Does AWS D1.1:2025 specify what instrument must be used to measure preheat?

No. AWS D1.1:2025 establishes the required minimum preheat and interpass temperatures but does not prescribe the measurement method or instrument. The choice of measurement tool — tempilstik, contact pyrometer, infrared thermometer, or thermocouple — is left to the contractor's quality control plan. What the standard does require is that the measurement is made before welding begins, that it is taken at the correct location (at least 3 inches from the joint on each side for thicker plate), and that the temperature meets the minimum required by the WPS.

Are infrared thermometers acceptable for preheat verification under AWS D1.1?

Infrared thermometers can be used but have a significant accuracy limitation on steel: their readings depend on emissivity, which varies with surface condition (mill scale, rust, paint, oxidation). A shiny steel surface will read much lower than the actual temperature on an infrared gun not corrected for emissivity. Contact methods — tempilstik or contact pyrometer — give more reliable readings on structural steel in typical shop and field conditions. If an infrared thermometer is used, the instrument should be calibrated for the emissivity of the steel surface being measured, and the QC plan should specify this requirement.

Where do you measure preheat on a groove weld joint?

AWS D1.1:2025 requires preheat measurement at least 3 inches (75 mm) from the edge of the weld joint, on each side of the joint in the base metal. For plate thicknesses greater than 1.5 inches (38 mm), the temperature should be verified on the opposite face or by through-thickness measurement to ensure the full section has reached temperature, not just the surface facing the heat source. The temperature must meet the minimum at all measurement points before welding begins.

How often should interpass temperature be checked and recorded during a multi-pass weld?

AWS D1.1:2025 does not set a fixed checking interval for interpass temperature, but common QC plan practice is to check at the beginning of each pass on fracture-critical or CVN-qualified welds, and at least every 15-30 minutes on standard structural welds. The WPS specifies the maximum interpass temperature — typically 450°F (230°C) for most carbon steel structural work, and lower for quenched and tempered steels. When the WPS has both a minimum preheat and a maximum interpass limit, the CWI must verify that the joint is above minimum before each pass and below maximum after each pass.

Preheat Temperature Measurement Methods for CWI: What's Acceptable

Tempilstik, contact pyrometer, or infrared gun — AWS D1.1:2025 doesn't prescribe the measurement method, but your QC plan must. Here's how CWIs verify preheat and document it correctly.

Preheat and interpass temperature requirements in a WPS are only as good as the measurement that backs them up. A CWI can sign off on a preheat that was never really at the required minimum if the measurement practice is poor. Understanding which measurement tools work, where to take readings, and how to document results is a core competency for structural welding inspection — not a procedural footnote.

What AWS D1.1:2025 Actually Requires

AWS D1.1:2025 specifies minimum preheat temperatures based on base metal group, thickness, and in some cases the carbon equivalent of the material. These values appear in the preheat tables of the code and are incorporated into the WPS preheat and interpass temperature fields. Rule library based on AWS D1.1:2025; verify against your governing edition.

The code requires that the base metal be at or above the minimum preheat temperature before welding begins. For multi-pass welds, the interpass temperature — the temperature of the previously deposited weld metal and adjacent base metal before the next pass — must not fall below the minimum preheat nor exceed the maximum interpass temperature specified in the WPS.

What the code does not dictate is how you measure these temperatures. The measurement method is left to the contractor and CWI, provided the results are accurate enough to confirm compliance. This is where judgment and practice matter.

Tempilstik (Temperature-Indicating Crayons)

Tempilstik — the common trade name for temperature-indicating crayons — remains the most widely used preheat verification tool on structural job sites. The crayon is formulated to melt at a specific temperature. The CWI marks the base metal approximately 3 inches from the weld joint. If the mark liquefies and smears when the metal reaches the rated temperature, the minimum has been achieved. If the mark stays solid and chalky, the metal has not reached that temperature.

Advantages: Simple, inexpensive, requires no calibration, easily observed at a distance, leaves a visible mark for photographic documentation. Each crayon is single-purpose — a 250°F Tempilstik confirms 250°F and nothing else.

Limitations: Single-point temperature only; to verify 300°F you need a 300°F crayon, not just a 250°F one. The mark can be misleading on very smooth or oily surfaces. The crayon should be applied before heating so the mark is on unpretreated steel.

For routine structural preheat at 50°F, 150°F, 225°F, or 300°F — the most common AWS D1.1 preheat minimums — Tempilstik verification is fast and reliable. Most shops stock a set of crayons covering their typical preheat range.

Contact Pyrometers

A contact pyrometer uses a thermocouple-tipped probe held against the steel surface to measure temperature directly. The instrument displays a digital reading in real time, typically within a few seconds of contact.

Advantages: Gives an exact temperature reading, not just a pass/fail at a set point. Useful when you need to know whether you're at 280°F or 300°F, rather than just confirming you've cleared a minimum. Reusable and can cover any temperature in its range. Newer instruments log readings with timestamps, which is useful for documented inspection records.

Limitations: Requires calibration on a defined schedule (typically per manufacturer recommendations, at minimum annually or after instrument impact). The probe must make firm contact with the steel surface — a poor contact reading can be 20-30°F low. Probe wear affects accuracy over time.

For projects with stringent documentation requirements — AISC Tier 2 special inspection, demand-critical CVN welds, bridge work under D1.5 — a calibrated contact pyrometer gives a defensible recorded temperature rather than a "met/not met" crayon observation.

Infrared Thermometers

Infrared thermometers (also called IR guns or non-contact pyrometers) measure surface temperature by detecting infrared radiation. Point and shoot — no contact required. They are convenient and popular in other industries, but structural welding is a context where their accuracy limitation matters significantly.

Steel's emissivity varies widely depending on surface condition:

Mill scale: approximately 0.80–0.85
Polished steel: approximately 0.07–0.20
Rusted steel: approximately 0.70–0.80
Steel with intact primer or coating: highly variable

An IR thermometer set for emissivity 0.95 — the standard factory setting — will underread a polished steel surface by 100°F or more at typical preheat temperatures. On structural steel with mixed mill scale and cut surfaces, the error is less dramatic but still significant. A 200°F reading on an IR gun pointed at a clean-cut steel edge could correspond to 250°F actual temperature or 180°F actual temperature depending on surface condition.

Acceptable use conditions: IR thermometers are useful for relative comparisons (is one end of the joint hotter than the other?) and for monitoring heat buildup trends. They can be used for preheat verification when:

The instrument allows emissivity adjustment and the value has been set appropriately for the actual surface
The result is cross-checked against a Tempilstik or contact reading during setup to confirm the IR reading is accurate for that surface condition
The QC plan documents this calibration check

Unless these conditions are met and documented, default to contact methods for preheat and interpass verification.

Measurement Location

Where you measure matters as much as what you measure with. AWS D1.1:2025 requires the preheat measurement to be taken at a minimum distance of 3 inches (75 mm) from the joint edge in the base metal. Measuring too close to the joint gives an elevated reading from direct flame or arc heat — not the actual material temperature.

For thick plate (greater than approximately 1.5 inches / 38 mm), surface temperature alone is not sufficient. A 2-inch thick plate heated on one face can have a surface temperature of 300°F while the opposite face is still at 50°F ambient. The code addresses this by requiring the CWI to verify temperature on both sides of the joint or by checking after a soak period that allows temperature to equalize through the section. For thick-section CJP groove welds, checking the preheat from both sides is standard practice.

Documenting Preheat for CWI Records

Preheat verification should appear in the CWI's inspection record for every weld where a minimum preheat applies. A useful entry includes:

Joint identifier or weld map reference
Required preheat per the WPS
Measured temperature and measurement location
Measurement method used (Tempilstik 300°F, contact pyrometer serial XXXXX)
Time of measurement
Welder ID
CWI signature

For photographic documentation, a Tempilstik mark on the steel with a ruler showing the 3-inch offset is clear and reproducible. Some shops photograph the pyrometer display screen alongside the joint. Either approach works — the point is that the record must be specific enough to confirm what was measured where, with what instrument.

Interpass Temperature Monitoring

Interpass temperature maximum is the other side of the control loop. Exceeding the maximum degrades CVN toughness in the weld metal and HAZ. On CVN-required welds (Table 6.8 supplementary essential variable scope), interpass temperature exceedance is a reportable event that may require the WPS to be requalified if the maximum is changed upward. Rule library based on AWS D1.1:2025; verify against your governing edition.

The practical control in production is to check the joint temperature before starting each pass on CVN welds, especially late in a thick-section joint where heat has accumulated. If the joint is above the maximum, the CWI should stop welding and wait. Document the stop time and the temperature that triggered it — this is part of the weld record.

For non-CVN welds on standard carbon steel structural work, interpass monitoring is still good practice: excessive heat input accelerates grain growth in the HAZ and can affect mechanical properties even without a formal CVN trigger.

Building a Measurement Plan into the QC Plan

The fab shop QC plan should address preheat and interpass measurement explicitly: which instruments are approved, calibration requirements, measurement location rules, recording format, and the CWI's authority to halt welding when temperature conditions are not met. Leaving the method to individual CWI preference produces inconsistent records and gaps in the audit trail.

For shops pursuing AISC certification or working on owner-specified special inspection programs, a documented and consistently followed preheat verification protocol is one of the first things an auditor checks. The WPS essential variable documentation and the CWI pre-weld inspection checklist articles cover how preheat fits into the broader pre-weld inspection discipline.

For shops building out their WPS documentation system, see our pricing page for what a platform-supported WPS and inspection record workflow looks like.