How a sauna hygrometer works

A hygrometer measures humidity, which means how much water vapor is present in the air.¹ In a sauna it's the gauge for steam.²

Most hygrometers don't count water directly. They watch some other change and work the humidity out from it.¹

The main sensing methods

• Hair tension. An old design. A hair tension dial hygrometer has a nonlinear scale, so the marks aren't evenly spaced.¹
• Capacitive. Modern electronic devices can sense changes in electrical capacitance, meaning how much charge a part can hold.¹
• Resistive. Others sense a change in electrical resistance. Resistive sensors are less sensitive than capacitive ones.¹
• Dew point. Some modern electronic devices use the temperature of condensation, called the dew point.¹ That's more of a lab method.

Which one a sauna gauge uses depends on the model. The maker's page or manual is where to look.

How a sensor copes with heat and steam

• Capacitive sensors. They hold up against condensation and temporary high temperatures, according to the source.¹ It also warns of contamination, drift and aging effects.
• Resistive sensors. The material depends on both humidity and temperature. So the sensor must be combined with a temperature sensor.¹
• The housing. Makers list gauges for wet areas. One lists its sauna thermo-hygrometer with the use environment "Interior - wet area".³

Why the reading moves when the heat changes

Humidity here means relative humidity. At low temperatures a lower mass of water per unit volume can remain as vapor than at high temperatures.¹ So a change in the temperature changes the relative humidity.¹

Put plainly:

• Same steam, hotter air: the percentage shown can drop.
• Same steam, cooler air: the percentage shown can rise.
• A gauge by the ceiling and one by the bench sit in different air temperatures. Their percentages may differ even if the steam is spread evenly.

That last point links to the readings page.

Dry heat and steam bursts

The hottest Finnish saunas have relatively low humidity levels, with steam generated by pouring water on the hot stones.² A hygrometer is there to show that swing. A jump after a ladle of water isn't a fault.

What the gauge shows

One maker's thermo-hygrometer shows the temperature and air humidity level of the sauna on a single face.³ A separate hygrometer is a different build. One recessed model is 120 x 120 mm with a 40 mm recess, and it goes in during wall paneling.⁴

That's a case where you plan the gauge before the walls close. Where on the wall? See the placement guide.

Reading one sensibly

• Note the temperature too. The humidity figure only makes sense next to it.
• Compare at the same spot each time.
• Check the maker's manual before you decide a gauge is off.