>
>The design depends on there being a difference between the air temperature
>and the water temperature, which may or may not be the case in general. If
>there is any risk of them being at the same temperature you could address
>the problem by attaching a heater resistor to each sensor. Then the water
>would cool the resistor more than the air would so you would be measuring
>the difference in the specific heat of the media more than the temperature.
>Wouldn't have to be much heat to work, I wouldn't think.
>
The same technique could be used to monitor the flow in the filter as well..
As long as the water is flowing, it will be removing heat from the resistor.
If the flow stops, you should see a positive step in temperature.
A simular method is used in spirometry, called a hot wire anemometer, where
you put a dc voltage across a fine wire.. The wire heats up, and you measure
it's resistance. Air flow over the wire lowers it's temperature, changing it's
resistance, and the flow rate can be determined by the change in temperature,
which is proportional to it's resistance.
Steve
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