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.
On a WBGT unit, WBGT is the number that decides alerts — the unit page marks it "drives alerts", shows the current WBGT flag beside the status badge, and its readings tables carry two level columns: the WBGT band (which alerted) and the heat-index level (reference). The heat index is still measured and recorded on every reading, but its level does not colour the page or trigger anything while a healthy globe probe is present. The two scales genuinely disagree sometimes — a room can read Caution on heat index while WBGT still says no flag — and the labelling exists so that disagreement reads as two scales, not as a contradiction.
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 two units 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.