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I have a water tank around 16m vertically above my basement. It is 2m high so there is minimum 16m, maximum 18m.

Therefore I want to turn my pump on at say 17.2m of pressure, off at say 17.8m.

As per the image the water enters my house via a 1" pipe, there is then a T-fitting and the digital gauge/switch is there, then the water passes through a cartridge filter, then a UV purifier, then various branches to the rest of the house.

When I turn on the kitchen tap or whatever, the pressure will fluctuate so for example when the tap is turned off it briefly jumps. Since the pressure switch us set to turn off at say 18m of head, this causes the switch to turn off prematurely. Since my pump is switched on and off by a combination of pressure AND manual switching (I receive a notification to my phone, then turn on the pump using WiFi based on visually checking the condition of the river is not dirty, so it is not particularly convenient if it turns off prematurely)

Is there any simple way to stabilize the pressure reading? Bear in mind that the tank stores 3000 litres and the fluctuation is caused by turning on the tap for say 1/4 litre - it doesn't reflect an actual reduction in the hydrostatic pressure due to the fullness of the tank.

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    Float in the tank to turn on/off the pump, pressure regulator valve to keep the pressure constant into the house.
    – Jon Custer
    Commented Oct 28, 2021 at 16:31
  • Thanks. Float is definitely an option. I have one but I have to check the wiring from the tank to the pump, it's a relatively simple solution, I originally thought the pressure switch was better but the float seems more reliable somehow. I don't know anything about pressure regulator valves.... Is that a backpressure regulator?
    – thelawnet
    Commented Oct 28, 2021 at 16:37
  • Examples like menloservice.sandia.gov/https://www.homedepot.com/b/… - I needed one on my house inlet since I'm not too far down the hill from the main water storage tank, resulting in >110psi on inlet to my house which is, well, really bad for standard plumbing fixture ratings.
    – Jon Custer
    Commented Oct 28, 2021 at 16:40
  • that website is blocked here. I wonder if a simple check valve images.tokopedia.net/img/cache/900/product-1/2020/7/27/39155014/… will work
    – thelawnet
    Commented Oct 28, 2021 at 19:59
  • No, not a check valve, actual pressure control (think scuba regulator if you know them). Adjustable pressure downstream. Fairly cheap try to keep your plumbing from failure ($150 or so).
    – Jon Custer
    Commented Oct 28, 2021 at 22:04

2 Answers 2

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Debounce!

It sounds like you have some sort of home automation system set up to turn the pump on and off. You need to add a debounce timer to the system.

The debounce timer will require that the value be stable for a period of time - anywhere from 500ms to 5 seconds or more (depending on your requirements for your project) before it determines what the current pressure is and acting on it.

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  • the problem with this is I don't have a 'system' per se, I have a Chinese pressure switch with no manual, which connects to a Chinese wifi switch. So if the pressure switch switches off then the power is cut, and the wifi switch is just 'off', no power to it at all. I can maybe change my pressure switch to be normally on and add some kind of computer in between the pressure switch and wifi switch, but that doesn't seem that elegant in that it's an extra device to do that.
    – thelawnet
    Commented Nov 2, 2021 at 12:47
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Pilot line!

This is a common problem we see all the time in electrics, hydraulics and pneumatics. A single supply line feeds both a large load, and a sensitive sensor. The turning of the load on/off causes pressure/voltage fluctuations, throwing off the sensor.

A trope over at electronics.se is an Arduino controller right next to the motor it is controlling, sharing one supply line from the battery. Starting the motor causes voltage drop, which shuts down the Arduino!

In all cases, the answer is the same: The sensor needs a dedicated "pilot line" run all the way from the source. That line is not subject to pressure swings. This seems redundant because it is parallel to the main line. But you see why it is important.

It should be the smallest piping that is readily available, it doesn't need any size since it has no flow rate to speak of. (you might want to add a blowdown valve so you can purge it of stagnant water from time to time).

The pilot line does not need to be at the bottom of the tank. It will suffice for it to be at the lowest position you want to allow (so 17.2m in your case).

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  • Hmm, I'm thinking that buying 50 metres of 1/4" pipe (or whatever) and burying it is not very practical given the cost of pipe, risk of fracture, etc., and other solutions which seem easier (the float switch, which just requires an electrical cable, which seems slightly easier.)
    – thelawnet
    Commented Oct 28, 2021 at 19:18
  • @thelawnet yeah that's true. I would say that Freeman is onto the answer, though. I would collect data as frequently as possible (100ms?) and look at the shape of the data. I would think that any overpressure indications would be transient, and that once it reaches constant flow e.g. a shower, the indication would settle to at or below actual. Only exception I can think is a toilet, where the valve slowly closes, the delta-flow could cause an uptick in indicated pressure for 20 seconds (toilet flushing time) Commented Oct 28, 2021 at 23:56

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