Am 07.07.2013 15:49, schrieb Peter Hollenbeck:
> I don't really understand why that works,
>
Usually, there is air between the heater and heat sensor. This will heat
the sensor, but only marginally as
* heat conduction through air is nearly always insignificant
* heat convection through air is insignificant if the air flow is
neglible or laminar (small gap between the two -> laminar flow at most)
* heat radiation is insignificant at room temperature unless your
heatsink is something really cold (e.g. night sky)
So the heat sensor will report room temperature if there's air between
it and the heater.
If there's a water drop between the heater and the sensor, heat
conduction is about 25 times better than with air, allowing the heater
to effectively heat the sensor. The sensor will report the higher
temperature almost immediately.
That's also a good method checking the presence of non-polar liquids
like gasoline or kerosene where moisture sensors depending on electrical
conductivity will fail.
---
Another possible method working with both water and non-polar liquids is
checking the dielectrical permittivity, same method as a capacitive
touchscreen uses. There are really cheap ICs for that purpose out there,
but they all have to be calibrated carefully and in your installation, I
doubt the calibration data needed is stable enough in the long run.
Kind regards
Jan
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