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> water has ONE boiling temperature – 100 Celcius This is not correct. The boiling point of water is a function of atmospheric pressure [1]. Weather changes th

by maho 13y ago

> water has ONE boiling temperature – 100 Celcius This is not correct. The boiling point of water is a function of atmospheric pressure [1]. Weather changes the atmospheric pressure by up to 50mbar, which translates to a couple degrees of variation in the boiling point. The pressure variation due to height is even more severe [2]. At an elevation of 3000m the atmospheric pressure is typically only 700mbar, which means that water boils at 90 degrees. How does the basic bimetallic strip know the outside pressure and the boiling-point-curve of water so that is knows when to stop the kettle? I don't think it knows anything about that. Instead I assume once the water starts boiling, the bubbling steam-"padding" around the heating element lowers the heat transfer rate from the heating element to the liquid. This means that the heating element will quickly get a lot hotter until it reaches the threshold temperature of the bimetallic switch. [1] http://www.engineeringtoolbox.com/boiling-point-water-d_926.html http://www.engineeringtoolbox.com/boiling-point-water-d_926.... [2] http://en.wikipedia.org/wiki/Atmospheric_pressure http://en.wikipedia.org/wiki/Atmospheric_pressure

dsuth 13y ago
Thermodynamics was never my favourite subject, but I suspect what happens is that the water boiling at the lower temperature created a higher pressure within the kettle (hence the screaming noise as pressure escapes the small nozzle), allowing the water to heat up to 100degC, and also the bimetallic strip.
- maxerickson 13y ago
  Somebody answered this elsewhere, there is a vent that directs steam past the strip. So it isn't heated directly by the water in the kettle, but by the steam that is given off after it boils.
asgard1024 13y ago
Interesting. I was also wondering about this; how the kettle knows what the boiling point is, since it cannot just measure temperature, because it would boil out the water in the mountains. I thought there is electronics which simply detects not only absolute temperature, but also change. Once the temperature stops changing and is around 100 degrees, it will stop.

by maho 13y ago

dsuth 13y ago

Thermodynamics was never my favourite subject, but I suspect what happens is that the water boiling at the lower temperature created a higher pressure within the kettle (hence the screaming noise as pressure escapes the small nozzle), allowing the water to heat up to 100degC, and also the bimetallic strip.

maxerickson 13y ago
Somebody answered this elsewhere, there is a vent that directs steam past the strip. So it isn't heated directly by the water in the kettle, but by the steam that is given off after it boils.

asgard1024 13y ago

Interesting. I was also wondering about this; how the kettle knows what the boiling point is, since it cannot just measure temperature, because it would boil out the water in the mountains. I thought there is electronics which simply detects not only absolute temperature, but also change. Once the temperature stops changing and is around 100 degrees, it will stop.