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:

  • Temperature – Thermometer
  • Moisture / Relative Humidity – Hygrometer
  • CO2 – CO2 Meter
  • Particulate Matter – PM Meter

 

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. 

Similarly, North American heaters have high-temp sensors that can cause unnecessary problems. I’ve a collection of photos of solutions 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 endeavors.

 

Thermometers – Analog

For most people a simple analog 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 might be 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. 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.

ThermadataExample01

 

 

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 (they are actually the worst digital thermometer I’ve ever 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.

 

Hygrometers – Analog

Analog Dial Hygrometers are notoriously 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 but not really of any use.

 

Hygrometers – Digital

Digital Hygrometers can be quite accurate but this accuracy can come with considerable cost, especially as air temps increase. 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 longer tube. 

 

Multi Devices (CO2, PM, etc.)

Most multi sensor devices include temp and humidity 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 accuracy is questionable as what CO2, PM and other elements are where we are breathing is likely quite different and higher than down lower in the sauna.

Gaslab (CO2Meter.com) 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 for sauna. Note that these are only good to 50°c so must be placed fairly low in the sauna. While not ideal this will provide somewhat accurate humidity and CO2 levels.

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.

 

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 somewhat depending on ventilation and air currents within the sauna.

IQAir AVP – Provides Humidity, CO2, PM1.0, PM2.5 and PM10 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 to avoid false measurements. I’m a big fan of these for general home and outdoor air quality monitoring and they work relatively well in saunas.

AWAIR Element – This is a fairly accurate monitor (avoid the v1 product 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. 

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.

 

Electricity

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. 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 more accurate data.

IotaWatt – 

 

 

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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.