K
Klaus Kragelund
- Jan 1, 1970
- 0
Hi
I wrote an earlier post regarding a water sensor for a pump (see below
for original post):
The first idea dealed with using a capacitive sensor, but that has
several problems for example the problem of the sensor being affected
by incoming water because of an enclosure that is not hermetically
sealed.
Well - another idea is to use an totally sealed enclosure by using a
sensor that will work through the barrier of the stainless steel
housing. It works by measuring the dynamic thermal resistance of a NTC
resistor. The NTC is placed just on the other side of the enclosure
wall next to the water surrounding the pump. The NTC has a good thermal
contact 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 to
measure the temperature and more importantly the negative slope of the
temperature when the power is off. A high slope indicates that water is
present to cool off the NTC through the thermal resistance of the
NTC/steel connection, the steel itself and finally the water. In case
of no water the slope is not as steep.
Thats all fine because a sensor like this may not be as sensitive as
the capacitive one because the impedance levels are lower and in case
of water that penetrates the enclosure the effect of the volume
conductivity will be minimal.
But - can anyone tell me how to calculate the thermal resistance of the
enclosing water and the thermal resistance of the water/steel junction
to get an indication of how good this sort of sensor would work through
a steel enclosure?
Thanks
Klaus
(Sorry for the lenghty post)
OLD POST:
"I'm working on a pump that is suspended in water and I need to add
some
electronics to it (not more than 1 meter water level)
My initial thought was to house the electronics in a sealed
plastic/rubber enclosure, but someone pointed out that plastic/rubber
might over time absorbe water and render the electronics un-usable
Anyone got details of the behavour of plastic enclosures exposed to
long
periods of total suspension i water?"
I wrote an earlier post regarding a water sensor for a pump (see below
for original post):
The first idea dealed with using a capacitive sensor, but that has
several problems for example the problem of the sensor being affected
by incoming water because of an enclosure that is not hermetically
sealed.
Well - another idea is to use an totally sealed enclosure by using a
sensor that will work through the barrier of the stainless steel
housing. It works by measuring the dynamic thermal resistance of a NTC
resistor. The NTC is placed just on the other side of the enclosure
wall next to the water surrounding the pump. The NTC has a good thermal
contact 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 to
measure the temperature and more importantly the negative slope of the
temperature when the power is off. A high slope indicates that water is
present to cool off the NTC through the thermal resistance of the
NTC/steel connection, the steel itself and finally the water. In case
of no water the slope is not as steep.
Thats all fine because a sensor like this may not be as sensitive as
the capacitive one because the impedance levels are lower and in case
of water that penetrates the enclosure the effect of the volume
conductivity will be minimal.
But - can anyone tell me how to calculate the thermal resistance of the
enclosing water and the thermal resistance of the water/steel junction
to get an indication of how good this sort of sensor would work through
a steel enclosure?
Thanks
Klaus
(Sorry for the lenghty post)
OLD POST:
"I'm working on a pump that is suspended in water and I need to add
some
electronics to it (not more than 1 meter water level)
My initial thought was to house the electronics in a sealed
plastic/rubber enclosure, but someone pointed out that plastic/rubber
might over time absorbe water and render the electronics un-usable
Anyone got details of the behavour of plastic enclosures exposed to
long
periods of total suspension i water?"