What is a Hi-Lo gauge used for in weld inspection?

A Hi-Lo gauge measures the internal root alignment (mismatch) between two pipe or plate edges before welding or in a completed root pass. Under AWS D1.1, the maximum misalignment at a CJP groove weld root is typically 1/16 in. (1.6 mm) for static loading and varies with thickness; the Hi-Lo gauge provides a direct read of the offset without arithmetic.

What does a Bridge Cam gauge measure?

A Bridge Cam (or Cambridge) gauge is a multi-function weld inspection gauge that measures fillet weld leg size, throat, weld reinforcement height, undercut depth, root opening, and groove angle—depending on the measurement setting. It is the most versatile single tool a CWI carries for AWS D1.1 visual inspection.

Do you need special certification to use weld inspection gauges?

No separate gauge certification is required beyond the CWI credential from AWS. However, AWS QC1 (the CWI certification standard) requires calibrated instruments for quantitative measurements. Weld gauges should be traceable to a calibrated standard; a gauge that has been bent, dropped, or is otherwise out of calibration is not acceptable for code inspection.

What is the difference between a fillet weld leg gauge and a throat gauge?

A leg gauge measures the linear dimension of each leg of the fillet weld from the weld toe to the root. A throat gauge measures the theoretical throat (0.707 × leg for a 45° fillet) or the actual throat through the weld cross-section. AWS D1.1 visual acceptance criteria are stated in terms of effective throat and minimum leg size; the Bridge Cam gauge reads both.

Nov 11, 20266 min read

CWI Weld Inspection Gauges: Hi-Lo, Bridge Cam, and Fillet

Practical guide to the weld inspection gauges every CWI uses under AWS D1.1: the Hi-Lo gauge, Bridge Cam gauge, and fillet weld gauges—what each measures, when to use it, and how to record findings.

CWI visual inspection weld gauges AWS D1.1 fillet weld groove weld inspection tools

New to procedures? Start with WPS welding: what a welding procedure specification is, with a filled-in example.

Walk a CWI through any structural steel job site and you will see the same handful of gauges hanging from their vest or kit bag. They are not interchangeable; each answers a specific question that the AWS D1.1 visual acceptance criteria require the inspector to answer before they can sign off a weld. Using the wrong gauge—or misreading the right one—means either passing a noncompliant weld or rejecting a sound one. Neither outcome is acceptable.

This article covers the three most commonly used weld inspection gauges for AWS D1.1 structural welding: the Hi-Lo gauge, the Bridge Cam gauge, and fillet weld gauges. For each one, the article explains what it measures, when it applies under D1.1, and how to document the reading in the inspection record.

The Hi-Lo Gauge

The Hi-Lo gauge (sometimes called a root alignment gauge or pipe fitting gauge) measures the radial mismatch—the offset in height—between two joined edges at the weld root. It was originally developed for pipe-to-pipe alignment at butt joints, and it remains the standard tool for checking root offset before and during welding on pipe and tube work.

In structural steel applications under AWS D1.1, the Hi-Lo gauge answers one critical pre-weld question: does the root alignment of a CJP groove weld meet the acceptable mismatch limit? Root offset creates a stress concentration at the weld root under tensile loading and can prevent the root pass from achieving full penetration. AWS D1.1 specifies maximum root mismatch as a function of plate thickness and loading condition; the Hi-Lo gauge provides a direct, numerical read of that offset without requiring the inspector to visually estimate it.

How to use it. Bridge the gauge across the two plate edges at the weld root. The step in the gauge body bridges the gap between the two surfaces; reading the gauge shows the height difference between the high and low sides in 1/16 in. or 1 mm increments depending on the gauge version. If the measurement equals or exceeds the allowable mismatch, it must be corrected—by shimming, re-fitting, or adjusting the joint—before welding proceeds.

Limitation. The Hi-Lo gauge measures surface-level root mismatch. It does not measure root gap (the space between the edges). A separate feeler gauge or pin gauge measures root gap to verify it is within the WPS qualified range and the prequalified joint tolerance. These are two separate acceptance criteria, and a CWI must check both.

Documentation. Record the measured offset and the location (weld number, joint ID, station or distance along a continuous joint). Note the allowable maximum for that joint. If a re-inspection was required after correction, record the corrected reading.

The Bridge Cam Gauge

The Bridge Cam gauge—manufactured under various brand names but universally called the Bridge Cam in the structural inspection world—is the most versatile single gauge a CWI uses. It was developed for comprehensive weld profile measurement and replaces several single-purpose gauges. A properly used Bridge Cam gauge can measure:

Fillet weld leg size (the length of each weld leg from root to toe)
Fillet weld throat (the theoretical or actual throat dimension)
Groove weld reinforcement height (the crown height above the base metal surface)
Undercut depth (the groove eroded into the base metal adjacent to the weld toe)
Root opening (the gap between the joint faces before welding)
Groove angle (the included angle of a V-groove joint)

Each measurement uses a different function of the gauge, so familiarity with the instrument is necessary. An inspector who sets the gauge to the reinforcement-height function and reads it as a throat measurement will record an incorrect value.

Fillet weld leg measurement. Set the sliding pointer to the leg measurement position. Place the flat base of the gauge against the base metal flat surface adjacent to the weld toe. Lower the pointer until it contacts the weld face. Read the leg size from the scale. Take readings for both legs—on a 45-degree fillet, they should be equal; unequal legs mean the weld cross-section is not balanced.

Reinforcement check. For groove welds, the Bridge Cam gauge measures whether the weld crown height exceeds the maximum reinforcement limit. Under AWS D1.1 visual acceptance criteria, groove weld reinforcement must not exceed 1/8 in. (3 mm) for most static-loaded structural applications. The Bridge Cam reading must stay at or below that limit; a weld with excessive reinforcement must be ground flush or dressed before it is accepted.

Undercut measurement. Set the pointer to contact the base metal surface on the unfused side of the weld toe, then slide it to touch the bottom of the undercut groove. The scale reads the depth directly. AWS D1.1 limits undercut to 1/32 in. (0.8 mm) for most transverse-loaded welds; a Bridge Cam reading over that limit requires the undercut to be repaired. See weld undercut acceptance criteria under AWS D1.1 for the full acceptance table.

Fillet Weld Gauges (Go/No-Go Gauges)

Fillet weld gauges designed as go/no-go comparators provide a faster check than the Bridge Cam in production inspection. A go/no-go fillet gauge is a stepped or profiled blade that corresponds to a standard fillet weld size—1/4 in., 5/16 in., 3/8 in., and so on. The inspector fits the gauge against the weld face; if the weld fills the step (is at least as large as the gauge), it passes; if it doesn't fill the step, it fails.

Go/no-go gauges are fast and require less calibration sensitivity than a vernier-read instrument, but they have one significant limitation: they measure whether the weld is at least the minimum size, not the actual measured size. For inspection reports that require a recorded dimension—rather than just a pass/fail notation—the Bridge Cam or a ruled measurement is needed.

Fillet gauge sets. A standard set covers sizes from 1/8 in. to 1 in. in 1/16 in. increments. For structural steel where Table 7.7 minimum fillet sizes start at 3/16 in. and the specified sizes on a typical connection design run up to 5/8 in. or 3/4 in., the set covers the full production range. See minimum fillet weld size under AWS D1.1 Table 7.7 for the size-by-thickness minimums.

Keeping Gauges Calibrated and Documented

AWS QC1 requires that inspection instruments be calibrated. For weld gauges, this typically means periodic verification against a certified measurement standard—not full metrology lab calibration, but enough to confirm that the sliding pointer or blade edge has not shifted from use or damage. Any gauge that has been bent, has a loose pointer, or shows obvious wear should be replaced before use in formal inspection.

Document gauge calibration status in the CWI's daily inspection log or the shop's quality system. For AISC-certified fabricators, the QC plan must address inspection instrument control. An auditor looking at the inspection records for a job may ask to see calibration documentation for the gauges used to accept or reject welds during production.

Inspection Sequence for a Fillet Weld

A practical visual inspection sequence using these gauges for a fillet weld under AWS D1.1:

Pre-weld: Hi-Lo gauge to check fit-up at root (for CJP joints); feeler gauge for root gap. Verify fillet weld application matches the WPS and drawing.
During welding (interpass): Visual check of bead profile, slag removal, and no visible cracks. No gauge required unless a discontinuity is suspected.
Post-weld final: Bridge Cam gauge for leg size (both legs), throat, and undercut. Go/no-go gauge as a quick pass on production runs. Record all measurements with weld ID, gauge size used, and acceptance/rejection decision.

This sequence matches the inspection requirements in the CWI daily weld inspection report framework. Keeping the records complete and legible at the time of inspection—not reconstructed later—is the standard an auditor and an EOR expect.

For a welding procedure management system that links weld inspection records to WPS and welder qualification data, see wpswelding.com/pricing.

Rule library based on AWS D1.1:2025; verify against your governing edition. Gauge manufacturers' instructions govern specific model setup; this article describes general inspection principles, not tool-specific operating procedures.

FAQ

Frequently asked.

What is a Hi-Lo gauge used for in weld inspection?: A Hi-Lo gauge measures the internal root alignment (mismatch) between two pipe or plate edges before welding or in a completed root pass. Under AWS D1.1, the maximum misalignment at a CJP groove weld root is typically 1/16 in. (1.6 mm) for static loading and varies with thickness; the Hi-Lo gauge provides a direct read of the offset without arithmetic.
What does a Bridge Cam gauge measure?: A Bridge Cam (or Cambridge) gauge is a multi-function weld inspection gauge that measures fillet weld leg size, throat, weld reinforcement height, undercut depth, root opening, and groove angle—depending on the measurement setting. It is the most versatile single tool a CWI carries for AWS D1.1 visual inspection.
Do you need special certification to use weld inspection gauges?: No separate gauge certification is required beyond the CWI credential from AWS. However, AWS QC1 (the CWI certification standard) requires calibrated instruments for quantitative measurements. Weld gauges should be traceable to a calibrated standard; a gauge that has been bent, dropped, or is otherwise out of calibration is not acceptable for code inspection.
What is the difference between a fillet weld leg gauge and a throat gauge?: A leg gauge measures the linear dimension of each leg of the fillet weld from the weld toe to the root. A throat gauge measures the theoretical throat (0.707 × leg for a 45° fillet) or the actual throat through the weld cross-section. AWS D1.1 visual acceptance criteria are stated in terms of effective throat and minimum leg size; the Bridge Cam gauge reads both.