoted this sequence as safer than reverse profile, the claim was not based on evidence, and has been rebutted after examination of the history of the claim and evidence of comparative decompression risk by a panel of experts in 1999.[10]

Saw-tooth profile

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In a saw tooth profile the diver ascends and descends a number of times during the dive. Each ascent and descent increases the risk of decompression sickness if there are any bubbles already in the diver's tissues.[2][11][12] The increase in risk depends on the ascent rate, magnitude and duration of the upwards excursion, the saturation levels of the tissues, and to some extent the time spent after returning to depth. Accurate assessment of the increase of risk is not currently (2016) possible, but some dive computers make an adjustment to the decompression requirement based on violations of recommended maximum ascent rate as an attempt to compensate.[13]

Decompression profile

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When no stop depth or time limits are exceeded the diver must decompress more extensively than allowed for in the recommended maximum ascent rate to reduce the risk of decompression sickness. This is conventionally done as decompression stops, which are pauses in ascent at specified depths for specified times derived from the decompression algorithm and based on the dive profile history and breathing gas composition. Depth and duration of obligatory decompression stops are specified by the decompression model used.[14][15] Stops are usually specified in 3-metre (10 ft) steps. The depth of the deepest (first) stop for the same profile history will depend on the algorithm, as some decompression models start decompression at lower supersaturation (lower M-values) than others. The duration of the shallower stops is generally more than the duration of deeper stops on a specific dive. Stops extend the dive profile graph along the time axis.

Bounce profile

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Bounce dive is a commonly used term, but the meaning of a bounce dive profile depends on context, and can vary considerably.

In recreational diving terminology, in a bounce dive the diver descends directly to the maximum depth, spends very little time at maximum depth and ascends directly to the surface, preferably at an ascent rate recommended by the decompression model used, usually without any obligatory decompression stops.

In technical diving, the ascent may include decompression stops, and the short bottom time may remain a feature. Depth record dives generally follow this profile type to minimise the decompression obligation, which is several hours.[16]

In commercial diving in general, and offshore diving in particular, a bounce dive is any surface oriented dive, in which the diver is decompressed to surface pressure at the end of the dive and does not transfer to a hyperbaric habitat where the diver lives at pressure between dives and only decompresses at the end of a tour of duty. The duration of bottom time is not relevant in this usage, and decmpression may be required for long periods.[17]

In New Zealand occupational diving, the term refers to "repetitive diving to depths shallower than 21 m with less than 15 minutes surface interval between consecutive dives". No reference is made to the duration of the dive.[18]

A bell bounce dive is a dive where the diver is transported to and from the underwater workplace in a closed diving bell or lock-out submersible, and decompressed to surface pressure after the dive, without the use of saturation techniques.[19][17]

Saturation profile

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A saturation profile is one which all the tissues considered by the decompression model have become saturated with inert gas from the breathing mixture. Most decompression models will take this to be at six tissue half-times for the slowest tissue considered. Further bottom time at the same depth will not affect the inert gas loading of any tissue and will not affect the decompression required.[20]

Excursion from saturation depth

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An excursion from saturation depth is an upward or downward change of depth during a saturation dive, usually from a closed bell[5] Excursions from a bell are usually limited to depth variations that do not require decompression to return to storage depth nor decompression in water to reach the upper extreme. Dives from an underwater habitat are more likely to involve decompression on downward excursions, as habitat internal pressure is usually not easily varied.

Uses of a dive profile

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A simple record of depth and time for a dive is useful as a legal record of a diving operation, where this is required, and in the case of an accident during the dive, an accurately recorded dive profile can provide useful diagnostic information for treatment of the injured diver and for analysis of the circumstances leading to the accident and the action taken during and after the incident.[21] A proposed dive profile is necessary for effective dive planning, both for estimating the required breathing gas composition and quantities, for planning decompression, and for choosing suitable diving equipment and other logistical aspects.

Calculation of gas requirements

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The breathing gas mixtures appropriate to a dive depend to a large extent on the maximum depth and the decompression obligations incurred by the planned duration of the dive and the time spent at each depth. The quantity of gases required for scuba will depend on the time spent at each depth, the breathing rate of the diver, the type of breathing apparatus to be used, and reasonable allowances for contingencies.[7][8]

Planning and monitoring decompression

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For planning and monitoring decompression using decompression tables, the input data usually consists of the maximum depth reached during the dive, the bottom time as defined by the dive table in use and the composition of the breathing gas. For repetitive dives it also includes the "surface interval" – the time spent at surface pressure between the previous dive and the start of the next dive. This information is used to estimate the levels of inert gas dissolved in the diver's tissues during and after completing a dive or series of dives. Residual gas may be expressed as a "repetitive group", which is an important input value for planning the decompression for the next dive when using tables. A more detailed and extensive set of residual gas data is stored in the memory of a dive computer, and automatically applied as initial conditions to subsequent dives.[22]

When decompression planning software is used to produce a schedule for a planned dive, the necessary input includes a definition of the dive profile. This may be in as much detail as the user is prepared to provide and the program is capable of using, but will always specify at least maximum depth and bottom time, and may go into detail regarding recent dive history, multiple levels, gas switches, altitude and personal conservatism factors.[23] Many dive computers have a dive planning function for which the diver selects a maximum depth and the computer displays the maximum bottom time for which no decompression stops are required.[24]

Ambient pressure at the surface

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Atmospheric pressure changes due to change of altitude before or after diving can have a significant influence on decompression risk.[25] Diving at high altitude requires special consideration in decompression planning.[26][27][28] Such variations in ambient pressure caused by flying or surface travel involving changes in altitude will affect decompression and should be considered during dive planning and therefore may influence a planned dive profile.[29]

Records

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The dive profile is often recorded in some way as part of a permanent record of the dive. Maximum depth, bottom time and any decompression required are routinely logged by most professional divers, for whom it may be a legal requirement,[30][31] and by many recreational divers, for whom it is usually a recommendation of the training agencies.[32]

Recreational diving paper logbooks frequently provide a simple graphic representation of a dive profile for recording the details of a dive which are necessary for planning a repetitive dive using a specified set of dive tables.[33] Digital diving logs such as the freeware Subsurface and various proprietary packages from diving computer manufacturers may display a graphic representation of the dive profile downloaded from the dive computer.[23][34]

References

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Jody Wenzel, DeFelsko Corporation

Abstract

This paper will evaluate methods used to measure surface profiles created by power tools, namely air needle scalers, bristle cleaners, and roto peen scalers.  Particular focus will be placed on the use of replica tape and its ability to characterize profiles of various surfaces using a digital replica tape reader with 3D imaging capability.  More specifically, this paper will determine if the measurement methods described in ASTM D4417 are appropriate for the measurement of profiles produced by power tools.

Introduction to Surface Profiling Power Tools

NAVSEA defines hand power tools as portable automatic devices used for surface preparation that can be broken down into three basic categories:

1. Impact cleaning tools (including air needle scalers)
2. Rotary cleaning tools (including bristle cleaners)
3. Rotary impact cleaning tools (including roto peen scalers)

‍While there are many standards relating to the measurement of surface profiles produced by abrasive-blasting of steel surfaces, there is little research or guidance for measuring profiles created by power tools.  

This paper will examine three common measurement methods for determining surface profile parameters and evaluate their efficacy on power tool prepared surfaces: spring micrometers using replica tape, depth micrometers, and stylus roughness instruments. Profiles produced by air needle scalers, bristle cleaners, and roto peen scalers on steel test panels were examined for this study.

Attention will be paid to each measurement method’s effectiveness to measure on all three power tool produced profiles and whether any of the power tools produce profile characteristics that present challenges.  Through evaluation of data, plotting subsequent results and the use of 3D surface imaging, a final recommendation will be made as to which measurement method is the most appropriate.  

The Importance of Power Tools in Surface Preparation

Surface Preparation directly affects the performance of protective coating systems.  Ensuring that a surface is clean of rust and mill scale, as well as surface contaminants such as dirt, oil, soluble salts, and grease is critical.  Equally important is the surface profile, the characteristics of which contribute to the coating lifespan and adhesion strength.  Evaluation of surface profile, then, becomes a critical exercise.

Power tools are frequently used to clean steel surfaces prior to the application of protective coatings.  While the profile of abrasive blasted surfaces is routinely measured with replica tape, depth micrometers, or portable stylus roughness instruments, coatings professionals are often uncertain which method is best suited for profiles created by power tools including air needle scalers, bristle cleaners, and roto peen scalers.

Abrasive impact produces complex, random patterns across the surface.  However, surface profiles produced by power tools exhibit repetitive patterns that present challenges to proper peak-to-valley height and peak density measurement.  

In an article written in February 2015, D. Beamish2 illustrated how replica tape could be used to determine critical surface profile parameters for blasted steel surfaces and related these parameters to pull-off adhesion strength.  Specifically, the article discussed how significantly more information was available through replica tape measurements over other measurement methods, allowing for Peak Density (Pd) and Developed Interfacial Area Ratio (Sdr) to be determined, which directly correlated to pull-off adhesion strength.  Further, it was shown that surface parameters measured using replica tape were closely correlated to established measurement techniques for blasted profiles, such as confocal microscopy and stylus profilometry.   This paper will take this analysis further and determine the suitability of replica tape to not only measure surface parameters of blasted profiles, but to measure surface profile across a variety of power tool prepared surfaces.

What type of surface profile measuring equipment is available?

Replica Tape

Replica tape has been used since the 1960s to measure the surface profile of blasted steel.  Widely used in the coating industry, replica tape consists of a layer of crushable plastic foam attached to a non-compressible polyester substrate of a highly uniform thickness of 2 mils ± 0.2 mils (50.8 microns ± 5 microns).  The foam thickness is dependent on the tape grade. Replica tape is available in two types, regular and optical, and two grades, Coarse and X-Coarse.  For most applications, regular replica tape is sufficient. Optical grade replica tape is used when producing 3D images of the tape surface.  The two tape grades are Coarse, which measures profiles from 0.8 – 2.5 mils (20 to 64 µm), and X-Coarse, which measures profiles from 1.5 – 4.5 mils (38 to 115 µm).

When pressed against a roughened steel surface, the foam forms an impression, or reverse replica, of the surface. The foam can collapse to about 25% of its pre-collapse thickness. Therefore, as the highest peaks on the original surface push up to the polyester backing, the fully compressed foam is displaced sideways. Likewise, the deepest valleys on the original create the highest peaks in the replica.

Placing the compressed tape between the anvils of a spring micrometer, like the PosiTector RTR H, and subtracting the contribution of the incompressible polyester substrate (2 mils / 50 µm) gives a measure of the average maximum peak-to-valley surface roughness profile (Fig.1).

 This method for surface measurement is rugged, relatively simple, inexpensive, and allows the user to retain a physical replica of the surface being evaluated.  It is one of the most common ways to determine peak-to-valley height of blasted surfaces in the coating industry.  

Replica tape provides added advantages over other measurement methods in that it measures the surface profile over a two-dimensional area, rather than a single point or straight line measurement.  The pointed probe tip on a depth micrometer measures a single point with a radius of approximately 0.05 mm (50 microns), for a sampling area of 0.007 mm2. The typical sample line of a stylus roughness instrument is 12.5 mm long and 4 microns wide, for a total measurement area of 0.05 mm2. The measurement area of replica tape is 31 mm2.  This represents a measurement area roughly 258 times larger than the measurement area of a stylus roughness instrument and approximately 4,400 times larger than the depth micrometer.  Further, when using an instrument such as the PosiTector RTR 3D, digital imaging of burnished replica tape can produce 3D images of surface profiles, allowing a user to visually observe the surface prior to coating application.

Surface Profile Depth Micrometers

A depth micrometer, like the PosiTector SPG, uses a flat base that rests on the peaks of the surface profile and a spring-loaded probe tip mounted inside the base which drops into the valleys of the profile.  The flat base rests on the highest peaks and each measurement is therefore the distance between the highest local peaks and the particular valley into which the tip has projected, as illustrated in Figure 2.  Depth micrometers have the advantage of being able to measure profile heights that exceed the range of many other instruments.

Stylus Roughness Instruments

A portable stylus roughness instrument utilizes a stylus that is drawn at constant speed across a surface and records the up and down movements to determine the Rt, or the vertical distance between the highest peak and lowest valley within any given evaluation length.  The instrument measures and records the vertical distance the stylus travels as it passes over the surface, as seen in Figure 3.

Typically, a predetermined evaluation length is divided into 7 sampling lengths and the instrument measures the peak-to-valley height within each sampling length, Ry, of each section, disregarding the first and last sections.  The average of the remaining Ry’s is used to calculate Rz. For this study Rz is equivalent to RzDIN, equaling the average of the distances between the highest peak and lowest valley in each sampling length, per ASME Y14.36M7.

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Summary of Power Tool Cleaned Surface Test

Test Media

Twelve steel plates were prepared using three different power tools: an air needle scaler, bristle cleaner, and roto peen scaler, illustrated in Fig. 4.  These panels were labeled 1 through 4 within each group.

 The following panels were evaluated:

• Four (4) Steel Plates prepared with a bristle cleaner; labeled BB1 through BB4
• Four (4) Steel Plates prepared with an air needle scaler; labeled NG1 through NG4
• Four (4) Steel Plates prepared with a roto peen scaler; labeled RP1 through RP4

Surface Profile Instruments

Surface profiles on the panels were evaluated with the following three instruments:

1. A portable stylus roughness instrument reporting Rz and Rpc (linear peak count). Per ASTM D7127, an evaluation length of 12.5 mm consisting of five 2.5 mm sampling lengths was used. Rpc, when squared, was used to estimate Pd.
2. A digital depth micrometer surface profile gage reporting R
3. Replica Tape and digital replica tape reader and imager reported:
   a. Average Maximum Peak Height (HL) by measuring burnished replica tape and applying a linearization algorithm
   b. Peak Density (Pd) in accordance with ASTM B46.18, measured as peaks/mm2

Test Methodology

Testing was performed with each instrument in the following manner:

1.     A portable stylus roughness instrument was used to determine Rz and Rpc for each panel.  Three trace measurements were performed at 5 locations on each panel with each trace measurement having an evaluation length of 12.5 mm, and a sampling length of 2.5 mm.

a.     Measurement locations are detailed on Diagram 1.  It should be noted that on the bristle cleaner prepared panels trace measurements 2 & 4 are in the direction of bias, while trace measurements 1, 3, and 5 are against the direction of bias.  For the roto peen scaler panels, trace measurements 2 & 4 are against the direction of the bias and trace measurements 1, 3, and 5 are in the direction of bias.

2.     A digital depth micrometer was used to determine Rt.  10 measurements were taken at 5 locations on each panel, for a total of 50 measurements per panel.  Per D4417, 10 readings per location were taken.  This study used 5 locations and the maximum values of the 10 readings in the 5 locations was recorded and averaged.  The average of the 50 individual readings was also recorded.  Sampling locations are detailed in Diagram 2:

3.     A digital replica tape reader and imager used replica tape to measure HL and Pd.  Four burnishings were taken per panel.  Three were taken using regular Replica Tape (Coarse and/or X-Coarse) and one was taken using Optical Replica Tape.  Measurement locations are shown on Diagram 3:

Initial Observations

1.     Patterns were seen in the results. When examining images of the surfaces, directional striations were visible on the bristle cleaner and roto peen scaler prepared surfaces. The readings taken by the stylus roughness instrument in the direction of this directional bias and against the bias confirmed clear differences in the surface parameters.  Further, images of the air needle scaler prepared surfaces showed they had few distinct peaks and valleys, leading to speculation that the depth micrometer may not have adequately captured true peak-to-valley heights.  It was hypothesized that results would improve with modified measurement techniques that accounted for bias/peak density:

a.     Bristle cleaner/roto peen scaler prepared panels showed directional bias that presented challenges for portable stylus roughness instruments.  A portable stylus roughness instrument may not be appropriate because readings are bias dependent.  Initial specifications (D7127) for measuring power tool cleaned surfaces do not account for bias and/or density of peaks.  This may lead to under or over reported values on surface profile.  Modifying the test method to ignore readings taken in the direction of the bias is necessary to produce meaningful results.

b.     It was observed that the air needle scaler prepared panels had very low peak density (peak frequency).  It was proposed that increasing the number of measurements taken with the depth micrometer might help to account for this decreased frequency and produce a more accurate result.  To evaluate this hypothesis, a second round of testing was done taking 20 measurements per spot in all 5 locations, for a total of 100 readings per panel.  The average of the 5 maximums was reported.

2.     Results indicated that replica tape could be used across all three power tool produced profiles. Results acquired with a digital replica tape reader were not significantly affected by the bias and density that present challenges to the other instruments, and there was no need to modify the test method. 

3.     The depth micrometer results showed that using the average of the maximums for the five locations produced results that correlated more closely with results from the other measurement methods when compared to the average of the 50 individual readings.  

Summary of Initial Results

Chart 1 shows the initial results of the three measurements methods.  The depth micrometer results are shown as both the average of all 50 readings and the average of the 5 maximums.  It was observed that the characteristics produced by the tools challenged some of the measurement tools and made their results less consistent.

The charts below illustrate this.  Individual traces are shown in Charts 2 and 4.  For the bristle cleaner panels readings 2 and 4 were consistently lower for all parameters than readings 1, 3, and 5.  Readings 2 and 4 were taken in the direction of the bias (shown in red), while readings 1, 3, and 5 were taken across the bias (shown in blue).  Charts 3 and 5 display results of all traces taken with the bias averaged together and all traces taken across the bias averaged together.

For the roto peen scaler panels, individual traces are shown in Chart 6 and 8.  Readings 2 and 4 were consistently higher for all parameters than readings 1, 3, and 5.  Readings 2 and 4 were taken across the direction of the bias (shown in blue), while readings 1, 3, and 5 were taken with the bias (shown in red). Charts 7 and 9 display results of all traces taken with the bias averaged together and all traces taken across the bias averaged together.

When peak densities of the three panel types were compared, the air needle scaler panel showed significantly lower measurements than the others, as seen in Chart 10.

Because of the lower densities, it was hypothesized that the depth micrometer readings could be detrimentally affected due to reduced probability of the instrument being placed in the lowest valleys. 

When comparing measurement methods, initial observations showed that amongst the power tool surfaces and measurement methods, Replica Tape was the least affected by influences such as bias or peak count. 

Directional Bias

After testing was complete, analysis of the data showed that results acquired with the portable stylus roughness instrument seemed to be significantly impacted by the directional bias of the panels.  This was most notably present in the panels treated with the bristle cleaner, and to a lesser extent with the panels treated by the roto peen scaler.  

The first 3D image below of a bristle cleaner prepared surface (Figure 6) shows striations from the left to the right, corresponding to the direction the bristle cleaner was applied to the panel, essentially making valleys and/or peaks that align in that general direction.  The second 3D image of a roto peen scaler surface (Figure 7) shows similar characteristics.

In order to confirm the effect this had on results acquired by the stylus roughness instrument, additional testing was performed on the bristle cleaner prepared panels with specific attention paid to the directional bias.

Follow Up Testing - Directional Bias

This additional testing was performed by taking four measurements with the bias and four measurements across the bias created by the bristle cleaner.  Two values, Rpc and Rz, were then compared between the horizontal and vertical testing.  The results are presented below on Charts 11 & 12:

Measurements taken with the bias and across the bias yielded distinctly different results.  Measurements taken by this method could lead to improper characterization of the surface if directional bias is not taken into account, or not known.  This could lead to incorrect or insufficient application of a coating.  Measurements taken by replica tape or depth micrometer instruments were not affected by directional bias.

The manufacturer’s user manuals were consulted for the bristle cleaner and the roto-peen scaler to determine if methods were discussed to address directional bias.  No specific instructions were found discussing directional bias or the development of surface striations in either manual.  The manual for the roto-peen scaler states, in regard to surface treatment, “To insure uniform peening coverage, use a circular or oscillating motion over the entire area.”

However, there are not any instructions or suggestions for use that address or prevent directional bias from occurring.

Peak Density

Measurement results showed that the panels produced by the air needle scaler showed low peak density when compared to the other power tool-produced panels.   Lower peak densities make it more challenging for the depth micrometer to find true peak to valley height.  This is due to a lower statistical probability that the point of the micrometer will land directly into the lowest point of the profile.  Unless the instrument finds the lowest depression of the profile, results will be erroneously low.  A 3D rendering of an air needle scaler prepared surface is shown below in Figure 8.  It can clearly be seen that the surface contains few distinct peaks and/or valleys and appears mostly rounded and flat.  

Follow Up Testing - Peak Density

After low peak densities were observed using digital imaging of replica tape, it was determined that increasing the number of measurements taken with the depth micrometer produced more accurate peak to valley measurements. To test this hypothesis, the number of readings taken in each spot was doubled to 20, for a total of 100 readings per panel.  By doubling the number of measurements the result is more representative of those found with a stylus roughness instrument and replica tape, as shown on Chart 13.

For three of the four panels that were examined, doubling the number of depth micrometer measurements per spot resulted in values more closely correlated with the stylus roughness instrument and replica tape. Although not investigated here, further increasing the number of readings per spot may result in readings that are more consistent with other methods.

It is hypothesized that the stylus roughness instrument will be similarly affected by lower peak density.  The stylus roughness instrument measures over a larger area than the depth micrometer, however, and this larger measurement area may be sufficient to capture the highest peak and lowest valley. Regardless, the probability of finding the true maximum peak to valley profile height on air needle scaler prepared surfaces is reduced, simply due to the lower number of distinct peaks and valleys.  

Observations of Replica Tape Results

Measurements derived from replica tape were not affected by directional bias or peak density unlike measurements taken by other measurement methods.  This method provided consistent results on all three power-tool cleaned surfaces.  

With panels affected by directional bias, the measurement area of the replica tape captures patterns in both directions of the bias. Since HL is measured as the maximum peak-to-valley height across the entire area, the bias has no effect.  Because of this, results from replica tape measurements were more representative of the surface and did not require modification to the measurement method to produce meaningful results.

Similarly, replica tape measurements were not negatively affected by peak density.   Since the replica tape measurement area is larger than the horizontal distance between peaks and valleys, the reduced density was not a factor.  Again, because of the large measurement area of replica tape, readings were more representative than with the micrometer or stylus roughness instrument.

Conclusions

Method C, replica tape and spring micrometers, measured the surfaces produced by all three tools most accurately, primarily because the surface area sampled is larger than that of the other methods.  The increased measurement area of replica tape accounted for the characteristics of power tool-created surfaces that negatively affected the other measurement methods.

Surfaces created by power tools exhibit characteristics that must be taken into consideration when selecting a surface profile measurement method. Tools that leave directional striations (bias) on the surface can result in inaccurate measurements of surface profile parameters when portable stylus roughness instruments are used.  Tools that result in surface profiles with low peak densities are not anticipated in the Standards (ASTM D4417) and as a result, lower peak-to-valley measurements are reported when using a depth micrometer in accordance with that test method.  Failure to account for these characteristics can result in inaccurate results.  

Although there are limitations when using portable stylus roughness instruments and depth micrometers on power tool prepared surfaces, there are modifications that can be made to the measurement methods that allow these instruments to measure these surfaces effectively. Method D stylus roughness instruments were not suitable for measuring surface profile on these surfaces unless care was taken to only measure across the bias.  Readings from Method B depth micrometers suggested that results would improve by increasing the number of readings to account for the low peak density.  

Additionally, results from the Method B depth micrometers clearly showed that using the average of the maximums for the five locations produced results that correlated more closely to results from the other measurement methods when compared to using the average of the 50 individual readings.  This supports other studies that have been done with the same findings, notably “Surface Profile – A Comparison of Measurement Methods” by D. Beamish9, where this method was first proposed.

Chart 14 shows results across all measurement methods after adjustments had been made to account for errors caused by directional bias and low peak density. Directional bias has been accounted for on the bristle cleaner and roto peen scaler prepared panels by removing the trace measurements taken in the direction of the bias.  This resulted in a result that was an average of 13.2% closer to the replica tape results for the bristle blasted panels, and an average of 8.9% closer to the replica tape readings for the roto peen treated panels.  

Low peak density on the air needle scaler panels has been accounted for by plotting the results with both 10 and 20 readings per spot, and reporting the average of the maximums for the five locations.  By doing so, the readings taken by the depth micrometer showed a 15.9% closer correlation to the portable stylus roughness instrument readings, and a 14.2% closer correlation to the replica tape readings.  

The replica tape results, however, are displayed as measured, with no modification for surface effects.  It is clear that the tape’s unmodified results are in line with other measurement methods.

These results strongly suggest that replica tape presents a simpler and more effective solution for measuring across surfaces created by multiple power tools.  The tape requires no special consideration for bias or peak density. Unlike other measurement methods that require power tool-created profile effects to be taken into account, replica tape results can be used as measured.

Related Standards

Within the painting and coating industry, there has been significant research into evaluating blast-cleaned profiles on steel substrates.  

ISO 8503-5—Preparation of steel substrates before application of paints and related products – Surface roughness characteristics of blast-cleaned steel substrates – Part 5: Replica tape method for the determination of the surface profile3

ISO 8503-5 describes the preparation of steel substrates before application of paints and related products and the surface roughness characteristics of blast-cleaned steel substrates:

1 Scope

This document describes a field method for measuring the surface profile produced by any of the abrasive blast-cleaning procedures given in ISO 8504-2. The method uses replica tape and a suitable gauge for measuring, on site, the roughness of a surface before the application of paint or another protective coating.

The method is applicable within the range of profile heights cited for a given grade (or thickness) of replica tape. The commercial grades currently available permit measurement of average peak-to-valley profiles of 20 μm to 115 μm. The method is valid for surfaces that have been cleaned with abrasives.

ASTM D7127—Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument4

ASTM D7127 describes the measurement of surface roughness of abrasive blast-cleaned metal surfaces using a portable stylus instrument:

1. Scope

1.1 This test method describes a shop or field procedure for determination of roughness characteristics of surfaces prepared for painting by abrasive blasting. The procedure uses a portable skidded or non-skidded stylus profile tracing instrument. The measured characteristics are: Rt and Rpc. Additional measures of profile height (Rmax and/or Rz) may also be obtained as agreed upon by purchaser and seller. 

NACE SP0287—Field Measurement of Surface Profile of Abrasive Blast-Cleaned Steel Surfaces Using a Replica Tape5

NACE SP0287 describes and characterizes one procedure for measuring the surface profile of abrasive blast cleaned steel. The measurement technique uses a tape that replicates the surface profile. Other common methods of measuring surface profile are not discussed:

1.1 This standard describes a procedure for on-site measurement of the surface profile of abrasive blast-cleaned steel surfaces that have a surface profile, as defined in Section 2, between 38 and 114 μm (1.5 and 4.5 mils)

ASTM D4417—Standard Test Methods for Field Measurement of Surface Profile Blast Cleaned Steel

ASTM D4417 describes three methods for evaluating the surface profile of blasted steel surfaces in Table 1 below:

1. Scope

1.1 These test methods cover the description of techniques for measuring the profile of abrasive blast cleaned surfaces in the laboratory, field, or in the fabricating shop.

SSPC PA 17—Determining Profile Compliance6

SSPC PA 17 provides additional guidance for determining conformance with surface profile requirements. Whereas ASTM standards describe how to take measurements, SSPC PA 17 focuses on where to take these measurements and how often.

Largely absent from these industry standards are procedures and descriptions for the evaluation of power tool cleaned surfaces.  As the use of power tools becomes more prevalent, it is increasingly important to determine the best and most accurate way to evaluate these surfaces.

Within these standards, there is only one mention of how to evaluate power tool prepared surfaces. ASTM D4417-14 paragraph 1.2 states, “Method B may also be appropriate to the measurement of profile produced by using power tools.”

Lacking standards and/or guidance, there is little information as to whether ASTM D4417 Method B is the best method for all power tool surfaces or if there may be other, more dynamic solutions for measuring surfaces.  Power tool prepared surfaces exhibit characteristics that are not present in blast-cleaned surfaces, namely directional bias and peak density variations between tools.  The impact of these characteristics on specific measurement methods is not well known. 

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References

1 ASTM D4417 “Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel” (ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428)

2 D. Beamish, “Replica Tape – Unlocking Hidden Information”, Journal of Protective Coatings and Linings, February 2015, pp. 1 – 6

3 ISO 8503-5 “Preparation of steel substrates before application of paints and related products — Surface roughness characteristics of blast-cleaned steel substrates — Part 5: Replica tape method for the determination of the surface profile” (International Organization for Standardization (ISO), 1 rue de Varembé, Case postale 56, CH-1211, Geneva 20, Switzerland) 

4 ASTM D7127 “Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument1 (ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428)

5 NACE Standard SP0287, “Field Measurement of Surface Profile of Abrasive Blast-Cleaned Steel Surfaces Using a Replica Tape”. (Houston, TX: NACE, 2016)

6 SSPC-PA-17 “Procedure for Determining Conformance to Steel Profile/Surface Roughness/Peak Count Requirements” (SSPC: the Society for Protective Coatings,800 Trumbull Drive, Pittsburgh, PA 15205, USA) 

7 ASME Y14.36M 1996 “Surface Texture Symbols” (The American Society of Mechanical Engineers, Three Park Avenue, New York, NY 10016-5990 USA) 

8 ASME B46.1-2009 “Surface Texture (Surface Roughness, Waviness, and Lay)” (The American Society of Mechanical Engineers, Three Park Avenue, New York, NY 10016-5990 USA) 

9 D. Beamish, “Surface Profile – A Comparison of Measurement Methods”, DeFelsko Corporation, January 2013