Can an IR thermometer be adjusted for accurate emissivity on steel?

Technically yes — but it requires knowing the exact emissivity for that surface at that temperature, which varies by surface condition. To determine it accurately, a thermocouple reference is needed at the same point, and the emissivity dial adjusted until both readings agree. This process is too time-consuming for high-throughput preheat inspection on a construction site.

Can Tempilstik® be used for high-temperature PWHT above 600°C (1112°F)?

Yes — Tempilstik® is available up to 2000°F (1093°C). For PWHT of carbon steel at 600–650°C (1112–1202°F) or P91 at 730–760°C (1346–1400°F), Tempilstik® remains accurate. In furnace or resistance-heating PWHT operations it is typically used as a supplementary spot-check alongside thermocouple dataloggers.

Can Tempilstik® be used for PWHT checks above 600°C?

Yes — Tempilstik® is available up to 2000°F (1093°C) and is routinely used as a supplementary spot-check at P91 PWHT temperatures of 730–760°C alongside thermocouple dataloggers.

Tempilstik® vs Infrared Thermometer — Technical Comparison for Industrial Preheat Inspection

Q: Does AWS D1.1 permit infrared thermometers for preheat measurement?

AWS D1.1 does not prohibit IR thermometers but requires that temperature measurement devices be calibrated and accurate. When emissivity is set incorrectly, the device may be fully calibrated yet still give an inaccurate reading — and the inspector has no way to detect this on site. Many O&G owner specifications explicitly prefer or require temperature indicating crayons (sticks) for the final preheat sign-off.

Published 20 Jan 2025 · Fast Group Engineering · 7 min read

When a QC inspector has two options in hand — a Tempilstik® temperature indicating crayon and an infrared (IR) thermometer — the question is not which one cost more. It is which one is more trustworthy under the actual conditions on this site. Both instruments measure surface temperature, but through completely different physical mechanisms, and that difference has significant implications for structural steel and O&G pipeline welding. This article is a technical analysis — not a sales pitch — to help QA/QC engineers and welding inspectors make the right choice for each specific situation.

Tempilaq® and Tempilstik® used together for direct-contact temperature measurement — independent of emissivity — Tempilaq® and Tempilstik® measure temperature by direct contact — independent of surface emissivity, no batteries, no calibration drift. These are the core advantages over IR thermometers.

Operating Principles — the Fundamental Difference

Infrared Thermometer

An IR thermometer measures the infrared radiation emitted from an object's surface and converts it to a temperature reading via the Stefan–Boltzmann relationship. More precisely, the device measures radiation from the surface — not the actual temperature of the material. Accuracy depends entirely on the emissivity value (ε) programmed into the device.

Simplified IR thermometer relationship: T_measured = f(ε, E_measured) Where: ε = surface emissivity (0.00 to 1.00) E_meas = measured infrared radiation Temperature error from incorrect emissivity setting: Each 0.1 unit error in ε → approximately 5–15°C error (varies with temperature and wavelength band)

Tempilstik® Temperature Indicating Crayon

Tempilstik® operates through pure physical chemistry: a wax compound formulated to melt at a precisely defined temperature. When the surface reaches that temperature, the crayon mark melts immediately — with no dependence on any surface parameter. No emissivity setting, no measurement angle, no spot-size factor, no background radiation.

Emissivity — the Primary Error Source for IR Thermometers

The emissivity of steel varies substantially with surface condition. This is the core vulnerability of IR thermometers in industrial welding applications:

Steel Surface Condition	Actual Emissivity (ε)	Temperature Error if ε Set to 0.95
Mirror-polished steel	0.07–0.10	Very large: 40–80°C (72–144°F)
Hot-rolled steel with mill scale	0.70–0.80	5–15°C (9–27°F)
Lightly rusted steel	0.80–0.85	3–8°C (5–14°F)
Epoxy primer-coated steel	0.85–0.90	2–5°C (4–9°F)
Brushed stainless steel	0.15–0.35	Large: 20–50°C (36–90°F)
Blast-cleaned steel (SSPC SP10)	0.90–0.95	Small: < 2°C (4°F)

On a typical offshore construction or fabrication site, steel surfaces are in a mix of states — mill scale, light rust, and primer — with actual emissivity in the range 0.70–0.90. A standard IR thermometer shipped with a factory default of ε = 0.95 will read 5–15°C low on the most common construction-site surfaces.

Field example: WPS requires a minimum preheat of 150°C (302°F) for 30 mm SM490 plate. Inspector uses an IR thermometer with ε set to 0.95 on a mill-scale surface (actual ε ≈ 0.78). Display reads 150°C — but actual surface temperature is approximately 138°C (280°F), 12°C below the WPS minimum. The inspector signs off "passed" — but the joint has not reached the required preheat. This is the origin of many invisible preheat deficiencies in the field.

Comprehensive 14-Criterion Comparison

Criterion	Tempilstik®	IR Thermometer
Measurement mechanism	Direct contact — physical chemistry	Infrared radiation — optical
Affected by emissivity	No — completely independent	Yes — primary error source
Accuracy (ideal conditions)	±1%°F / ±3%°C	±1–2°C (with correct ε)
Accuracy (mill-scale surface)	±1%°F — unchanged	±5–15°C — surface dependent
Outdoor wind / rain effect	No effect on measurement	Background radiation and moisture can affect reading
Reflected heat interference	None	Furnaces or flames nearby cause false high readings
Calibration requirement	None — chemical mechanism is inherently stable	Periodic calibration required (typically annual)
QA/QC documentation	Visible melt mark — photographable evidence	Number on a display — requires separate logging
Accepted by standards	AWS D1.1, ASME, API 1104, EN 1011-2	AWS D1.1, ASME, API 1104 (with correct ε)
Initial cost	Low — purchase by grade	High — electronic instrument
Cost per measurement	Consumable (each stick)	Near zero (reusable device)
Power requirement	None	Battery — fails at remote sites without spares
Equipment failure risk	No electronics → no failure modes	Drop damage, moisture, extreme heat affect accuracy
Non-contact measurement	No — must touch surface	Yes — useful for inaccessible surfaces

Tempilaq® colour change on metal surface — unambiguous visual confirmation of temperature reached — A clear colour change when the rated temperature is reached — unambiguous visual evidence that cannot be disputed, at a time when an IR thermometer may be reading 5–20°C low due to surface emissivity.

When to Use Tempilstik® — When to Use an IR Thermometer

✅ Use Tempilstik® for:

Final preheat confirmation at the weld joint per WPS

This is the primary application. Tempilstik® delivers an unambiguous, physically verifiable result confirming that a specific joint has reached the exact WPS-required temperature before the arc is struck. The melt mark is visible evidence that can be photographed and recorded in the QA/QC file without ambiguity. Reliable in outdoor conditions — wind, rain, uneven surfaces.

✅ Use Tempilstik® for:

Preheat of high-alloy steels — P22, P91, duplex stainless

For steels with low or variable emissivity (blast-cleaned Cr-Mo, stainless steel), IR thermometer error can reach 20–40°C. For P91 with a mandatory minimum preheat of 200°C (392°F) where ±10°C means the difference between pass and fail, the risk of relying on an IR thermometer alone is unacceptable on a strictly-monitored project.

✅ Use IR thermometer for:

Rapid screening of the heating zone before final confirmation

An IR thermometer is very useful for observing which parts of the heating zone are reaching temperature and which still need more heat input — before the definitive Tempilstik® check. Use the IR to guide heating, then Tempilstik® to confirm before welding.

✅ Use IR thermometer for:

Inaccessible surfaces or remote measurement

For welds in narrow gaps, small-bore pipe below DN 50, or areas where personal safety prevents close approach, an IR thermometer may be the only practical option. In these cases, emissivity must be carefully corrected or emissivity tape applied to the measurement point to improve accuracy.

Conclusions — Choosing by Actual Field Conditions

Tempilstik® is the better choice when:

Steel surface has mill scale, rust, or primer coating
Outdoor conditions — wind, rain, high humidity
High-alloy steel: P11, P22, P91, duplex SS
Visual, photographable QA/QC evidence is required
No batteries available at the work location
Absolute reliability is needed for each individual joint
Third-party inspector is witnessing preheat sign-off

IR thermometer is the better choice when:

Clean surface with accurately known emissivity
Rapid scanning of multiple points in sequence
Remote measurement — surface is inaccessible
Controlled indoor fab shop environment
Continuous furnace temperature monitoring
Preliminary screening to guide heating operations

Recommended practice for O&G and offshore projects: Use both tools together — the IR thermometer for rapid observation during heating, Tempilstik® for the definitive threshold confirmation at each weld location per WPS before striking the arc. This is standard practice on EPCi projects in Vietnam, including work by PTSC, Technip, and international contractors at major offshore facilities. The cost of Tempilstik® per weld joint is negligible compared with the cost of a repair weld caused by insufficient preheat.

Inspector using Tempilstik® to confirm preheat at a pipe weld on an offshore construction site — On an offshore site, Tempilstik® delivers an immediate, reliable result — unaffected by strong light, reflective surfaces, or mill scale, conditions where an IR thermometer routinely gives errors of 5–20°C.

Frequently Asked Questions

Can an IR thermometer be accurately adjusted for steel emissivity in the field?

Technically yes — but to do so correctly requires knowing the exact emissivity for that specific surface condition at that temperature. The only reliable method is to place a thermocouple at the same point, heat the surface, and adjust the emissivity dial until both readings agree. This is too time-consuming for high-throughput preheat inspection on a construction site. On a surface that is half mill scale and half rust, the emissivity also varies across the measurement spot, adding a further uncertainty that cannot be corrected with a single ε setting.

Does AWS D1.1 permit infrared thermometers for preheat measurement?

AWS D1.1 does not prohibit IR thermometers but requires that temperature measurement devices be calibrated and accurate. The problem is that a device can be fully calibrated yet still give an inaccurate reading when the emissivity is set incorrectly — and the inspector has no way to detect this on site. Many owner QA procedures for O&G projects explicitly require temperature indicating crayons (sticks) for the final preheat pass/fail decision, precisely because of this limitation.

Can Tempilstik® be used for high-temperature PWHT checks above 600°C (1112°F)?

Yes — Tempilstik® is available up to 2000°F (1093°C). For carbon steel PWHT at 600–650°C (1112–1202°F) or P91 PWHT at 730–760°C (1346–1400°F), Tempilstik® remains accurate and is routinely used as a supplementary spot-check alongside thermocouple dataloggers — confirming that specific locations on the weld actually reached the required temperature rather than relying solely on thermocouple placement.

Need technical guidance on choosing the right preheat measurement tool for your project? Contact the Fast Group technical team — tempil.vn is the authorised Tempil® distributor in Vietnam, with over a decade of O&G welding support experience.

View Tempilstik® Part Numbers +84 938 888 958