The commercial vacuum switches typically used on the suction side of a centrifugal water pump to detect loss of prime are extraordinarily expensive ($70) given that they appear to be nothing more than a plastic diaphragm that bows inward under negative pressure which in turn causes a microswitch to activate.
My requirements are pretty basic. This is for chlorinated salty water therefore the diaphragm or bellows arrangement needs to be non-metallic. The max negative pressure is 1 Atm and I would guess that the max positive pressure due to water hammer would be 2 Atm.
I only have hand tools so a fancy bellow arrangement isn't possible. I could imagine a disk arrangement like a hollow hockey puck where the faces deflect inward. For the actual sensing I plan to use a magnet stuck to one of the faces and an analog hall sensor to measure the reduction in field strength as the magnet moves away under vacuum. I have some 0.5 mm fiberglass sheet that could possibly act as the diaphragm.
Does this sound like a workable solution? Has anyone done anything similar?
Have you considered repurposing a water level switch from a clothing washer, it should work about exactly as you want. Mount it so the liquid cannot drain into it.
gilshultz:
Have you considered repurposing a water level switch from a clothing washer, it should work about exactly as you want. Mount it so the liquid cannot drain into it.
They are only designed to cope with a fraction of an Atm of pressure and work by sensing positive pressure as the water rises in a tube compressing the air above.
mikb55:
The commercial vacuum switches typically used on the suction side of a centrifugal water pump to detect loss of prime are extraordinarily expensive ($70) given that they appear to be nothing more than a plastic diaphragm that bows inward under negative pressure which in turn causes a microswitch to activate.
My requirements are pretty basic. This is for chlorinated salty water therefore the diaphragm or bellows arrangement needs to be non-metallic. The max negative pressure is 1 Atm and I would guess that the max positive pressure due to water hammer would be 2 Atm.
I only have hand tools so a fancy bellow arrangement isn't possible. I could imagine a disk arrangement like a hollow hockey puck where the faces deflect inward. For the actual sensing I plan to use a magnet stuck to one of the faces and an analog hall sensor to measure the reduction in field strength as the magnet moves away under vacuum. I have some 0.5 mm fiberglass sheet that could possibly act as the diaphragm.
Does this sound like a workable solution? Has anyone done anything similar?
Perhaps you should consider that the suction side of a centrifugal water pump will still have some vacuum on the input side if the pump is turning. May I suggest you monitor the pressure on the output side as it will only exist while water is actually being pumped.
Paul
Yes, washing machines are good sources of solenod valves and pressure/vacuum switches.
Hardly worth the effort of making one.
Dig around for airline equipment suppliers.
I do know that some pressure and vacuum swithes are protected by hydrophobic membranes if you don't want liquids getting in contact with the switch mechanism.
Another homemade solution might be a commercial vacuum gauge with a very small magnet on the idicator I use one on a pressure gauge together with a reed switch.
Cheap vacuum gauges are made from copper and brass which will corrode in these conditions.
The professionals use gauges made from expensive Hastelloy, or use 'gauge savers' which are probably the same thing as the hydrophobic membrane you mentioned. Again the cost is at the professional level, not the hobby level.
I also wonder about your thought processes. Your vacuum/pressure switch/sensor does not need to be in contact with your solution. Let gravity help you. Put a Tee in the line you are testing and add a vertical pipe perhaps 6 inches long and then attach your switch/gauge to the top of the pipe. The air in the pipe will keep the solution away from the sensor and you will still have the same pressure/vacuum at the top of the pipe.
Paul
You could use a semiconductor pressure transducer, they are not that expensive but again keep it out of the moisture. With that sensor you will know the level. NXP is a good place to start looking. There are a lot of different ones available.