Water sensor - dynamic thermal resistance sensor

Klaus Kragelund · 2005-10-05T17:53:21+00:00

HiI wrote an earlier post regarding a water sensor for a pump (see belowfor original post):The first idea dealed with using a capacitive sensor, but that hasseveral problems for example the problem of the sensor being affectedby incoming water because of an enclosure that is not hermeticallysealed.Well - another idea is to use an totally sealed enclosure by using asensor that will work through the barrier of the stainless steelhousing. It works by measuring the dynamic thermal resistance of a NTCresistor. The NTC is placed just on the other side of the enclosurewall next to the water surrounding the pump. The NTC has a good thermalcontact to the stainless steel.The sensor works by dissipating a known power in the NTC resistor.After some time the power is turned off and the NTC resistor is used tomeasure the temperature and more importantly the negative slope of thetemperature when the power is off. A high slope indicates that water ispresent to cool off the NTC through the thermal resistance of theNTC/steel connection, the steel itself and finally the water. In caseof no water the slope is not as steep.Thats all fine because a sensor like this may not be as sensitive asthe capacitive one because the impedance levels are lower and in caseof water that penetrates the enclosure the effect of the volumeconductivity will be minimal.But - can anyone tell me how to calculate the thermal resistance of theenclosing water and the thermal resistance of the water/steel junctionto get an indication of how good this sort of sensor would work througha steel enclosure?ThanksKlaus(Sorry for the lenghty post)OLD POST:"I'm working on a pump that is suspended in water and I need to addsomeelectronics to it (not more than 1 meter water level)My initial thought was to house the electronics in a sealedplastic/rubber enclosure, but someone pointed out that plastic/rubbermight over time absorbe water and render the electronics un-usableAnyone got details of the behavour of plastic...

Oct 05, 2005 43 Replies

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keith 20 years ago

When the milkman gives the milk for free why marry the cow?

As a family story goes, during WWII, my dad told female coworkers that one of my brothers (a red head at the time) was the ice-man's. ...only later telling them that he was the ice man.

Keith

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Klaus Kragelund 20 years ago

The

Depending on the sensor offcourse - but if the circuitry is switchable (simple analog switch to toogle which sensor to use), then the cost of adding a second sensor is almost none

Regards

Klaus

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Klaus Kragelund 20 years ago

Yes, that might be an option - good idea :-)

Thanks

Klaus

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Klaus Vestergaard Kragelund 20 years ago

Yes, that might be an option - good idea :-)

Thanks

Klaus

PS Posting via the browser and google groups didn't present this previously send message correctly

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