I bought two thermometers, which exceeded expectation for a modest price. These two agreed within 1 degree F with one another and with an industrial temperature monitor. I was puzzled that the scales were quite different between the two, and at first thought that I got the last from one batch and the first from a new batch. But I saw two small tick marks on the glass at the same numerical values (but not the same positions) on the two. So, I am pretty sure that the glass instruments are given a two point calibration leading to customized scales, which is surprisingly fine for such modestly priced and rustically presented instruments. I with that the product descriptions gave technical specs, including accuracy and speed of response. The weakest quality is that the fluid height is only clear for a small range of viewing angle. I'm pretty sure that is inherent in this sort of glass tube.
The important issues for measuring sauna temperature go far beyond these particular thermometers, and it's a shame there's no clear authoritative discussion to be found. Speed of response is as important as accuracy, since a sauna session may be only 5 minutes and seldom more than 30 minutes (mine are 25 minutes). I'm not sure whether I get totally correct readings in that time frame.
Then there is a huge temperature variation from the top of the room to seat level. I am pretty sure that there is 2 or more degrees F difference between the top and bottom of the thermometers. I expect that the reading is essentially for the bulb temperature. I set my sauna for the maximum legal setting in US/Canada: 194F. The control sensor and my industrial temperature probe confirm that temperature about 2 inches from the ceiling. The liquid thermometer hung below the ceiling eventually passes 180F at bulb level. Liquid theremometer and another bimetal meter show only 160-170F at navel height above the bench, and 140F at the bench itself. I am worried that there is also a difference from one side of the room to the other, but I am not so sure of that.
In order to fiddle with ventilation and try to achieve greater heat at body level, I need clear information regarding the actual temperature. I intend to hang one thermometer as high as possible, and other with bulb around seated body level, but I will still be unclear how long to wait for the result of an adjustment.
Then there's the humidity. I am going to try to adapt one of the thermometers to wet bulb, but not sure how reliable that can be with a static instrument, since meteorological wet bulb is usually taken with a thermometer whirled rapidly on a sling, and is still reported to run 5F high.
I think that my rambling info is relevant to anyone who wishes to really measure sauna temperature. But the bottom line for these liquid thermometers is still that they are as accurate as you need, comparable to much more expensive instruments, and *probably* responding a bit faster than most more expensive instruments.