My P-trap water sensor

@BinaryRhyme & DaveE...Yes, I think Dave is on to something ..it's not normal for water in a p-trap to evaporate, but more likely a drain line vacuum problem..If the drain line is not properly vented, because the vent is blocked, or poorly designed, when emptying the tub, the water creates a vacuum as it pulls the water through the line..that what the vent is for, to equalize the pressure. Not a very common problem with modern homes, but sometimes those vacuum breaker valves stick closed..Anyway, a good plumber can find it and fix it pretty quick...one other thing, check the vent stack on the roof ( that's where they're usually located).. make sure it's not plugged up with a birds nest or some such thing...lol

good luck..

regards,

LouisR

Hi @binaryrhyme,

  Fair enough - I would not want to be responsible for causing unneccesary expense -- and even if it is a venting problem, it might be due to something blocked, broken, etc., rather than a 'design' issue. You might want to try to find or figure out if it is a problem that just appeared or was it like that from (say) first installation or following some changes to the installation.

Good luck with your research. I hope you get it sorted, and in the meantime I hope you get some electronics, etc. to play with ...  generally more fun than plumbing .. except perhaps if your next project dissipates so much power it needs to be water cooled.  😀 Dave

@davee It's not about the expense - I've probably spent more building this sensor than the plumber would cost - it's just our experience from trying to fix exactly this problem in the past elsewhere that the debugging skills on this vary wildly, plumber to plumber. The sensor is to enable correlating events and the trap going empty, and how frequently. We don't have reliable data, so hopefully this will give some clues.

Posted by: @binaryrhyme

The sensor is to enable correlating events and the trap going empty, and how frequently. We don't have reliable data, so hopefully this will give some clues.

Perhaps you should consider using an Arduino and an RTC (real time clock). The Arduino can monitor your sensor and, when the trap is empty, it can record when (year, month,day,hour,minute and second) directly into the Arduino's built in memory. That'll let you store dozens of "events" that you can later retrieve for analysis and matching against external events.

@will Yeh, I might go all next gen on it - the arduino programming is no huhu, but at the moment, my smarter half checks the trap using a wick and sees if there's water when she visits every other month (a feat the elderly resident is unable to perform), and its usually dry - so version 1 is just to enable the resident to check independently. She pushes a button, sees green or red (momentary activation reduces the risk of lead degradation).

If I were to build a continuous monitoring solution, it'd use a capacitive sensor fed a square wave from a 555 to prevent lead degradation (good video on that from the swiss accent fellow), and some kind of logging output...

Edit: the sensor video: #207 Why most Arduino Soil Moisture Sensors suck (incl. solution) - YouTube

... but for now, we might find out that it goes dry on laundry day, we run some experiments with the washing machine, and we call the plumber. 😉 Fingers crossed.

@binaryrhyme

We have a number of ensuite's and other bathrooms with only my wife and I in our house, meaning that some rarely get used.  Eventually evaporation will reduce the water in the various u bends. The shower traps seem the worse I expect as they are designed to be rather shallow.  'Eventually' can mean a year or so, but eventually the taps need to be turned on for a brief squirt or a wc may need a flush.

Maybe that instead of setting up a sensor to sense when the u bend may be running  low, you just need a microprocessor board that can wake up every 6 months to briefly open a valve on a water container plonked into the washbasin or shower to give a cup full of water down the plug hole.  After all, you still have to actually do something if your sensor indicates a lack of water in the trap.

Having read at least the last few posts on this thread its prompted me to put a solution in place for our house.  Its a low tech repeating calendar entry year from today as I've just done the rounds to ensure all plug holes and w'cs are fully primed.  Now I should be prompted into action before some  nasty niffs assail my delicate senses. 😎 

@byron The situation here is similar - an empty basement apartment in a larger house. In this case, we suspect some kind of interaction between the trap and other plumbing may also be at play, as it vacates much more often. We're not sure how often, as we can only test it atm when we visit - but < 2 months for sure.

The automatic re-fill has been suggested. 😉 My take on how to best implement such a thing would be a reservoir of some sorts with a digitally controlled solenoid valve, but before we settle on that approach, we want more frequent data - and this widget enables a daily (or more frequent) check.

Data in hand, we'll have a better idea what's going on. 😉

@binaryrhyme It is not unheard of for a poorly engineered vent or a blocked vent to allow a trap to be emptied due to a vacuum downstream. The vacuum is usually another line being flushed. In the deep recesses of the part of my mind devoted to plumbing I recall there are design rules along the line of thou shalt place the vent x distance from the trap. Too far or near (I don't recall) will cause some sort of problem.

If you are sure it's just evaporation, add a little oil like corn oil to prevent it. I did that for a 50 gal water drum and the water stayed over an entire winter.

@ronalex4203 Nope, not sure it's just evaporation. The house has three and a two half baths, a shower, a kitchen and a laundry room with washer and sink - the permutations of causal possibilities are vast, lol. Venting issues may entirely be the issue. Hence the need for some idea when the trap goes dry.

@binaryrhyme The key is as you probably already figured out is to determine if the water goes away slowly (evaporation) or instantly which is what a vent/vacuum issue will do. Good luck.

Water Probe v01 ready to deploy tomorrow. Much to be learned on the mechanical/industrial design front, lol.

@binaryrhyme 

Good luck, hope you trap lots of P 🙂