Humidity and the Löyly Index

When you are locked down at your home for almost two years, have lonely sauna sessions five times a week and have humidity sensors, you start pondering the scientific side of löyly. Or I have…

The basic concept of löyly is of course that you pour water on the stove and it vaporizes and increases air humidity. Finns judge saunas by the quality of löyly. Good löyly is often subjective assessment and can be based on anything: hot scorching steam that burns your skin or gentle warm humid air that slowly hits you. Or anything in between. Bad löyly seems to be bad either because the löyly is so weak that you don’t feel it or it won’t last long enough or it is too hot and scorching so called “dry electric sauna löyly”.

But what determines technically the state of löyly in sauna? When does it start and when does it end?

One approach to answer this question would be to determine some relative humidity range in sauna where you could say you are in the löyly. You may have heard someone saying that their sauna is very humid because the humidity is 40%, for example. The challenge is that relatively humidity is the percentage of maximum humidity in certain temperature. So 40% humid air at 65C air means that there is about 70-75 grams water in a cubic meter or air. 40% humidity at 85c means 155-160 grams water in a cubic meter. This is discussed in great detail in my earlier About Sauna Temperature and Humidity post.

To the Sauna Lab!

Below are temperature, absolute humidity (AH) and relative humidity (RH) readings of three different sauna days of mine in Texas between August and October 2021. You will also see that naturally the outside air temperature and humidity varies day-by-day, which has an impact on the readings in my outside sauna.

The on the left (sauna day 1) was a hot and humid day with 40% RH on floor level and the temperature inside the sauna was about 40c when I started heating it. As you can see, the RH starts dropping quickly as we heat up the sauna but interestingly, absolute humidity increases. I don’t know why that happens though. This was my typical three-session sauna day where I went in at 70c and poured maybe two cups of water. Absolute temperature went from about 90g to 110 after the first ladle and then to 120g. Then I enjoyed löyly and cooled off. When I went back to sauna, the AH was back at about 90g and went up to 110. I had switched off the stove at this point. At thir sessions, the starting humidity was maybe 80g and went to about 100g as the stones got colder and could no longer give the steam.

The second sauna day was also hot but less humid as RH was “only” 40% in the beginning. This time I went in a bit later and the sauna had switched off. You can see how the temperature started dropping after adding löyly. I then switched the stove on again so you can see the temperature climbing up again. This time it seemed that the AH atthe beginning was about 70G and only got up to 95g. Maybe the stones were already bit colder because I eventually got AH to almost 120g.

The third sauna day was a “cold” fall day in Texas. The outside temperature was in low twenties in centigrade. This timel, you coul observe quite radical AH increase from 60g to about 110g during the first löyly. The temperature also rose by 5c so the stove was on all the time. It seems that the relative humidity change was less radical but that is because the scale is different on the third picture – in all sessions, the relative humidity went from low 30’s to mid 40’s. On the third sauna day, you can see that the stove was off during the third session because the temperature kept falling.

Löyly Index

What I can see of the graphs (and I have tens of these), I can determine that a good löyly begins when absolute humidity increases by 20 g per cubic meter or more in say 5 seconds. It is more difficult to say when the löyly ends by reading the graph but technically probably when AH drops back to the starting AH. In practise, most people would feel that the löyly ended earlier though. Maybe it is fair to say that löyly ends when AH drops about 20g/m3 from the peak. Every sauna is different though and I feel that the löyly lasts very long in my sauna and it is more intense than in many other saunas I have been to.

Why would all this matter? Two reasons:
1. The readings illustrate why you cannot tell pretty much anything about sauna air based on relative humidity meter. Also if you have one those sauna thermometers, the hygrometer probably shows whatever anyway. In these test, the sensors were tested and should have been +/- 3% accurate.

2. These findings can lead to scientific breakthrough in the area of sauna automation as you can read below.

Me and my fellow sauna scientist buddy @momanse have been trying to figure out when the löyly begins and ends to use this information on some sauna automation applications.

The first prototype is complementing a good löyly is already in rigorous testing. When the sensor thinks that we got some löyly in the room, the speaker plays authentic Finnish-language complement on a nice löyly. No words necessarily but an “Aissss!” sound.

The next phase is to play an audio file to ask for more löyly when we determine that the previous löyly has ended. Also very common small talk in a Finnish sauna.

I know that some sauna purists hate all kinds of technology in sauna but these applications would have been appreciated by my forefathers hunders of years ago. The technology eliminates the need to talk and outsources all social interaction to a computer . Silence is gold but if there needs to be communications, let the computers handle that.

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