Failed...magnet? Never seen that one before! - Page 1
Forum section: electronics / general

author: cprobertson1
author_rank: Frequent Contributor
timestamp: July 25, 2018, 07:39:56 am
content: I thought I'd share this as it's a failure mode I've never really seen before!So a gent at work recently gave me some flow switches from his spa which he says weren't working.They are super-simple - just a flap with a magnet on it - and as the water pushes past it, the flap moves against the side, where it triggers a liddle reed switch.The spa has two flow switches - one for the inflow and the outflow: and one wasn't working.So I grabbed the multimeter and tested away, and sure enough, one of them didn't trigger... so I replaced the reed switch and... huh - still didn't work. Swapped the reed switches with the working sensor, and it still didn't work - so then I swapped the magnets and, there it is! Works a treat!So I removed the two flaps, and sure enough, one of them is about half as magnetic as the other (according to my very cheap and nasty magnetometer - 200uT for the good one, <100uT for the bad one)The magnet appears to either be molded into the plastic (as a ferrite cement one would assume) or the plastic molded around the magnet - so it's possible they just haven't charged the magnet properly.Still, it struck me as an interesting failure mode - certainly not one I've encountered before (but then again, this is the first failed magnetic switch I've ever encountered!)Is this a common failure mode with magnetic switches that I was just unaware of? Anybody else encountered similar issues?

author: BradC
author_rank: Super Contributor
timestamp: July 25, 2018, 07:46:35 am
content: Quote from: cprobertson1 on July 25, 2018, 07:39:56 amI've had a few magnets fail when a fault has exposed them to high temperatures. I haven't ever seen any just demagnetize though.

author: firewalker
author_rank: Super Contributor
timestamp: July 25, 2018, 08:24:05 am
content: I had magnetic switches fail. All of them was magnetized steel magnets and not real ceramic ones. They didn;t suddenly fail. Over time the magnetic field gave up.Alexander.

author: borghese
author_rank: Regular Contributor
timestamp: July 25, 2018, 08:47:55 am
content: Permanent magnets lose their magnetism when subjected to high or low temperature; even repeated blows or falls are fatal

author: cprobertson1
author_rank: Frequent Contributor
timestamp: July 25, 2018, 09:03:57 am
content: Quote from: borghese on July 25, 2018, 08:47:55 amYou might be onto something there! I wonder if this was on the hot-water (output) side pipe...I've seen small neodymium magnets fail (over the space of ten years) - but those were all very small (<5 x 1mm). Not sure how long these magnets have been in use.Quote from: firewalker on July 25, 2018, 08:24:05 amI suspect this is magnetised steel as well - though it looks like it was injected into a void in the plastic and then magnetised, as the plastic completely encases it save for a orifice in the upper surface, which is where I assume it was injected into.Hopefully should be an easy fix though (once I get a magnet). I'll just 3D print a little flap, glue a magnet onto it and then some enamel varnish should stop it corroding too quickly.Alternatively, they are £>25 to buy a new one! impressive!

author: amyk
author_rank: Super Contributor
timestamp: July 25, 2018, 11:31:14 am
content: How hot is the hot water? The Curie temperature for common magnets is in the several hundred degrees C range... a lot of other things would have melted and burnt long before they get hot enough to demagnetise.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 25, 2018, 11:37:07 am
content: I had plenty of problems with failed magnets of all sorts over the years.In one case the Curie point as per datasheet was at 150 degree C and these magnets failed at 80 degree CIt caused a very expensive recall for the manufacturer.Temperature is a huge problem for many type of magnets!!!What I learned...- Don't rely and believe datasheets!- Test the magnets yourself!

author: Brumby
author_rank: Unknown
timestamp: July 25, 2018, 11:41:40 am
content: Don't forget the effect of AC EM fields... built in degaussing!

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: July 25, 2018, 01:33:51 pm
content: What if it cracks in two? Now you have an air gap...

author: edy
author_rank: Super Contributor
timestamp: July 25, 2018, 03:05:17 pm
content: I had one of these flow switch/sensors "fail" on me but it wasn't a magnet issue. It was installed vertically so that gravity would normally pull the magnet or some ferrous core material down, and when fluid flowed through the sensor it would carry the core up and thereby activate or provide a change in the signal to the microprocessor. There was a little rubber "tip" like a stopper to prevent backflow against the sensor when the pump wasn't flowing and after a while it started degrading and became "sticky". That was enough to get it to stick long enough in position to cause the microprocessor to think the flow failed when it started the pump... and would shoot off an error code. The unit needed to make sure that detergent fluid was properly dispensed into the wash cycle at the right time, and the pump was a peristaltic type so over time the tubing that ran through the pump needed replacement.Anyways, the issue was never adequately resolved by the company, they hid it from users and wouldn't admit anything wrong or sell you the parts but was part of some internal bulletin which eventually surfaced (see PDF below). When I told them of the issue they did not want to fix it no charge, it was 6 months warranty only and this happened around year 2. The way I fixed it was after opening up the sensor, I removed that sticky rubber tip and replaced it with a light-cured resin material, the hell with the backflow issue (it wasn't an issue anyways). The sensor no longer "stuck", has been working fine ever since. I had to change the peristaltic tubing once, just bought a length for $30 and did it myself (the service technicians and suppliers otherwise charge hundreds $$$).Personally I believe this problem stemmed from the fact that the FCS-X008A - Gentech flow sensor was designed to handle WATER ONLY, and this company was using it in an application that used a basic detergent (see PDF link below) that probably degraded the rubber and made it become "sticky". Either that or residue from the detergent built up on the rubber, but there was not way to remove it and the surface was definitely not healthy looking!Flow switch bulletin, sensor (no longer being sold):https://www.scicancanada.ca/site/scican/assets/pdf/04-TSB-171_Hydrim_L110wd_Flow_Switch_and_Tube_Modifications.pdfhttp://www.te.com/usa-en/products/sensors/position-sensors/position-switch-sensors.html?tab=pgp-storyDetergent formulation:https://www.scicanusa.com/site/scicanusa/assets/pdf/hip_hydrim_eng_-_eu_us_20151211.pdf

author: cprobertson1
author_rank: Frequent Contributor
timestamp: July 25, 2018, 09:09:02 pm
content: Quote from: edy on July 25, 2018, 03:05:17 pmThanks! That's my bedtime reading sorted!These are at least installed in a semi-sensible way - gravity holds a flap down - flow pushes it upwards - magnet triggers the reed switch - super simple - nothing to go wrong (or so I thought!)Well, apart from fouling of the flap of course - or the reed switch sticking... it was actually a bit of a fluke that I even considered the possibility of the magnet being the problem!Quote from: HighVoltage on July 25, 2018, 11:37:07 amWhoops! I never said where it was from! This is from a Lay-Z Spa hot-tub; temperatures are roughly "human body temp" - perfect for bacterial growth (which is why you chloraminate the heck out of them)! This one was spotless though - no biofouling at all - I like itThere are two of these in series - one for the outlet, the other for inlet - system checks that they are down to start with and then that they are up once the flow starts (can't just short them out! Sneaky)Anyway, I doubt 37-ish degrees C is going ruin the magnetsthatquickly - is there a chance it would accelerate the deterioration of the field or would it be negligible at these temps?

author: james_s
author_rank: Super Contributor
timestamp: July 26, 2018, 04:56:38 am
content: I've heard of this happening with motors in vintage model train locomotives and slot cars and such. Some people build devices to remagnetize them.

author: Cyberdragon
author_rank: Super Contributor
timestamp: July 26, 2018, 05:19:25 pm
content: Quote from: james_s on July 26, 2018, 04:56:38 amFor this (a simple flat magnet), it would be as simple as placing it on top of a strong electromagnet for a few hours, maybe overnight.

author: Gyro
author_rank: Super Contributor
timestamp: July 26, 2018, 05:52:00 pm
content: Quote from: james_s on July 26, 2018, 04:56:38 amIt is (was?) universal practice to magnetize AlNiCo magnets after assembly, certainly for loudspeakers and probably for model trains and cars etc. They would certainly loose strength if the magnetic circuit was disassembled. The same process was used with anisotropic ferrite magnets too. It makes assembly much easier, because nothing 'sticks'. I don't know if the same procedure is used with Neodymium though.Magnets are funny things, Alnico for instance will operate happily when red hot, but will demagnetize quite easily by rough handling. Neodymium is pretty much the reverse.Lest we forget, one of the earliest ways of making a magnet was to line up a piece of iron bar with the Earth's magnetic field and wallop it with a hammer!

author: Nusa
author_rank: Super Contributor
timestamp: July 26, 2018, 05:58:20 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: July 26, 2018, 07:20:09 pm
content: 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

author: metrologist
author_rank: Super Contributor
timestamp: July 26, 2018, 08:15:36 pm
content: Quote from: BradC on July 25, 2018, 07:46:35 amYeah, I ruined a nice magnet trying to bake some paint onto it. Just a little too hot and it lost its motivation.

author: cprobertson1
author_rank: Frequent Contributor
timestamp: July 27, 2018, 10:03:54 am
content: Quote from: Nusa on July 26, 2018, 05:58:20 pmGood 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 = 6gArc = 90 degreesArc Radius = 20mmFlap area = 625 mm^2Pipe diameter = 40mmPipe area≈ 1250 mm^2Pipe - Flap area = 625 mm^2 - huh, that's convenient.Flow rate = 1600 L/h ≈ 0.4L / secondFlow velocity = 318 mm/sIf 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.AFd = drag forcep=mass density of the fluidu=flow velocityA=areaRearranging that gives us Fd=(Cd.pu^2.A)/2Cd=1.05, p=1kg/m^3, u=318 mm/s, Area = 625 mm^2So 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^2The 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 holdWork = mass * gravity * height, so that would be the equivalent of dropping a 2p coin (~6g) from a height of 1.75cmObviously 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!Itprobably ismay beentirely 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!Quote from: floobydust on July 26, 2018, 07:20:09 pmI considered that but swapping the reed switches around that the magnet was at faultFor magnets M1 and M2, and Switches S1 and S2:S1S2M1FalseFalseM2TrueTrueIf 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!).