Temperature (internal sensor) 1 degree too low: Calibration possible?

Yup. I would have responded not at this level of detail, but that it's probably expecting a bit much of a watch - never mind how it is designed - to give you not only this degree of accuracy, but also repeatability. A bit like training for an Olympic level marathon and relying on a GPS watch for exact information on pace...

During swim, 1C is much important for me! I cant' swim for example at 30C in summer but i can swim at 29C, i can swim at 27C but i can't at 26C! Maybe none of you is a swimmer... It also important for me to know the exact temperature of a room leaving the watch on the table... To me is very important an accurate thermometer not everyone have the same needs.

Thanks! I couldn´t explain it better!
@tmk2: thanks for your good explanation! I don´t expect 100% exact temperature, but it should be possible to calibrate the internal sensor if it differs so much. I hoped for a trick to calibrate it, but apparently there is no way to do this. :(

Calibration would not improve things, because the factors I mentioned are not constant, so offsetting the thermometer by a constant value would not give you a usable reading anyway.

During swim, 1C is much important for me! I cant' swim for example at 30C in summer but i can swim at 29C, i can swim at 27C but i can't at 26C! Maybe none of you is a swimmer... It also important for me to know the exact temperature of a room leaving the watch on the table... To me is very important an accurate thermometer not everyone have the same needs.

Ok. Fine. You need an accurate temperature measurement. Got that.

But if we assume that the watch could measure the temperature with the required accuracy, how would you actually use it for this purpose?

One approach could be:

• Take off the watch
  
• Tie it to a piece of string
  
• Lower it into the water, tying the string to something, so the watch doesn't get lost.
  
• Wait 10-15 minutes for the temperature in the watch to equalize.
  
• Pull the watch out of the water and look at the temperature.
  
• Put on the watch.
  
• Start swimming.
  

But then wouldn't it be easier and faster to have a separate thermometer for this purpose?

And if you don't do the above steps or something similar, I wouldn't trust the watch temperature anyway.

Ok. Fine. You need an accurate temperature measurement. Got that.

But if we assume that the watch could measure the temperature with the required accuracy, how would you actually use it for this purpose?

One approach could be:

• Take off the watch
  
• Tie it to a piece of string
  
• Lower it into the water, tying the string to something, so the watch doesn't get lost.
  
• Wait 10-15 minutes for the temperature in the watch to equalize.
  
• Pull the watch out of the water and look at the temperature.
  
• Put on the watch.
  
• Start swimming.
  

But then wouldn't it be easier and faster to have a separate thermometer for this purpose?

And if you don't do the above steps or something similar, I wouldn't trust the watch temperature anyway.

No need to take off the watch from your arms, while swimming, when the watch is underwater, heat exchange between your body temperature and the watch it's negligible ( +0,1C max, i have done some tests)

No need to take off the watch from your arms, while swimming, when the watch is underwater, heat exchange between your body temperature and the watch it's negligible ( +0,1C max, i have done some tests)

Ok. That surprises me. But I haven't done any tests myself.

Anyway, you will still need to jump into the water and wait for the watch to adapt to the water temperature before you can make your decision about swimming today or not, based on the temperature reading from the watch.

Isn't that impractical, compared to throwing a thermometer into the water and then staying above water yourself while you wait for the thermometer to tell you if you should swim today?

I don't decide to swim or not by reading temperature from my Fenix 5X, but some days i felt too cold or too hot and i simply want to know if i'm sick or it's the water too hot/cold... :D

A quick check of ebay found me some floating pool thermometers for less than $5. Convenient and built-for-purpose, and far more convenient than jumping into the water with your watch to try to work out whether the water is too warm.

The sensor is designed to measure the internal temperature of the watch to calibrate the baromoter/altimeter. It is not designed to tell you the water temperature. And using it for a purpose it wasn't designed for is of course likely to yield inaccurate results.

But I am intrigued - you say the watch measures a degree too low, so obviously you are using another method to measure temperature. Why not just use that other method in the first place.

A quick check of ebay found me some floating pool thermometers for less than $5. Convenient and built-for-purpose, and far more convenient than jumping into the water with your watch to try to work out whether the water is too warm.

The sensor is designed to measure the internal temperature of the watch to calibrate the baromoter/altimeter. It is not designed to tell you the water temperature. And using it for a purpose it wasn't designed for is of course likely to yield inaccurate results.

But I am intrigued - you say the watch measures a degree too low, so obviously you are using another method to measure temperature. Why not just use that other method in the first place.

A quick check of ebay found me some analogue compass for less than 1€, some barometer and altimeter for less than 20€ and some gps for less than 50€ ecc... What's the point? I paid 700€ for this watch i don't want to carry with me some other stuff for my activity! My 100€ Casio watch has the ability to calibrate altimeter, barometer and thermometer, why is not possible to add this function on a 700€ watch? Some software release ago they add ability to calibrate barometer, maybe for someone is not important that barometer is +/- 2Mb off, but for someone it is. I think that in 10 minutes developers could add this with no problem...
I always compared watch temperature with official pool swim thermometer (when i swim i a pool that has it) and always my fenix 3 was spot on with it. So, when there was discrepancy between my fenix 5X and pool swim thermometer, i compared also with my weather station at home and i found that fenix 5's internal thermometer was off... Swim log reported temperature too, so all swim log will be wrong...