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how to differentiate between water flow and air flow

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polarized

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circuit to detect water flow in motor pump?

Hi all,


I have an electric 12V pump intended for water pumping, but when the water tank is empty it pumps air through the pipe which is not good and there is a possibility that the electric pump can be damaged due to overheating if it works in this abnormal situation more then 2 minutes aproximately. Is it possible to detect the air flow and somehow differentiate between normal water flow and forbiden air flow by the usage of a simple sensor or some different method? I have constructed an electronic circuit which permits a programable maximum working time of the pump from the first moment it starts to draw current, but this method is not so intellegent, so I would like to differentiate between water flow and air flow.



Regards.
 

Boardstiff

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must admit this i have not done myself , but one possible way may be to monitor the current also

usually if you are pumping water or any fluid , if it then draws air their is an immediate sudden change both in the rate of flow and subsequently the current drawn by the motor , perhaps this may give you some ideas

sorry i cannot be any more help, but thinking hard!
 

carrotts

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Could you use two electrodes spaced apart. When water was flowing the contacts would make, when air flows there would be no path. You could even measure the capacitance, this would give a good idea of what was flowing. Try a google search on Rain Detector Circuit, I seem to remember a few examples out there that you could adapt
 

brmadhukar

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The best option is to measure the capacitance, but rain detector circuit is simple. Take care that the water is not used for non drinking circuit if rain detector circuit is used.

I do not know whether anyone has tried to measure the magnetic field. If used do let me know of the feasibility.
 

GrandAlf

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Would it not be possible to have a float switch near the water intake, like a pond pump?
 

sadat007

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U can use temperature sensor.
For pips the temp will change if the water start flow (shift to low for hot contries).
If you place the sensor where the mechanics that move the water. It becomes hot if no water and clod if water.
 

cruxader

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Depends on whether you can tolerate a short period of pumping some air (few seconds) and other factors, eg what sensors you have access to.

The most straightforward is to measure the current/power consumed by the pump. Water is heavier than air and thus will impose more load.

You did not state how far up is the pumping distance.

Assuming the height is sufficient, you should be able to detect a measureable difference between pumping water/air by either an in-line current sensor or just the voltage on the pump (if the difference is big enough).

When a motor is free running, it draws very little current and so its back-EMF is almost equal to the supply voltage. When the load is heavy, the motor slows down and its back-EMF drops. This creates a voltage drop across both the pump internal resistance/impedance as well as the resistance/impedance of the power supply. If the power supply is a simple one with no regulation, you may experience significant enough voltage swing to just detect the voltage at the pump terminals.

For example, lets say both the pump and power supply have equal internal resistance of 10 ohms. And the power supply ideal voltage source is 12V. When the motor is free running, insignificant current flows so the voltage at the motor terminals will be close to 12V. When the pump is loaded, let say the speed drop significantly so the back-EMF is now 10V. The 2V voltage drop will be sl=plit equally between both internal resistance and you should see 11V at the motor terminals!

In other wards, just put a voltmeter on the terminals and record voltage readings when pump is pumping water/air. If you can see significant diiference in voltages, you can use the voltage method. If not, plug in an amp meter instead and repeat experiment. If the amp meter shows detectable changes, use a current sensor.
 

GrandAlf

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I Agree with boardstiff/cruxader, I think that there will be a significant change in current consumption when pumping water. If it is only a low current current pump, you may be able to use a series low volt relay, make sure insulation is ok if this is a mains pump. Also if AC you will need a rectifier and possibly smoothing cap. You would need to experiment a bit, and monitor the coil voltage. Could also use a series resistor with opto across it. Also current transformer if AC.
 

crono

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hi,

if there is some resistance in the flow of liquid you can try to use a pressure sensor at pump output.
 

pico

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If the intake water has pressure, you can use a pressure switch (like the one they use for home use RO water system)
 

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