Trying to detect when a drain pipe, filled with water from a washing machine pump, starts to get backed up.

Since the water is being pumped through, a simple water detector is not enough. And since the water is being pumped and draining, but slowly (Too much water volume for the pipe in a small amount of time), I don't know if a float sensor will work (downward movement from pump, upward movement from backing up).

Also, the pipe is about 2.5" in diameter. The sensor should interface to a 3.3v or 5v level, or simply open drain, but I can figure it out from there if I need to level shift. It should be water tight to prevent shorts or anything (Though I don't know the proper safety method for this).

Is there any standard way of doing this? The microcontroller (a msp430) will not be submerged or anything, just the sensor.

Edit: Picture/Diagram as requested.
enter image description here
Setup is essentially exactly the same. Pumped water goes down "standpipe", as does the backflow up. My main access is only at the section marked "Indirect Connection, a 2~3" diameter pipe. The Waste Stack is probably 4~5" diameter. There is a sink that is in parallel with the washer machine, and water sometimes backflows out of there instead of the "indirect connection", as it is a few inches lower.

Ideally, the sensor could fit and work right above the trap in the diagram, without clogging the pipe itself. I might be able to add a t-pipe between the trap and waste pipe, but I want to avoid that if I can.

  • possible duplicate of Measure Water Flow in a Swimming Pool – NickHalden Dec 18 '13 at 22:53
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    @NickHalden I don't believe this is a duplicate. The other question was about measuring a continuous flow of water, this one is about water backing up in a standpipe. – Joe Hass Dec 18 '13 at 23:16
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    @Passerby A mechanical/architectural drawing of your drainage system would make this question clearer. There may be some important nuances. What kind of pump do you have? What are the height gradients? What's the caliber of the tubing? – Nick Alexeev Dec 19 '13 at 0:00
  • @JoeHass Ok, that's fine. I just wanted to point it out as a possibility. – NickHalden Dec 19 '13 at 20:10
  • Will sketch a drawing of the system today. Also, @NickHalden also, that other one required indirect measurement of total flow, from a large body. Here I have limited access to a pipe, and need to know when the forward movement and backup meet. It did have one good suggestion in it, but the situations are completely different. – Passerby Dec 20 '13 at 13:50
up vote 3 down vote accepted

I once had the exact same problem. My drain pipe was only just a little bit slower than the drain pump in the washer, so the water rose relatively slowly, but fell quickly when the machine was shut off for 5 seconds. My switch would activate several times on every drain cycle, and effectively managed the problem. (And yes, before building my gadget, I did try to unclog the drain itself, but as it ran some 20 feet across the basement, embedded in the floor slab, I didn't have much luck with that. I suspect that it simply hadn't been installed with enough slope, and fixing that would have required jackhammering it out and installing a new one.)

I used a float switch that came (IIRC) from an old dishwasher. The wires were kind of short, so I simply extended them by soldering and then heat-shrinking the joints. The switch has a square base with four mounting holes, so I fashioned a simple L-bracket to hold it upside down inside the top of the drain pipe, with the L-bracket simply taped/zip-tied to the outside of the pipe. This meant that the switch became normally-closed, and it opened if the water rose too high.

The control box plugged into the wall, and then the washing machine plugged into the box. It contains a 25-amp SSR and a small isolated 12VDC power brick, so the control circuit is completely isolated from the line power. The float switch triggers a 555 timer that cuts off power to the washing machine for 5 seconds each time it opens, which in my case was about right to allow the pipe to drain sufficiently. Obviously, this was an older machine with an electromechanical timer, which was unfazed by such treatment. I don't know if this would work with a modern microprocessor-based washing machine.

We used it for quite a while, until we moved out of that house. That was 20 years ago, but I still have the switch and the control box. If you're having trouble visualizing any of this, I can take a picture or two.

  • So the downward force of the pump wasn't enough to keep the float switch inactivated? Neat. Pictures would be nice. I am planning on adding a relay in parallel (or series, need to check the wiring) with the lid switch (The pump and everything is cut off when the lid is opened) , so simple control logic levels instead of messing with high voltage/high current ssrs. – Passerby Dec 20 '13 at 13:40
  • Perhaps your setup is different from mine. I had an open pipe (with a trap and a vent) sticking up about 4 feet from the floor, along the wall, and the J-shaped drain hose from the washer was simply hooked over the edge. The discharge from the pump was therefore below the float switch (no force directly on the float), and the water only reached the float when the drain backed up sufficiently. – Dave Tweed Dec 20 '13 at 14:32
  • Diagram added is essentially 95% what I have. Trap included, no vent afaik. – Passerby Dec 20 '13 at 15:19
  • That was my situation exactly. In your case, the vent is at the top of what you're calling the "waste stack". In my case, I had nothing else on that side of the house, so it merely went directly to the roof vent on that side. In any case, I had enough room at the top of the pipe (at the "indirect connection") for both the washer drain hose and the float switch. If you don't, it would be easy to add a PVC "Y" just below "indirect connection" (above the trap) to create a second opening for it. – Dave Tweed Dec 20 '13 at 16:36

Stak Enterprises makes a ''Washer Watcher'' that attaches to the drain pipe and when water backs up, it cuts power to the washer (spin cycle), waits and restarts a while after it is safe. It also beeps. I bought one after repeated floods in the laundry room. The drain goes to a standpipe in a recessed box (for hot, cold, and drain ) about 30 inches up from the floor. You plug the device in the wall and plug the washer into the device. Attach the sensor from the device to the end of the 1 1/4'' washer drain tube. I didn't pay $69 for mine. After repeated snaking of the plumbing drains from several point, the problem has stopped but I still have it all connected.

  • It's good to know a commercial product exists. – Passerby Apr 19 '15 at 17:33

Take a wooden clothes pin. Wrap a piece of bare wire around each end of the clamping side of the pin. These will be your leads to interface your device. Take a pill of aspirin and clamp it in the clothes pin (insulating the ends wrapped in wire). Jam the clothes pin in the pipe with the Washer drain hose. If the water backs up the pipe, it will dissolve the aspirin, and allow the two ends with the wire wrapped on them, to contact each other and complete your circuit.

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    I really don't think this is practical advice. The clothespin-and-aspirin hack is only good for slow accumulations of water because it takes time for the aspirin to dissolve. If your washing machine drain backs up it does so in a hurry and you will have a significant flood on your hands before the aspirin dissolves. – Joe Hass Dec 18 '13 at 23:18
  • Wow I didn't think this idea was that impractical. I've heard it a couple of times and even thought of sanding the aspirin pill thinner to make the reaction time quicker. In the end a lot of the projects such as this are home tinkering and not Industrial Automation. Brainstorming with Rube Goldberg ideas can lead to all sorts of interesting things. In all practicality the only thing that would stop water from getting on the floor will be automating the shutdown of the washer in reaction to the sensor. – Tinkerer Dec 18 '13 at 23:28
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    It is also not self-resetting. – Dave Tweed Dec 18 '13 at 23:29
  • I didn't intend to be too critical because not everyone has experienced the problem of a washing machine drain that backs up, so you might not envision the sudden geyser of water that results. The last sentence of your response, "In all practicality..." is exactly what the OP needs. – Joe Hass Dec 18 '13 at 23:32
  • I bet, this approach was inspired by a fuse of naval mine from a pre-electronic era. – Nick Alexeev Dec 19 '13 at 0:05

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