A sauna thermometer is cheap and almost every cabin has one, which is why so few people think about it. Two things are worth knowing: the placement changes the reading far more than the instrument does, and a digital unit in a hot cabin is a consumable rather than a fitting.
Placement is most of the reading
Air in a sauna stratifies steeply, so a thermometer at bench level and one near the ceiling will disagree by a large margin and both will be correct. Convention is to hang it at the height of a seated head on the upper bench, on a wall away from the heater, because that is the air you are actually sitting in.
Analogue, combined and digital
A bimetallic dial is the standard: no battery, no electronics to cook, readable through steam and accurate enough for the job. A combined dial pairs it with a hygrometer, which is worth having because humidity changes felt heat more than a few degrees do. Digital units read precisely and tend to fail in a hot cabin within a season or two.
What accuracy you need
You do not need laboratory precision. What you need is a consistent reading you learn to interpret, because the number that matters is the one you have come to associate with a good session. A dial that is a few degrees out but repeatable is more useful than a precise unit mounted somewhere the air is not representative.
Materials in a hot cabin
Wooden or stainless housings are the ones that survive. Plastic bezels craze, printed faces fade and adhesive backings let go. Screw fix rather than stick, keep it clear of the heater's direct heat path and out of the way of a ladle, and it should outlast several sets of benches.
Questions people ask about sauna thermometer
Where should a sauna thermometer go?
At the height of a seated head on the upper bench, on a wall away from the heater. That is the air you sit in.
Is a digital one better?
More precise and less durable. Most owners are better served by a bimetallic dial that needs no battery and tolerates the heat.
Do I need a hygrometer too?
It is useful. Humidity changes how hot a given temperature feels more than a few degrees on the dial do.