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Q & A: Soap and water mixer

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Most recent answer: 10/22/2007
Q:
I AM INVENTING A SYSTEM THAT WILL MIX LIQUID SOAP WITH WATER BEFORE EXITING A FAUCET- OR WHATEVER CONTRAPTION I COME UP WITH- HOW CAN I SET IT UP SO THE SOAP IS PULLED OUT USING THE ENERGY CREATED BY RUNNING WATER TO FORM A VACUUM
- Zach (age 9)
oakdale elementary school, barberton, ohio, summit
A:
Hi Zach,

You actually have to do surprisingly little! Simply running a pipe containing liquid soap into a pipe containing the running water is enough (use a "tee"). But -- and here's the hard part, the liquid soap has to be at a higher pressure than the water in the pipe. Otherwise the water will push the soap back up into the soap container. A vacuum will not pull the soap -- rather, pressure elsewhere pushes the soap to fill a vacuum.

It appears as if the important features of the design are:

1) Making soap flow into the water when the water is flowing.

2) Keeping the soap from flowing when the water is shut off.

3) Keeping the water from pushing into the soap container.

It is easiest to build and refill the soap container if it is not under high water pressure all the time but is open to the air. You may want to mix the soap and water at a point where the water pressure is low, then, downstream of the valve used to turn the water on and off. Then problem number 2 comes up -- you may need two valves hooked together -- one for the soap and one for the water, or one downstream of the mixing point, to keep soap from running out when the water is off.

You can use gravity to your advantage to adjust the pressure of the soap at the mixing point. Putting the soap container higher will increase the pressure lower down.

You can restrict the rate at which the soap flows into the water by getting a "tee" with a very small side pipe, or by having an adjustable valve on the soap line before the mixing point. You can also get "one-way" valves which allow fluid to flow in one direction only, to prevent water from flowing back up into the soap container.

Good luck with your invention!

Tom.

(published on 10/22/2007)

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