How a sauna thermometer works

A thermometer in a sauna measures temperature. A hygrometer measures humidity or steam instead.¹ The humidity side has its own page, the hygrometer explainer.

Underneath, every thermometer does one trick. Something inside changes as it warms, and the gauge turns that change into a reading.

The dial type: a bimetal strip

A mechanical dial needs no power. Here's the chain, one link at a time.

• Two metals, bonded. A bimetal strip is two strips of different metals that expand at different rates as they're heated.²
• The strip bends. Those different rates make it bend one way when heated and the other way when cooled. It converts a temperature change into movement.²
• A coil does the turning. Household dial thermometers use a strip wrapped into a coil in the most common design.²
• The needle moves. One end of the coil is fixed to the housing. The other end drives an indicating needle inside a circular indicator.²

That source describes household dial thermometers in general. It doesn't describe any one sauna gauge. Your maker's manual says what's inside yours.

The electronic type: resistance

Some sensors use a material whose electrical resistance changes with temperature. Resistance is how hard it is for current to flow through a part. A circuit reads that resistance and a display shows a number.

• Thermistor. An NTC thermistor has less resistance at higher temperatures. It's commonly used as a temperature sensor.³
• What it's made of. Generally a ceramic or polymer.³
• RTD (resistance temperature detector). Many RTD elements are a length of fine wire wrapped around a heat-resistant ceramic or glass core.⁴
• RTD metals. The wire is typically platinum, nickel or copper.⁴
• Thermistor versus RTD. Thermistors use ceramic or polymer, while RTDs use pure metals. Thermistors typically achieve a greater precision within a limited temperature range.³

Not sure what yours uses? Check the maker's model page or manual.

What the sensor senses

It reacts to what touches it. That's the air at its own spot. Nothing more. And a sauna isn't the same temperature everywhere, so what the sensor feels depends on where you've hung it, which is the whole reason placement gets so much attention. The higher benches are hotter, and the lower benches are cooler.¹

So a gauge near the ceiling and one by the bench will honestly disagree. They're both right about where they are. The page on differing readings goes through it.

Questions to ask about any gauge

• What does it sense with? Metal strip, ceramic bead or metal wire. The manual or model page should say.
• Does it need power or a cable? A cable or battery is one more part sitting in the heat. Ratings for those are in the heat-rating notes.
• Where does the maker say it goes? Distances are listed in the mounting notes.
• Is it for a sauna at all? Plenty of dial thermometers are made for patios or kitchens. A sauna listing is the sign to look for.