Thermometers and other measurements

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There are several things that we’d ideally like to know about the environment in our sauna:

  • Ambient Air Temperature – Thermometer
  • Moisture / Relative Humidity – Hygrometer
  • CO2 – CO2 Meter
  • Particulate Matter – PM Meter
  • Radiant Heat – Globe MRT Thermometer


Truth In Advertising

Before venturing further I’d like to propose a bit of Sauna Truth In Advertising. Every manufacturer of any kind of pre-made or kit sauna should publish the following for each product or product variation (and wouldn’t be a bad idea for all custom sauna builders to do this for the saunas they build):

Recommended temperature logger: Thermadata 4-Channel Logger along with either ‘PTFE/FEP Tip Probes’ or ‘high temp wire probes’

Recommended CO2 logger: Gaslab CM-501 or Aranet4.

Short Protocol:

  1. Line chart of actual continuously measured temperatures over a period of time at the floor, foot bench, sitting bench and 1m above the sitting bench with probe placed at least 1” from any solid surface.
    1. Protocol:
      1. Warm up to 105°c or max temp.
      2. 30 minutes with sauna empty
      3. Until sauna has cooled to 1/2 max temp.
    2. All measurements taken simultaneously
    3. In a 68° room for inside units
    4. In shaded -7°c (20°f) outside temperature for outside units
    5. In shaded -25°c (-13°f) outside temperature for outside units

Full Protocol (or as much as possible):

  1. Photo showing, for the advertised number of people, that number of average size American adult men sitting in the unit.
  2. Line chart of actual continuously measured temperatures over a period of time at the floor, foot bench, sitting bench and 1m above the sitting bench.
    1. Protocol:
      1. Warm up to 105°c or max temp.
      2. 30 minutes empty
      3. 2 rounds – 
        1. All enter
        2. One stays 10 minutes, one 13, one 16
        3. All enter 25 minutes after all entered for the first round
        4. One stays 10 minutes, one 13, one 16
      4. Wait until sauna regains max temp
      5. Until sauna has cooled to 1/2 max temp.
    2. All measurements taken simultaneously
    3. In a 68° room for inside units
    4. In shaded -7°c (20°f) outside temperature for outside units
    5. In shaded -25°c (-13°f) outside temperature for outside units
  3. Line chart of CO2, temperature, % relative humidity (and barometric pressure if available) measured at 1m above the sitting bench over a period of time with the advertised number of adults and the sauna at 50°c.
    1. Warm up to 50°c or max temp. Ventilation set to ‘warm-up’.
    2. 30 minutes empty with ventilation in ‘use’ mode.
    3. 3 rounds – 
      1. All bathers in sauna for 15 minutes
      2. All bathers out of sauna for 15 minutes
    4. Wait until sauna regains max temp
    5. Until sauna has cooled to 1/2 max temp.
    6. Accompanying video of the measurement being taken and showing how long the door was opened each time.
  4. Mean Radiant Temperature measurements for each seating position; 700mm above the front edge of the sitting bench (bather face) and 200mm in front of the bench (bather knee), with the heater continuously heating from 70°c to 80°c.
  5. Line chart of relative humidity over a period of 30 minutes in a 90°c sauna measured simultaneously at the middle (side to side and front to back) of the sitting bench and middle of the foot bench. Water thrown on stones @ 5, 10, 15 and 20 minutes. Both should rise to the same level, approx 20-35% RH, and both should then decline to the base level within about 3-4 mins. If the lower bench does not attain the same level then that’s an indication of steam not reaching down to bathers feet. If the RH does not decline back to the base level that’s an indication of poor ventilation.

Some target values might be:


This should provide consumers with some accurate objective information upon which to base decisions. Tests should ideally be conducted by an independent agent to insure impartiality and consistency but measurements from the manufactures would still be helpful.



Thermometers – Location

Temps vary throughout the sauna so where you put your thermometer matters. The generally accepted official location is at the mid point of and 1m (39”) above the upper sitting bench opposite the heater – so at the head of a bather sitting in the middle of the bench. When someone in Finland says that they like 96°c, this is what they are referring to.

Thermometers elsewhere in the sauna can tell you what the temp is at that location but that is not ‘the sauna temp’ and could be off by ± 30°c or more. However, it can be useful to know temps elsewhere. I have 4 semi permanent probes; 1) Official 1m above the middle of the upper bench, 2) At the foot bench below #1, 3) 1m above the upper bench at the far side from the heater and 4) at floor level in the middle of the room.

Heater Thermostat – Outside of North America the thermostat sensor for electric heaters is typically placed about 20cm below the ceiling (or 1m above the upper bench height) and at least 20cm away from the heater (so that it stays out of the rising hot air from the heater). UL guidelines in North America complicate this.  Many people in North America find that they need to place the sensor lower down and further away in order to achieve proper sauna temps. 

North American heaters also have a UL mandated High-Temp Limit Sensor that can cause unnecessary problems. I’ve a collection of photos of solutions people have tried including pulling these further from the heater if mounted on it, placing a fresh air vent as close to the sensor as possible, having a fan blowing air on to the sensor, and other not so successful or more dangerous endeavors.


Thermometers – Analog

For most people a simple analog dial wall thermometer made for a sauna will do the trick. They’re inexpensive, look good and are usually accurate enough.

Single Temp Accuracy – These are generally only accurate at one temp (and will ‘drift’ high or low the further away from that temp you are). Most are adjustable though so we adjust ours to be accurate at 100°c (212°f). We usually like our sauna to be about 94-96°c (201-204°f) and maybe once per week in the 105-115°c (221-239°f) range so we know that it’s pretty accurate for these. 

To check and adjust analog thermometers the best option is to buy, beg, hire or borrow a NIST calibrated digital thermometer. A glass tube candy thermometer may work for this as well but I’d not want to keep it in there permanently as cleaning up glass shards out of soft wood from the tube breaking is nearly impossible. 

Drift – We know that one of our analog thermometers looses about 1°c per 6°c diff from 100°c so we know that if we want 85°c actual that we should look for about 88°c on the thermometer.

Slow – Analog thermometers are slow responding. Your sauna may get to your preferred 94°c but your thermometer still says 83°c and will take another 10 minutes to get to 94°. It’s kind of telling you what the average temp has been over the past 20 minutes (or sometimes what it was 10 minutes ago) rather than what the current temp is.. Similarly, they also miss peaks and valleys. However, if you maintain a consistent temp then it will eventually settle in and be close enough.

Oven Thermometers – These are generally not very accurate and being off by 30°f or more is not unusual. In one test of 4 Taylor oven thermometers they were off by 13°f among themselves but ALL of them were above the actual temp as measured by a NIST calibrated digital logging thermometer with the best one 14°f high and the worst reading 27°f above actual.


Thermometers – Digital

Digital have a number of advantages including being more accurate at all temps and faster response time. With a good one – what you see is what you’ve got. The downside is that electronics generally don’t like the higher temps of a sauna.

The solution is a digital thermometer with external probes so that the electronics can be in a cooler location and the probe where you want it – such as 1m above the upper sitting bench, which would be too hot for electronics.

A few of many options:

Thermoworks DOT with pluggable probes – These are inexpensive and accurate. Available in round or square, various colors and with or without bluetooth. The DOT can be placed low (where temps stay below 50°c) inside the sauna or on the wall in the changing room. Air probes will be somewhat more accurate than meat probes but not significantly.

Thermoworks Thermadata WiFi or Thermadata 4-Channel Logger – These is the most accurate for the money devices we’ve found. We use the PTFE/FEP probes in saunas (though air/cooking probes work relatively well also). I’d recommend getting K-style probe loggers if accuracy is important. The logger itself sits near the floor or on the wall in the changing room and probes can be placed wherever we want. The wire for the probes is rather stiff so get only as long as you need. One 7m and a few 2m & 3m work well if you want to experiment. One drawback is that it only does automatic WiFi updates once per hour (or when the Audit button is pressed which is a bit of a problem for remote monitoring). Temps displayed on the logger itself are current and accurate however.

One benefit is that pressing the ‘start/audit’ button generates an ‘audit’ marker at that point in time and forces a cloud update via WiFi. In the graph below I did two rounds in the sauna, pressing the audit button when I entered and left the hot room each round. For the 1st round head temp was 100.7°c and foot temp was 86.5°c on entering and 97.6°c / 84.7°c leaving 11.2 minutes later. The second round was 96.9°c / 84.5°c entering and 89.6°c / 81.1°c leaving.



Onset HOBO 4-Channel Logger – This has two advantages over the Thermoworks 4-channel; 1) the logger is good to 70°c so can be placed higher in the sauna and in many cases on the foot bench and 2) the logger includes a thermometer so you actually get 5 channels of logging.

Ruuvi Tag – These have proven to be slow, inaccurate and are limited to only 70°c (or 85°c for the Pro or with a special battery). Neither can be placed at head height (you’ll have to guess what the actual temp is at head height). I do not recommend these as a thermometer (they are actually the worst digital thermometer we’ve seen).

Inkbird IBS-TH1 – The device and it’s internal sensor is only good to 60°c and the external probe is only good to 100°c. The probe temp limit might be OK for some, but people should be cautious and know that once the probe has gone over temp it is no longer ‘reliable’.  Accuracy is OK but not great at ±1°c. Humidity accuracy is poor at 25°c / 77°f and so presumably quite bad at sauna temps.

BBQ / Meat Thermometers can work fairly well. Note that air probes are somewhat more accurate than meat probes though the difference is usually rather small.

All-In-One Digital Probes like Thermapens, Thermopops or the myriad of these available anywhere and everywhere are not accurate when the device is above 50°c so are at best questionable in a sauna.


Thermometers – Radiant

Ideally a bather should experience no noticeable radiant heat in a sauna, particularly from the stove.

Radiant heat is measured by using two temperature probes; a standard ambient air temperature probe and a globe probe. The globe will measure radiant plus ambient, so to determine just radiant energy you subtract the ambient temperature (probe should be placed about 1” from the globe). For calibration both probes should be placed together where they are as far from or protected from radiant heat as possible.

This is primarily for manufacturers and professional sauna builders. 

Testo Type-K Globe Thermometer – This is a $500 temp probe that should work with any Type-K logger including those from Thermoworks, Onset, Testo and others. 

DIY – You can make one yourself for a bit less money. A ping pong ball won’t work so sell in a sauna and in particular anywhere near the stove so the best alternative is to use a hollow copper sphere. A 2” hollow copper float can work well. Insert a temp probe, such as a PTFE high temp, through the 1/4-20 thread in to the center (as close to center as possible is critical) of the sphere and then glue in place with a high melt point glue. 


Thermometers – IR

IR Thermometers measure the temperature of a solid surface. They DO NOT measure or indicate air temperature. Surface temperatures can be affected by convective, conductive and radiant heat. So, an IR thermometer is not necessarily a good way to determine air temperature. If there is very little radiant heat from the heater then the surface temperature of a bench should be about the same as the air temp if the sauna has been at the same temp for a while, perhaps 30 minutes. However, if the heater is giving off a lot of radiant heat then the surface temperature of a bench could be much higher than the ambient air temperature. A bench surface being 20°c (36°f) warmer than ambient air temp is not unusual. Ideally though there should be very little measurable radiant heat at the benches so the surface temp of the benches should be about the same as the ambient air temp. 

Walls are trickier because they not only gain heat via convection and radiation (and perhaps a tiny bit via conduction) but also loose heat via conduction to outside. Wall temperatures will almost always be somewhat colder than air temp. How much colder depends primarily on insulation and outside air temp.


Hygrometers – Analog

Analog Dial Hygrometers are inaccurate and realistically worthless. This is largely a nature of the beast thing more than any kind of manufacturing problem. Interesting to have on the wall as decoration but not really of any use.

A wet-bulb + dry-bulb (psychrometer) along with a chart to calculate humidity should be fairly accurate though.


Hygrometers – Digital

Digital Hygrometers can be quite accurate but this accuracy can come with considerable cost at sauna temps. Our MAC 125 works well for measuring moisture in a sauna but costs about $3k.

A digital hygrometer with an external port that you can use a tube with might be a good solution. Note that you’ll need a pump if the hygrometer does not have one. Also be careful of the head pressure limits of internal pumps as some are only good for 24” or less of tube and do not work well with the longer tube needed in a sauna. Also pay attention to max sample temperature and any condensation on the inside of the tube.

Some multi-sensor devices and IAQ monitors have somewhat accurate digital hygrometers but these all need to be kept in lower temps below the Löyly cavity, its convective loop and thus the steam (image below) so are pretty much useless.

However, if your foot bench is up in the löyly cavity (well above the top of the stones and/or cold zone) and has good air gaps and no skirt then there’s an option. The Ruuvi Tag Pro may be a really bad thermometer but it’s hygrometer will at least work at foot bench temps so placing a Tag just above the foot bench is an option so long as it’s in the convective loop. (note: our Ruuvi Tags appeared to work well for a while but now no longer provide reliable moisture information. Hopefully this is correctable. More to come.)



Professional Multi Sensor Devices (CO2, PM, etc.)

Most multi sensor devices include temp and humidity (and ideally barometric pressure) as these are necessary for reporting accurate CO2 measurements. Note that the device itself will not tolerate higher sauna temps so must be placed lower in the sauna. This will provide an indication of the levels of these elements for that location in the sauna but you’ll have to guesstimate/calculate for higher up where we and our breathing are in the löyly cavity. 

Gaslab ( CM Series – The CM-501 and CM-505 are useful for sauna. The 501 provides accurate temp, humidity and CO2 levels while the 505 adds a number of measures including PM which is valuable. Note that these are only good to 50°c so must be placed fairly low in the sauna when the sauna is at normal temps. However, you can also experiment with heating your sauna to 50°c and placing these up higher to get an idea of humidity, CO2 and PM up higher. These measurements will not be the same as at temp measurements but can still be useful.

TSI 7585 & 7545 – These are higher quality devices that are a bit more accurate than the 501/505 but also come with higher price tags of about $2k. These also must be kept below 45°c so well below the foot bench of most saunas. These are also widely available for rent. Be careful about heat though as if the logs indicate that it’s been in too hot of an environment the warranty (own or rental) is void. FWIW, we use a lot of TSI gear and have found it to be quite exceptional (and FWIW we’ve also learned that these appear to function well up to about 75°c (though that WILL void your warranty)).


Consumer / Home IAQ Monitors:

Many home IAQ monitors can be useful in a sauna to measure humidity, CO2, Particulate Matter and other stuff. The thing to keep in mind is that they are not designed for sauna temps so must remain where temps are lower which will affect accuracy for what’s happening higher up where we are.

Many consumer devices do not accurately compensate for temperature and barometric pressure with their CO2 measurements so that’s something to watch out for. There is some valuable review information at Southcoast AQMD.

IQAir AVP – Provides Humidity, CO2, PM2.5 and other data. Device is only rated to 40°c / 104°f so must be very near the floor of the sauna but should be at least a few inches above the floor if temps will allow to avoid false measurements. I’m a big fan of these for general home and outdoor air quality monitoring and they do work relatively well in saunas.

AWAIR Element – This is a fairly accurate monitor (avoid the v1 product that had errant PM measurements though). The only temp spec they provide is “-40 to 125°C (-40 to 257°F) / ±0.2°C” which I assume is only for the internal probe itself as producing a device with electronics that can function in those temps would be extremely expensive. 

Aranet 4 – Provides Temp, Humidity, Barometric Pressure and CO2. Works up to 50°c so should be placed below the foot bench in most saunas. One advantage of these is that they are small and battery powered so easy to carry around. We’ll often take one with us to restaurants, plane trips and other things where we might be in close contact with a lot of other people. Since CO2 is a good proxy for ventilation and pathogen levels we get a good idea how risky certain situations are.  

Good indoor ventilation for CO2, but more importantly for pathogens such as Covid, is below 700 ppm though even as high as 1,000 ppm is likely fairly safe. Above 1,000 is increasingly risky and like many others we’ll leave anywhere with CO2 levels in excess of about 1,900 ppm.

Foobot – Avoid. Great marketing but exceptionally poor accuracy. It does not have a CO2 sensor but rather calculates CO2 from other inaccurate data.

URAD A3 – Avoid. Lots of promise, disappointing results. These have so far proven to quite inaccurate and the company does not seem willing to fix them. 

Other Home AQ monitors such as AirThings may work as well.



If you want to monitor how much your electrical sauna (or any other device in your home) is costing there are a variety of options available.  The best is a system integrated in to the electrical panel by the manufacturer. While these have been around a while in Europe they are fairly new to the U.S. market but are becoming more available. 

IotaWatt – We looked at a number of aftermarket options and found IotaWatt to be the best for us. It requires a little more knowledge to setup but provides more useful data and accurate data than other alternatives. There is a valuable active community of IotaWatt users and developers to help.




CO2 Accuracy

An NDIR CO2 sensor counts the number of CO2 particles in a measured sample of air and from this is able to determine the number of CO2 particles per million air particles.  The trick is knowing the number of air particles in the sample as that varies by temperature and barometric pressure. If these are not accurately measured and used to calculate the correct number of air particles then the denominator in our calculation is off which throws the entire measurement off.

Here’s an example.  This is from a URAD A3 in our sauna.


The A3, a Gaslab 501 and an IQAir AVP (along with other instruments) were on a shelf below our foot bench that does not get above 50°c. Note above the charts for CO2 and Temperature. This was in a closed sauna that had not had any humans in it for at least 24 hrs nor during the test. The first thing to note is that the 501 and AVP, which are checked against a TSI 7545 for accuracy, both showed a CO2 level of about 410 throughout this test. 

During the first part of the test the URAD A3 indicated CO2 levels above 700 ppm, much higher than actual.  When the sauna began heating up the URAD A3 indicated declining CO2, exactly what would be expected as raw uncompensated data from the sensor.  As the sauna cooled the A3 indicated increasing CO2 levels which is what would be expected from an uncompensated raw measurement.

The most disappointing bit of this is that the manufacturer refused to correct it even after saying that they would and after we paid to ship it back to them.

The moral of the story is to be careful what you believe in marketing literature. While consumer products like the IQAir AVP have proven fairly accurate and reliable, many others like those from URAD and Foobot have proven extremely inaccurate.



Old discussion from Notes on Building…

Analog Thermometers – Many people say that accuracy is not important and you need a thermometer only for your own relative level. I don’t disagree with that and if you are the only one using your sauna and your thermometer works for you then that’s good enough. If you often have guests though, it’s good to have accurate temperature readings so that if you know that you are setting up your sauna to the temps they like. And for the data junkies among us… 🙂 

Digital Thermometers – The most accurate temp measurement will be a NIST calibrated digital thermometer with K style temp probes. A Thermadata 2 channel logger with FEP probes is about the least expensive option I’m aware of. Remember that the logger must be kept in the lower and cooler part of the sauna. I keep one or two loggers in our sauna almost all of the time for data collection. 

Hygrometers – Analog hygrometers are well known for being quite inaccurate. A digital hygrometer with appropriate tubes and pumps to accurately measure moisture in a sauna is expensive.