Two different measurements answer two different questions, and HeatAlert shows whichever one your unit can actually produce.
- Heat index combines air temperature and humidity into what the air feels like to a person. It needs two sensors, which every HeatAlert unit has.
- WBGT (wet bulb globe temperature) adds radiant heat — direct sun, a pizza oven, a hot roof. It needs a black globe probe as well.
WBGT is the more honest number for a working kitchen or a loading dock, because a person standing next to an oven is absorbing heat the air temperature never sees. It is also the measure OSHA's heat guidance is written around, which is why it is the one we report when it's available.
Both are computed from one shared implementation — the same code on every model of sensor, and the same method again on our side, which recomputes every reading and is what your records are built from. So a Wi-Fi unit and a cellular unit in the same room agree with each other, and both agree with what the compliance record says. A device showing one severity while the record shows another would be the worst kind of error in a product like this, and it is designed out rather than watched for.
Wet-bulb temperature is solved from the psychrometric relation (ASHRAE Fundamentals, Ch. 6) rather than estimated from a fitted curve, at standard sea-level pressure. Readings taken before 10 August 2026 used an earlier approximation and can differ by up to 1.9°F; they are kept exactly as recorded, because the record of what was observed at the time is the one that matters. The OSHA report states this in its methodology section.