An IR reflective temperature gun should not be used to accurately measure the temperature of a tacking iron.
IR thermometers are basically “brightness meters” for infrared light, not magical temperature mind-readers.
Shiny brass messes with them because it’s a low-emissivity, high-reflectivity surface. Here’s what’s going on:
- IR thermometers assume a certain emissivity.
Emissivity (ε) is “how well a surface emits thermal IR compared to a perfect blackbody.” Many handheld IR guns assume something like ε ≈ 0.95 (good for matte paint, skin, wood, etc.). - Shiny brass has low emissivity.
Polished metals (including brass) often have ε down around ~0.05–0.2 depending on finish and wavelength. That means brass emits way less IR than the thermometer expects at that true temperature. - So the thermometer reads too low… or just wrong.
Because the gun is expecting lots of emitted IR but gets little, it interprets that as “must be cooler.”
Even worse: because brass is reflective, the sensor also picks up reflected IR from the surroundings (your hand, overhead lights, hot furnace nearby, the sky, etc.), so the reading can drift high or low depending on what the brass is “mirroring” in infrared.
A quick mental model:
Matte black surface = “talks” (emits its own IR).
Shiny brass = “echoes” (reflects everyone else’s IR).
How to measure the tacking iron accurately if you must us an IR Gun
- Put a small piece of matte black tape (electrical tape works) on the brass, let it come to the same temperature, measure the tape.
- Or dab a spot with flat black paint (high-temp paint if needed).
- If your IR thermometer allows it, set emissivity appropriately—but tape/paint is often more reliable in real life.
- For best accuracy on metals, use a contact probe (thermocouple/RTD).
Extra nerdy detail: oxidized/tarnished brass usually measures better than polished brass because oxidation increases emissivity—so “uglier” brass is ironically more honest to an IR gun.
The melting point of electrical tape typically ranges from 80°C to 105°C (176°F to 221°F), depending on the specific formulation and manufacturer. Generally, electrical tape begins to degrade at temperatures above 80°C (176°F).
