Tempilstik® vs Thermomelt® — Detailed Comparison to Choose the Right Temperature Indicator

Tempilstik® and Thermomelt® are both solid-wax temperature indicating crayons from Tempil® (now part of Illinois Tool Works — ITW), Made in USA. Both work on the same principle: touch to a hot metal surface — if the crayon melts, that surface has reached the rated temperature. But they are not the same product: the body size differs, the application focus differs, and the level of national-standards traceability differs. Choosing the wrong one does not compromise technical accuracy, but it may cause practical inconvenience or fail to meet a project's documentation requirements.

The Core Physical Difference: Body Size and Grip

Thermomelt® is approximately 33% larger in volume than Tempilstik®. The thicker body is designed for use with heavy welding gloves — fingers wrapped in thick material are less able to control a slender object. During wide-area PWHT operations (heating an entire pipe spool or large plate assembly), the operator often stands back and reaches into the heated zone; the larger body of Thermomelt® reduces the risk of it slipping from a gloved hand.

Tempilstik® is slimmer and lighter, well suited for spot-checking at a precise point and working in confined spaces — gaps between pipe runs, structural corners, hard-to-reach locations. This is why Tempilstik® is the dominant product for daily preheat checks on construction sites: compact, pocket-sized, and manageable with one hand.

NIST Traceability — When This Matters

This is the most important documentation difference between the two products. Thermomelt® is NIST-traceable — meaning its accuracy is traceable to the US National Institute of Standards and Technology. Each box of Thermomelt® comes with a Certificate of Conformance (C/C) bearing the production lot number and a traceability statement linking it to NIST measurement standards.

This does not mean Tempilstik® is less accurate — both products meet the same tolerance of ±1%°F / ±3%°C. The difference is documentation, not physics. For projects requiring NIST-traceable documentation under contract terms — nuclear, defence, aerospace, or any project where the contract specification explicitly requires NIST-traceable temperature indicating devices — Thermomelt® satisfies that requirement while Tempilstik® does not.

For the vast majority of O&G, offshore structural, and power-plant projects in Vietnam operating under AWS D1.1, ASME B31.1/B31.3, or EN 1011-2, Tempilstik® is fully compliant and there is no requirement — or justification — for paying the premium associated with NIST-traceable documentation.

Thermomelt® — multiple grades

Heatstik® for furnace/PWHT

Tempilstik® compact

10-Criterion Comparison Table

Cross-Reference: Equivalent Part Numbers

Thermomelt® and Tempilstik® share ratings at many temperature levels. The table below lists the most commonly used equivalent pairs in welding and heat-treatment applications:

Conclusion: Which Product for Which Situation

The decision rests on three factors: the project's documentation requirements, the physical conditions of use, and the specific temperature range needed.

Use Tempilstik® when:

• Daily preheat verification per AWS D1.1 or ASME B31.1/B31.3
• Interpass temperature checks between weld passes
• Electrode oven temperature verification for E7016/E7018 rods
• Precise spot-checking in confined spaces, multiple checks per shift
• Commercial, industrial, and offshore projects — no NIST-traceable documentation required by contract

Use Thermomelt® when:

• Contract or client specification explicitly requires NIST-traceable documentation
• Wide-area PWHT with operators wearing heavy duty welding gloves
• Nuclear, defence, or aerospace projects where national standards traceability is mandatory
• Lot-specific Certificate of Conformance is required for the third-party inspector's QA package

Frequently Asked Questions