Detecting water level and pumping it out

[isis91]

Okay, so i'm all out of ideas. I live on a boat, which sadly leaks. Not very much, only about 50 litres per 24h. We collect this water in a 8 litre tank, which is the biggest one we can fit. We want this tank to empty itself. For this purpose we have installed an electric pump which runs from a relay. The relay is controlled by two switches, one in the top of the tank, and one at the bottom, When the top switch is closed, the relay activates and keeps itself opened until the bottom switch opens again. This runs on 12v. In theory. In practice, i make some hydrogen gas (i think) and consume about two amps.

The problem is that my "switches" are two copper wires that are supposed to be connected by the water when it rises, but this doesnt seem to work. When i put voltage on the wires, they consume ca 2 amps and create lots of bubbles. I can get ca 40 mA through the water and into the relay, but this is not enough to turn it on. Im considering amplifying the signal with transistors, but cant get hold of any here in cape verde. I also wouldnt know what kind of transistor Id need.

I have tried building switches that connect wires when a cork floats up in a tube, but they are unreliable, especially when the boat rocks.

Does anyone have an idea of how i can build a "no moving parts" switch or in another way solve this problem? Id be very grateful for any ideas!

 

[duke37]

Three things.

1.Stainless steel would corrode less than copper. What are metal guitar strings made of?

2. A transistor can have a current gain of 500 or so.
A Darlington transistor can have a gain of thousands.
A fet can have almost infinite current gain but they are a bit too delicate.

3.A float pushing a microswitch would do the job

What is the container made from, metal or plastic?
There are lots of transistors in dead TVs or radios.

 

[isis91]

That makes sense, of course. less corrosion would mean lower resistance over the gap aswell i assume. I will try with stainless electrodes and meassure how much current i can get through then. hopefully that will be enough to open the relay, but well see. Why do you ask about guitarr strings? i actually have a recently broken guitarrstring onboard so i could use that for electrodes.

That was exactly the kind of information that is very obvious, but still hard to find when you dont know that it exists. So even with copper wires i should be able to fix this with darlington transistors.

That would work to. Can microswitches usually be found in electronics?

Worst case scenario we will have to fix this in the caribbean where ill be able to buy more things, but hopefully trying different electrodes might do the trick, in combination with some transistors i might find in an old navtex receiver that we have onboard.

Thanks for your help.

 

[jpanhalt]

How about using a float switch for when to start. Using a few volts and high current will, as you have noticed, cause problems. Switching to stainless and using AC will help avoid those problems. Also, going to low voltage, e.g., 0.1 volt DC and using a comparator to drive the relay may stop the electrolysis too.

As for when to stop the pump, you could monitor current to the pump motor. Presumably, it is a submersed (self-priming), centrifugal pump, and when dry should draw less current. If DC, you can measure the voltage across a low-value resistor and use a comparator to drive the motor relay. Depending on what parts you have available, the comparator can drive a transistor to drive the relay.

John

 

[OLIVE2222]

Hi,

Because of the boat rocks you can may be thing differently. Wht don't start the pump on regular basis (let's say 5' a hour) and check the water flow at the exhaust pipe with something like this http://www.adafruit.com/blog/2012/05/09/new-product-liquid-flow-meter-plastic-12-npt-threaded/

Still need to think a bit to avoid running the pump "dry"

Olivier

 

[isis91]

That would propably work aswell, but its not ideal because the leakage differs with rudder usage. Thus when we are in port, we sometimes dont leak at all. Its also the problem with not being able to order anything online as we are constantly moving. Its still not a bad idea though, to check the exhaust for the close signal. I will think on it.