How violently can the flaps be moved by water flow turning on or off? It could be the magnet was taking a lot of mechanical shocks over time.
Good question! Let's see... the arc of travel is 90 degrees, radius of the arc is 20mm, and the weight on the end is about 6g. THe pipe is a little bigger than an inch, let's say an inch and a half - which we'll round to 40mm for arguments sake.
The flap has an area of around a square inch so let's say around 625 mm^2. Finally, the pump has a flow rate of - well it varies. Let's say 1600 litres/hour.
Please forgive me if I do something dumb! Fluid dynamics is NOT my strong point. It is my weak point. In terms of proficiency, it's right up there with "dancing" for me. I suck at it.
Mass = 6g
Arc = 90 degrees
Arc Radius = 20mm
Flap area = 625 mm^2
Pipe diameter = 40mm
Pipe area≈ 1250 mm^2
Pipe - Flap area = 625 mm^2 - huh, that's convenient.
Flow rate = 1600 L/h ≈ 0.4L / second
Flow velocity = 318 mm/s
If we treat the flap as a cube, the drag coefficient is approximately 1.05 (in a fluid with reynolds number of approx 10^4)
The drag coefficient, Cd is equal to 2Fd/p.u^2.A
Fd = drag force
p=mass density of the fluid
u=flow velocity
A=area
Rearranging that gives us Fd=(Cd.pu^2.A)/2
Cd=1.05, p=1kg/m^3, u=318 mm/s, Area = 625 mm^2
So 0.5 * 1.05 * 1g/mm^3 *0.318^2 m/s * 0.000625m^2 = 0.03318 Newtons.
That... doesn't seem like a lot... uh-oh... let's run with it anyway. Maybe I should have used bernoulli's principle applied to the annulusbetween the flap and the pipe walls instead...No that still wouldn't work... oh well.
As per f=ma, a=f/m: 0.03318 /0.006 = 5.53 m/s^2
The distance (displacement), U, traveled by the flap is ≈ 31mm (pi*diameter / 4 - it's actually equal to 10pi exactly
)
If you plug that into 1/2at^2+ut-s=0, you get a time of 0.106 seconds for it to close (by which I mean the contact is closing - the flap is open in this case)
After accelerating at 5.53m/s^2 for 0.106 seconds it will be travelling at 0.586 m/s; it will have kinetic energy, E, equal to 1/2mv^2 - so 1/2 0.006*0.586^2 = 0.00103J if the above numbers hold
Work = mass * gravity * height, so that would be the equivalent of dropping a 2p coin (~6g) from a height of 1.75cm
Obviously I haven't accounted for the drag changing as the angle-of-attack of the flap changes, which will of course generate lift: I am effectively calculating how much force the flap will experience if you dropped it into the flow (and assuming that no other forces act upon it as it strikes a surface - which obviously neglects lift and water hammer from the flow starting).
It doesn't seem like a great deal of energy/force - but it's still going to be repeated perhaps hundreds of times - every time the pump switches on - minimum. If the flow is turbulent it may oscillate in the flow and repeatedly strike the side - possibly many times a second.
In fact, let's assume they run it for 6 months straight - the flow will start/stop four or five times day (probably more - so that's 900 impacts a year... imagine dropping that coin on your finger 900 times from a height of ~2cm... hmmm - that might be enough to make it lose it's moxy!
It
probably ismay be
entirely wronginaccurate, but that was fun anyway!
I do wonder though why both magnets don't have equal "wear" on them - since they are on the same piece of plumbing (if water comes in the inlet, it must go out the outlet: one switch each...). In fact, I might have already answered that in my conjecture regarding turbulent flow - the inlet or outlet might be more turbulent and cause the flap to bounce up and down in the flow - while the opposite pipe may be less turbulent!
I find it's the reed switch getting magnetized and sensitivity drops.
I take an AC degaussing coil to the reed switch and nearby steel parts, demagnetized those parts and that restores things.
But when I magnetize screwdrivers, rubbing them on HDD magnets, the screwdrivers don't stay magnetized for more than a few months 
I considered that but swapping the reed switches around that the magnet was at fault
For magnets M1 and M2, and Switches S1 and S2:
| S1 | S2 |
| M1 | False | False |
| M2 | True | True |
If it were a failed reed switch I wouldn't bat an eyelid (though I wouldn't have thought to degauss it - I'd just have assumed the contacts were corroded/stuck!).
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