Contrast on Apple Watch Ultra
Apple Watch Ultra gets the most out of Contrast. Its water temperature sensor lets a bath or plunge round start on its own the moment you get in, and the reading decides whether the round is hot or cold. Ultra is also the only Apple Watch that Apple allows in a sauna at all, and only up to 55 °C, so it is the one watch that can record heart rate in a cooler sauna or an infrared cabin.
What the sensor changes
Every Ultra carries a water temperature sensor, and Core Motion reports both a temperature and whether the watch is under water. Contrast uses the second to open a round and the first to classify it. Water at or below 20 °C is cold, at or above 33 °C is heat, and the reading has to settle and agree three times running before a phase commits, so a splash does not start a plunge.
Series 10 and later carry the same sensor. On every other Apple Watch, Contrast works the same way except that you start each round yourself with the Switch button.
The sauna, and Apple's limit
Apple advises against wearing any Apple Watch other than Ultra in a sauna, and against wearing Ultra in one above 55 °C (130 °F). It advises against wearing any model in a steam room. A traditional Finnish sauna runs 70 to 100 °C, which is over that limit.
So an Ultra is the one watch that can stay on your wrist in a sauna, and only a cool one: an infrared cabin, or a sauna run below 55 °C. In a hotter sauna, tap Switch, leave the watch outside, and Contrast times the round from the tap without heart rate.
What it records in the water
Heart rate for every round, with the average, minimum and maximum, and the water temperature through the round. Time is split into six bands from Icy, below 8 °C, to Very hot, 41 °C and above, so a plunge you remember as uniformly brutal can turn out to be forty seconds of Icy and two minutes of Cold.
The sensor is specified to 40 °C. Contrast records hotter water and labels the reading as less certain up there rather than leaving it out.
Keeping the session running
A session runs as a workout on the watch, which is how watchOS keeps an app going with your wrist down and the screen off. Water Lock comes on when you get in, so splashes cannot tap anything. When you climb out, the watch asks whether you are out of the water, and the round closes at the moment you left it, whether or not you answer.
Common questions
Does Contrast detect rounds automatically on Apple Watch Ultra?
Yes, for baths and plunges. Getting into water starts a round, and the water temperature decides hot or cold. Sauna and shower rounds never submerge the wrist, so you start those with one tap.
Can I wear Apple Watch Ultra in a sauna?
Apple advises against wearing Ultra in a sauna above 55 °C (130 °F), and against wearing any model in a steam room. Most traditional saunas are hotter than that.