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Water sensor - dynamic thermal resistance sensor

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?"

Reply to
Klaus Kragelund
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No offense intended, but that\'s all total bullshit.

What is it that you want to do?  Start the pump when the water falls
to a certain level and then stop it when the water rises to another?

Or something altogether different?

Tell us what you want to do without telling us how smart you are 
(but how you can\'t quite work out a solution to your problem) and
maybe you\'ll get some good answers.

Grrr...
Reply to
John Fields

snip

Calm down Mr Fields. I've just finished my water divining stick with USB/Wifi interface. I'll put the extra software in that will handle the this, no more than 100K of code extra. The Leyline detection code will have to go though, it was a bit buggy around the summer solstice anyway :(

martin

Reply to
martin griffith

The system NTC/stainless steel tube is too slow to detect no-water when the pump runs dry. This condition should be detected in miliseconds.

Rene

Reply to
Rene Tschaggelar

LOL, that's funny!

--
 Thanks,
    - Win
Reply to
Winfield Hill

Hi. :-)

...

...

Well, if it was me, I wouldn't rely on the thermal impedance of water, unless it's chilled and/or flowing (like in a hot-wire anemometer) - I'd think the thermal conductivity of the steel would swamp out any effect from presence or absense of water, but I'm guessing.

What I might do is use ultrasonics. Get a small transducer, like they'd use in an ultrasonic thickness gauge, and I'd think that the difference in acoustic impedance between air and water would be much easier to detect. But, like I said, it's just one of my crazy ideas. (I have a little passing familiarity because I once got a job at RS by fixing an ultrasonic "fish finder" that the other two techs had been scratching their head over for about a month. There was a significantly higher return echo when the transducer was in water vs. in air. :-) )

Good Luck! Rich

Reply to
Rich Grise

Hello Rene,

Tell me about it. The booster pump for our pool sweep committed suicide within a minute when something in front failed and it got no water. I could repair it but that was a messy one-hour job.

Best would be to alert when cavitation or air bubbles occur. Ultrasound, inductive, whatever works best. Cavitation can clearly be heard so acoustic sensing might work as well. Ours sounded like a gear box that ran out of oil, very loud.

Another feature that would have saved the pump from seizing would be a thermal sensor. When that happened I ran down to the pool house and the enclosure of the pump (not motor) had gotten too hot to touch. That may be something for Klaus to try.

Regards, Joerg

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Reply to
Joerg

Joerg, one approach would be a precision current monitor. When the pump runs dry, the torque is much lower and this should be detectable.

The problem with these pumps is :

1) they are using slide bearings not ball bearings. And the slide bearing runs on a water film, not oil. 2) they may depend on the case being watercooled.

If they could cost more than a few dollars, they could be made to run dry without problem.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Reply to
Rene Tschaggelar

Hello Rene,

That's dicey. I tried that with ours. The difference isn't that great unless you are running a pump motor at almost full torque which tends to overheat it in summer.

The motors are usually ball bearing types.

Except for the gasket which is two ceramic disks spring loaded to hold them tight.

Regards, Joerg

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Reply to
Joerg

Thanks - and we do actually have a thermal sensor inside the motor (an UL demand due to the new rules that an a software system needs a redundant satefy system - total crap)

Regards

Klaus

Reply to
Klaus Kragelund

We have this sort of current sensor designed and working inhouse, but we are attempting other more cheaper methods

Thanks

Klaus

Reply to
Klaus Kragelund

I'm not so smart - otherwise I would be posting to this NG

No bullshit Q:

Need a waterlevel detector for a pump - must cost nothing, and no fancy electronics must be used (costly), must be slim (placed vertically along side the metallic chassis of the pump)

Thanks

Klaus

Reply to
Klaus Kragelund

[snip]

I take it that pump is "submersible", totally under the water in normal operation?

Couldn't you simply measure output pressure?

Around here we periodically drain our swimming pools (to get rid of mineral concentration due to evaporation). A submersible pump is used, which turns off automatically when the pool is empty. Wonder what they use?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

I guess it's "cheaper" to replace burned out motors ?:-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

The water here is VERY HARD, so every few years it's necessary to do a draining, or the calcium becomes uncontrollable.

Up where you live the evaporation is probably quite low compared to here.

(My reef tank, INSIDE, loses ~2-3 gallons per day. I'm contemplating giving it its own RO system ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

From about 100 miles away where the lakes, that catch the rainwater, are located.

There is no significant groundwater until about 15 miles from my location.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

I have a float valve. Remember the "hysteresis" post back a year or so when it partially failed ?:-)

See "WaterFloatValveHysteresis.jpg" on the S.E.D/Schematics page of my website.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Qualified pool service people seem hard to keep. In the 11 years I've been in this house I'd guess we've had at least 7 different pool service people; though the latest, a husband-wife team, seems to work quite well... she does the ordinary maintenance, sweep-down and chemical balance... he does the equipment repairs/replacements... lost a DE filter last month, bottom corroded out :-(

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

What about a pressure switch on the output side?

Reply to
jtaylor

Actually I would guess that just over 1500' would hit water. We're not all that far from the Sea of Cortez.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

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