Base Metal Reading Effects on Magnetic Thickness Gauge Accuracy for Hot-Dip Galvanized Coatings

The Coating Thickness Measurement Imperative

Hot-dip galvanized coating thickness verification serves as the primary quantitative quality assurance measure confirming adequate corrosion protection for galvanized steel articles. ASTM specifications—A123 for structural products, A153 for hardware and fasteners, A767 for reinforcing steel—establish minimum coating thickness requirements varying by material category and steel thickness range. Compliance verification requires accurate, repeatable measurement methodology enabling objective acceptance determination. While four distinct measurement techniques exist—magnetic thickness gauging, chemical stripping and weighing, weigh-before-and-after galvanizing, and optical microscopy—magnetic thickness gauges provide the predominant field and facility measurement tool due to rapid non-destructive assessment, minimal operator skill requirements, and adequate accuracy for specification compliance evaluation.

However, magnetic gauge technology demonstrates inherent sensitivity to base metal substrate properties creating systematic measurement errors potentially overstating coating thickness by 0.5 to 1.5 mils. Understanding the base metal reading (BMR) phenomenon—the magnetic interaction between gauge probe and substrate surface irregularities producing false thickness indication—enables informed measurement protocol development, appropriate error analysis, and realistic compliance determination avoiding unnecessary article rejection while maintaining specification integrity.

Coating Thickness Measurement Methods: Overview

Before examining BMR effects specifically, understanding alternative measurement methods provides context for magnetic gauge advantages and limitations:

Method 1: Magnetic Thickness Gauge

Principle: Measures magnetic attraction force or magnetic flux between gauge probe and ferromagnetic steel substrate through non-magnetic zinc coating

Advantages:

• Non-destructive testing
• Rapid measurement (seconds per reading)
• Portable field-capable equipment
• Minimal operator training requirements
• Real-time results enabling immediate decisions

Limitations:

• Sensitive to substrate properties (BMR effects)
• Requires calibration verification
• Accuracy affected by surface conditions
• Edge effects and curvature influences

Standards: ASTM E376, ASTM D7091

Typical Accuracy: ±(0.1 mil + 1-3% of reading)

Method 2: Stripping Method (Weigh-Strip-Weigh)

Principle: Weigh coated article, chemically strip coating, weigh again; calculate coating weight and convert to thickness using zinc density and surface area

Advantages:

• Eliminates BMR concerns
• Accurate average thickness over entire article
• Not affected by surface conditions
• Definitive verification for disputes

Limitations:

• Destructive testing consuming test articles
• Time-consuming (hours for complete process)
• Requires careful weighing and stripping
• Not practical for routine quality control
• Surface area calculation complexity for irregular shapes

Standards: ASTM A123 Appendix, ASTM A767

Typical Accuracy: ±0.1-0.3 mil for careful execution

Method 3: Weigh-Galvanize-Weigh

Principle: Weigh article before galvanizing, weigh after galvanizing; calculate coating weight from difference

Advantages:

• Simple concept
• No BMR effects
• Average coating over entire article

Limitations:

• Requires pre-galvanizing weighing (operational complication)
• Flux and moisture absorption affects accuracy
• Handling damage between weighings introduces error
• Not suitable for post-galvanizing quality control

Standards: ASTM A123 Appendix

Typical Accuracy: ±0.2-0.5 mil depending on article size and weighing precision

Method 4: Optical Microscopy

Principle: Cut cross-section through coating, polish, examine under microscope measuring coating thickness directly

Advantages:

• Direct visual measurement
• Shows coating structure and metallurgy
• Not affected by BMR
• Useful for metallurgical investigation

Limitations:

• Destructive testing
• Laboratory equipment required
• Time-consuming sample preparation
• Single location measurement (may not represent average)
• Requires skilled metallographer

Standards: ASTM E3, ASTM E1382

Typical Accuracy: ±0.05-0.1 mil for careful measurement

Magnetic Thickness Gauge Operating Principles

Understanding BMR requires comprehending magnetic gauge measurement physics:

Two Primary Technologies

Magnetic Attraction (Pull-Off) Method:

Operating Principle:

• Permanent magnet in probe attracts to ferromagnetic steel substrate
• Magnetic attraction force decreases with increasing coating thickness
• Spring or electronic force sensor measures attraction
• Calibration correlates force to thickness

Gauge Types Using This Method:

• Mechanical pencil gauges
• Some digital gauges

Characteristics:

• Simple, robust technology
• Sensitive to substrate mass and surface condition
• Requires firm contact with surface

Magnetic Flux (Induction) Method:

Operating Principle:

• Electromagnetic coil in probe generates magnetic field
• Magnetic flux travels through coating into steel substrate and returns
• Flux density or inductance changes with coating thickness
• Electronics measure flux change and calculate thickness

Gauge Types Using This Method:

• Electronic/digital gauges
• Mechanical dial (banana) gauges

Characteristics:

• More sophisticated electronics
• Less sensitive to surface variations than pull-off method
• Faster measurement capability
• Requires electronic power source

Magnetic Gauge Types and Characteristics

Type 1: Mechanical Pencil Gauge

Physical Description: Pen-shaped instrument with spring-loaded magnetic probe tip

Operation:

• Place probe perpendicular to surface
• Apply specified pressure until gauge "clicks" or reaches defined position
• Read thickness from graduated scale on barrel

Advantages:

• No batteries or power required
• Simple, durable construction
• Low cost ($50-200)
• Long service life with minimal maintenance

Limitations:

• Operator technique sensitivity (pressure, angle)
• Slower measurement speed
• Coarser reading resolution (0.1-0.5 mil increments)
• Reading scale interpretation subjectivity

Typical Applications: Field inspection, rough screening, backup verification

Type 2: Mechanical Dial (Banana) Gauge

Physical Description: Curved body with spring-loaded probe and analog dial indicator

Operation:

• Place probe on surface
• Apply until clicking stop mechanism engages
• Read thickness from dial face

Advantages:

• No batteries required
• Good readability from dial face
• Moderate cost ($200-500)
• Reasonable accuracy and repeatability

Limitations:

• Requires two-hand operation
• Bulkier than pencil gauge
• Dial mechanisms can wear affecting accuracy
• Slower than electronic gauges

Typical Applications: Shop floor quality control, moderate-volume inspection

Type 3: Electronic/Digital Gauge

Physical Description: Hand-held electronic instrument with probe cable and digital display

Operation:

• Place probe firmly on surface
• Gauge automatically measures and displays thickness digitally
• Data logging and statistical functions often included

Advantages:

• Rapid measurement (1-2 seconds)
• High accuracy (±0.1 mil + 1-3%)
• Digital readout eliminates interpretation error
• Statistical processing (averaging, min/max, standard deviation)
• Data storage and transfer capabilities
• Self-calibration checking functions

Limitations:

• Battery power required
• Higher cost ($500-2,000+)
• More complex electronics can fail
• Requires periodic calibration verification

Typical Applications: Production quality control, high-volume inspection, statistical process control, documentation-intensive applications

Base Metal Reading: Definition and Mechanism

The BMR Phenomenon

Definition:

Base Metal Reading (BMR) is the apparent thickness indication produced by a magnetic thickness gauge when measuring on uncoated steel substrate, resulting from substrate surface properties affecting magnetic interaction between gauge and base metal.

ASTM D7091 Section 3.1.2:

"base metal reading (BMR), n—the thickness indication of an instrument when it is placed on the uncoated metal."

Physical Cause:

Magnetic gauges respond to the distance between the probe sensing element and the effective magnetic interaction plane within the substrate. Surface roughness creates peaks and valleys with varying distances from probe contact point to solid metal bulk:

Smooth Surface:

• Gauge contacts metal surface directly
• Short, uniform distance to bulk metal
• Minimal BMR

Rough Surface:

• Gauge contacts peak of surface irregularity
• Greater distance through air gaps and valleys to bulk metal
• Magnetic field must traverse these gaps
• Produces positive thickness reading despite no coating present
• This false reading is the BMR

Measurement Geometry

Coating Thickness Definition:

True coating thickness = Distance from original steel surface (defined by average roughness plane) to coating outer surface

Magnetic Gauge Measurement:

Indicated thickness = Distance from probe contact point (typically on coating surface peak) to effective magnetic interaction depth in substrate

The Discrepancy:

When substrate has surface roughness:

Indicated Thickness = True Coating Thickness + BMR

The gauge "sees" coating plus the effective substrate roughness depth, overstating actual coating thickness.

Factors Affecting BMR Magnitude

Several substrate characteristics influence BMR value:

Surface Roughness

Primary Factor:

Surface topography irregularities directly create BMR:

Smooth Mill Scale Surface:

• Roughness (Ra): 50-200 microinches typical
• BMR: 0.1-0.3 mil typical

Light Blast Cleaning:

• Roughness (Ra): 100-300 microinches
• BMR: 0.3-0.7 mil typical

Aggressive Blast Cleaning:

• Roughness (Ra): 200-500 microinches
• BMR: 0.7-1.5 mil typical

Correlation: BMR approximately proportional to surface roughness average (Ra)

Steel Composition and Magnetic Properties

Substrate Permeability:

Different steel grades demonstrate varying magnetic permeability affecting magnetic field penetration:

Low-Carbon Structural Steel:

• High magnetic permeability
• Strong magnetic response
• Moderate BMR sensitivity

High-Alloy Steels:

• Lower magnetic permeability
• Weaker magnetic response
• Different BMR characteristics

Practical Impact: Generally minimal for common structural steels, but mixing steel types in single lot can create measurement inconsistency.

Substrate Thickness and Mass

Edge Effects:

Measurements near edges or on thin sections:

• Reduced substrate mass affects magnetic field pattern
• Lower magnetic attraction or flux density
• Can produce lower readings (opposite BMR direction)

Minimum Substrate Thickness:

ASTM E376 specifies minimum substrate thickness requirements for accurate measurement:

• Generally >1/16 inch (1.6 mm) for reliable measurement
• Thinner substrates require calibration adjustment

Residual Surface Contaminants

Rust and Oxides:

Heavy rust scale before pickling:

• Creates false thickness indication
• Combined with surface roughness increases BMR
• Should be removed before BMR measurement

Pickling Conditions:

Normal Pickling: Removes scale and oxides, reveals actual steel surface

Over-Pickling (Excessive Exposure):

• Can etch steel surface increasing roughness
• Increases BMR value
• Generally uncommon in properly controlled operations

ASTM Standards Governing BMR

ASTM E376

Title: "Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Testing Methods"

Scope: General methodology for electromagnetic coating thickness measurement on any conductive coating/substrate combination

Key BMR-Relevant Provisions:

Paragraph 4.4 - Interferences:

"The composition, surface roughness, and other properties of the substrate metal can affect the accuracy of the measurement."

Paragraph 5.7 - Surface Roughness Effects:

"Any appreciable surface roughness may cause gauges to read high. When unusually rough surfaces are encountered, measurements should be made on a sufficient number of points to establish a reliable average. If the surface of the uncoated base metal is appreciably rough, measurements on it should be made before coating, and such measurements should be subtracted from the measurements made on the coating."

Calibration Requirements:

Section 7 establishes calibration verification procedures using traceable thickness standards

Paragraph 7.3 - Substrate Effects:

"If the properties of the base metal, such as composition and surface roughness, affect the calibration, adjustments shall be made."

ASTM D7091

Title: "Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals"

Scope: More specific guidance for paint and coating thickness measurement (includes zinc coatings)

Key BMR-Relevant Provisions:

Section 3.1.2 - BMR Definition: Formally defines base metal reading term

Section 7.5 - Base Metal Readings:

"7.5.1 If the substrate is significantly rough, or if the most accurate results are desired, base metal readings should be taken on representative uncoated areas of the substrate.

7.5.2 For coated surfaces, the BMR should be subtracted from coating thickness readings to obtain true coating thickness."

Procedure:

• Measure uncoated representative sample at minimum 10 locations
• Calculate average BMR
• Subtract BMR from all coated article measurements

Practical BMR Considerations in Galvanizing Facilities

Operational Challenges

Production Environment Constraints:

Safety: Measuring articles after chemical cleaning (pickling, fluxing) but before galvanizing immersion presents safety concerns:

• Hot flux-coated articles difficult to access
• Measurement delays expose personnel to bath area hazards
• Articles suspended on handling equipment limiting access

Throughput Impact: Additional measurement steps between processing stages:

• Slow production flow
• Require dedicated personnel
• Extend cycle time

Practical Difficulty:

• Finding truly representative uncoated areas after flux application
• Ensuring same articles measured pre- and post-galvanizing
• Managing data correlation across production batches

Typical Reality:

Most galvanizing facilities do NOT routinely measure BMR for standard production due to these constraints. BMR compensation typically reserved for:

• Disputed measurements near specification minimums
• High-value or critical applications
• Blast-cleaned articles with known high roughness
• Customer-specific requirements

When BMR Compensation Proves Unnecessary

Adequate Coating Thickness Margin:

When measured coating substantially exceeds specification minimum, BMR becomes negligible concern:

Example Calculation:

Given:

• ASTM A123 minimum required: 3.9 mils (for structural shapes, 1/4"-5/8" thickness range)
• Measured coating: 5.2 mils
• Gauge accuracy: ±(0.1 mil + 2% of reading) = ±0.20 mil
• Estimated BMR: 0.5 mil (moderate roughness)
• Combined maximum error: 0.5 + 0.20 = 0.70 mil

True coating (worst case): 5.2 - 0.70 = 4.5 mils

Comparison: 4.5 mils > 3.9 mils required → Specification satisfied despite maximum conceivable negative errors

Margin Analysis: Measured thickness (5.2 mils) exceeds required minimum (3.9 mils) by 1.3 mils. Combined maximum error (0.7 mils) falls within this margin, confirming compliance without BMR measurement necessity.

General Guideline:

When measured coating exceeds specification minimum by more than sum of (gauge accuracy + estimated maximum BMR), BMR compensation unnecessary for compliance determination.

Typical Hot-Dip Galvanizing Thickness:

Standard galvanizing commonly produces 4-7 mils coating thickness even where minimums are only 2-4 mils, providing inherent margin absorbing BMR and gauge errors.

When BMR Compensation Becomes Important

Marginal Compliance Situations:

Scenario 1: Thin Coating on Minimum Threshold

Given:

• Required minimum: 3.0 mils
• Measured coating: 3.2 mils
• Margin: Only 0.2 mils

Analysis: With gauge accuracy ±0.2 mil and potential BMR 0.5 mil, true coating could be as low as 2.5 mils—below specification minimum.

Resolution: BMR measurement necessary to confirm compliance.

Scenario 2: Blast-Cleaned Substrate

Articles blast cleaned before galvanizing for:

• Paint adhesion improvement (duplex systems)
• Uniformity enhancement
• Special specifications

Blast cleaning increases surface roughness substantially (Ra 200-500 microinches), creating BMR values 0.7-1.5 mils.

Requirement: BMR compensation generally mandatory for blast-cleaned articles near specification minimums.

BMR Measurement and Compensation Procedures

For Mechanical Gauges (Pencil and Dial Types)

Procedure:

Step 1: Sample Selection

Select representative uncoated article:

• Same steel grade and thickness as production articles
• Similar surface condition (pickled and fluxed, not just raw stock)
• From same steel heat/lot when possible

Step 2: Measurement Locations

Measure at minimum 10 randomly distributed locations on sample:

• Avoid edges and corners
• Flat representative surfaces
• Distributed across article surface

Step 3: Record Readings

Document all individual measurements

Step 4: Calculate Average BMR

BMR = (Reading₁ + Reading₂ + ... + Reading₁₀) ÷ 10

Step 5: Apply Correction

For each coating thickness measurement on galvanized articles:

True Coating Thickness = Measured Thickness - BMR

Example:

• BMR average: 0.4 mil
• Measured coating on galvanized article: 4.2 mils
• Corrected coating thickness: 4.2 - 0.4 = 3.8 mils

For Electronic/Digital Gauges

Option 1: Zero Adjustment Method

Procedure:

Step 1: Calibration Verification

Verify gauge accuracy using manufacturer-supplied traceable standards:

• Check zero using uncoated steel standard
• Check span using coated thickness standards (typically 2-3 mils, 5 mils)
• Adjust if readings deviate from standard values

Step 2: BMR Zero Adjustment

Place gauge on representative uncoated sample:

• Make multiple measurements (5-10 locations)
• If gauge shows positive reading (BMR), activate zero adjustment function
• Electronic gauge adjusts internal calibration to read zero on this substrate

Step 3: Measure Galvanized Articles

All subsequent measurements automatically compensated for substrate BMR

Advantages:

• Simple single-step compensation
• No manual calculation required
• Gauge automatically adjusts all readings

Limitations:

• Zero adjustment modifies gauge calibration affecting all measurements until reset
• Must re-zero for different substrate conditions
• Can introduce errors if zero adjustment performed incorrectly

Option 2: Manual Subtraction Method

Procedure:

Same as mechanical gauge procedure:

• Measure representative uncoated sample at 10+ locations
• Calculate average BMR
• Manually subtract BMR from each galvanized article measurement

Advantages:

• Maintains gauge factory calibration
• Explicit BMR documentation
• No risk of incorrect zero adjustment

Limitations:

• Manual calculation required
• Potential for arithmetic errors

Error Analysis and Measurement Uncertainty

Understanding total measurement uncertainty enables realistic compliance determination:

Error Sources and Magnitudes

Gauge Calibration Error:

Specification: ±(0.1 mil + 1-3% of reading)

For 5.0 mil reading: 0.1 + (0.02 × 5.0) = 0.1 + 0.1 = ±0.2 mil typical

Base Metal Reading:

Range: 0.1-1.5 mil depending on surface condition

• Smooth mill scale: 0.1-0.3 mil
• Normal pickled surface: 0.2-0.5 mil
• Blast-cleaned surface: 0.7-1.5 mil

Operator Technique:

Variability from:

• Probe placement angle (should be perpendicular)
• Contact pressure consistency
• Reading location selection

Magnitude: ±0.1-0.3 mil for trained operators

Edge Effects:

Measurements near edges show reduced readings:

• Avoid measurements within 1/2 inch of edges
• Follow ASTM E376 edge distance requirements

Combined Uncertainty:

Root-sum-square method for random errors:

Total Uncertainty = √[(Gauge error)² + (BMR error)² + (Operator error)²]

Example: √[(0.2)² + (0.5)² + (0.2)²] = √[0.04 + 0.25 + 0.04] = √0.33 = ±0.57 mil

Conservative Approach:

Simple addition of maximum values: Total = 0.2 + 0.5 + 0.2 = ±0.9 mil

Measurement Best Practices

Calibration Verification Frequency

Before Each Use:

• Verify gauge zero and span using standards
• Document verification results
• Adjust if necessary

Daily: For production inspection, verify multiple times per shift

After Drop or Impact: Immediately re-verify calibration

Annual: Third-party calibration certification for quality system compliance

Measurement Technique

Surface Preparation:

Clean Surface:

• Remove loose scale, dirt, moisture
• Dry surfaces if wet
• Brush off surface dust

Avoid:

• Measuring on wet coatings
• Measuring through dirt or debris
• Measuring over temporary marks or contamination

Probe Placement:

Perpendicular Contact:

• Hold probe at 90° to surface
• Firm, consistent pressure
• Avoid rocking or angling

Location Selection:

• Flat representative areas
• Minimum 1/2 inch from edges
• Avoid weld areas (different roughness)
• Distribute across article surface

Multiple Measurements:

ASTM A123 requires minimum 3 measurements per article (more for larger articles)

• Distribute across accessible surfaces
• Average for compliance determination
• Note any unusually high or low readings for investigation

Special Considerations

Duplex Systems (Paint Over Galvanizing)

Challenge:

Measuring through paint to galvanizing on steel creates complex magnetic interaction:

• Paint layer (non-magnetic)
• Zinc coating (non-magnetic)
• Steel substrate (magnetic)

Standard Gauge Limitation:

Conventional magnetic gauges measure total non-magnetic thickness (paint + zinc), not individual layer thicknesses.

Solution:

Specialized duplex coating thickness gauges using dual measurement technology:

• First measurement: Total coating (paint + zinc)
• Magnetic adjustment determines zinc thickness
• Subtraction yields paint thickness

Manufacturers:

Several companies produce duplex gauges specifically for this application

Cost: $1,500-3,000 typical

Very Thin Coatings

Wire and Small Diameter Articles:

Minimum coating thicknesses (1.4-2.0 mils for wire) approach gauge accuracy limits:

• BMR becomes significant percentage of total thickness
• Measurement uncertainty increases proportionally
• Extra care in calibration and BMR compensation required

Recommendation: Consider weigh-strip-weigh verification for critical thin-coated articles.

Reactive Steel Coatings

Silicon-Bearing Steel:

Steels with silicon content 0.04-0.25% develop thick coatings (5-10+ mils) through enhanced zinc-iron reactions:

• Coating includes thick intermetallic layers (hard, brittle)
• Magnetic properties of intermetallics differ from pure zinc
• May affect gauge calibration

Typically: Effect is minimal for standard magnetic gauges calibrated on zinc-steel systems.

Documentation and Quality Records

Essential Documentation

Calibration Records:

• Date and time of verification
• Standards used (traceability)
• Gauge readings on standards
• Adjustments performed
• Personnel performing verification

BMR Measurements (When Performed):

• Date and conditions
• Sample description
• Individual measurement locations and values
• Calculated average BMR
• Articles to which BMR applies

Coating Thickness Measurements:

• Article identification
• Measurement locations
• Individual readings
• Average thickness
• BMR correction (if applicable)
• Pass/fail determination
• Inspector identification

Record Retention

Quality system requirements (ISO 9001, customer specifications) typically mandate:

• Minimum 5-10 year retention
• Traceability to specific production lots
• Retrieval capability for audits or disputes

Specification Language Recommendations

For Projects Requiring BMR Compensation

Explicit Requirement:

"Coating thickness measurements shall be corrected for base metal reading (BMR). BMR shall be determined per ASTM E376 Paragraph 5.7 or ASTM D7091 Section 7.5 on representative uncoated samples before galvanizing. BMR shall be subtracted from all coating thickness measurements."

Standard Specification Without BMR

Typical Language:

"Coating thickness shall be measured per ASTM E376 using calibrated magnetic thickness gauge. Minimum average coating thickness shall meet ASTM A123 Table 1 requirements."

Implication: Standard measurement without explicit BMR compensation—adequate when measured coatings substantially exceed minimums.

Base metal reading (BMR)—the false thickness indication produced by magnetic thickness gauges when measuring on uncoated steel substrate—results from surface roughness creating apparent distance between gauge probe contact point and effective magnetic interaction plane within the substrate, causing gauges to overstate coating thickness by 0.1 to 1.5 mils depending on surface condition with smooth mill scale surfaces producing 0.1-0.3 mil BMR, normal pickled surfaces yielding 0.2-0.5 mil BMR, and aggressive blast-cleaned surfaces generating 0.7-1.5 mil BMR. ASTM E376 and D7091 standards explicitly recognize BMR effects and recommend subtracting BMR from coating thickness measurements for most accurate results, requiring measurement of representative uncoated samples at minimum 10 locations before galvanizing to establish average BMR value subsequently subtracted from all galvanized article measurements. However, practical galvanizing facility constraints including safety concerns, throughput impacts, and operational complexity make routine BMR measurement impractical for standard production, with BMR compensation typically reserved for situations where measured coatings marginally exceed specification minimums, blast-cleaned articles with known high surface roughness, or customer-specific requirements demanding maximum measurement accuracy. When measured coating thickness substantially exceeds ASTM A123 minimum requirements by margins greater than combined gauge accuracy (±0.1-0.2 mil) plus estimated maximum BMR (0.5 mil typical), BMR compensation proves unnecessary because specification compliance remains assured despite maximum conceivable measurement errors—a common situation as standard galvanizing typically produces 4-7 mils coating where minimums are only 2-4 mils. Proper BMR compensation procedures for mechanical gauges involve measuring representative uncoated samples at 10+ locations calculating average BMR and manually subtracting from galvanized measurements, while electronic gauges enable either zero adjustment directly on uncoated substrate automatically compensating subsequent readings or manual subtraction methodology identical to mechanical gauges maintaining factory calibration. Total measurement uncertainty combining gauge calibration error (±0.1-0.2 mil), uncorrected BMR (0.2-0.5 mil typical), and operator technique variability (±0.1-0.3 mil) approaches ±0.5-1.0 mil, requiring realistic compliance evaluation recognizing measurement limitations while maintaining specification integrity and avoiding inappropriate rejection of conforming articles. Best practices demand daily gauge calibration verification using traceable standards, proper measurement technique including perpendicular probe placement on clean dry surfaces avoiding edges, multiple measurements per article distributed across surfaces, and comprehensive documentation of calibration records, BMR determinations when performed, individual measurements, and pass/fail determinations supporting quality system requirements and providing dispute resolution evidence. Visit the original AGA resource article for more information.