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Is there a non intrusive way to measure if water is flowing through a copper pipe or just standing still in it? The copper pipe belongs to my heating system and is always under pressure with water, flowing or not.

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    $\begingroup$ You could sense for a sudden change in temperature on the outside of the pipe. If the water isn't flowing, the pipe will slowly cool to ambient. When the water is flowing, it will very quickly heat to the temp of the water. $\endgroup$
    – Dan Laks
    Jan 14 '17 at 22:37
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    $\begingroup$ Might just consider measuring the temperature on the surface of the copper pipe at a distance from the water heater. No flow? The pipe cools down to some equilibrium state. Flow? Warms up very quickly as the hot water flows. No idea if the time constants will be good enough for you. But you haven't said why you care, so maybe? Won't tell you how much. But it may tell you IF. (I've used the "put-hand-on-pipe" to feel it heat up (or cool down for cold water) to tell if water is flowing, from time to time.) $\endgroup$
    – jonk
    Jan 14 '17 at 23:04
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    $\begingroup$ You could try attaching a microphone to the pipe in case there is a distinct signal assiciated with the water flowing, maybe next to a bend if one is available. $\endgroup$ Jan 15 '17 at 15:17
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    $\begingroup$ @Tim You'd need to analyze the frequency content and do some filtering to focus in on what is unique about the water flow. $\endgroup$
    – DLS3141
    Jan 16 '17 at 16:48
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    $\begingroup$ Your comments to the proposed answers suggest you are asking rather the wrong question. $\endgroup$ Jan 16 '17 at 18:55
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Non Invasive flow meters typically use ultrasonic sensors clamped to the pipe. The keyword is simply non-invasive flow meter.

But regular flow meters are much cheaper. All you need is the appropriate size, cut the pipe and thread it, attach t he meter and done. It's done all the time.

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  • $\begingroup$ Are there any known DIY solutions to the ultra sonic sensor ? All the commercial options are really expensive. Installing an invasive flow meter is not an option. $\endgroup$
    – user2021331
    Jan 14 '17 at 22:05
  • $\begingroup$ If all you are doing is flowing not flowing, I guess a ultrasonic distance sensor may give different results when flowing or not. $\endgroup$
    – cde
    Jan 14 '17 at 22:29
  • $\begingroup$ Ultrasonic time of flight flow meters work very well. You are looking at $500+ though. davis.com/Product/… $\endgroup$
    – ericnutsch
    Jan 15 '17 at 7:41
  • $\begingroup$ @cde I ordered the widely avaible ultrasonic distance sensor HC-SR04 and taped it to my pipe. It doesnt make a difference if water is flowing trough the pipe or not and it only makes a small differencef a pipe is filled or not. I believe because the sensor only analyses the time the sound takes to travel i won`t be able to get a usefull result using this solution. I propably have to find a way to analyze frequency changes when the sound travels trough the pipe. $\endgroup$
    – Tim
    Jan 17 '17 at 11:55
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You can use two piezo ceramic transducers in combination with a TDC1000 AFE from TI to measure the flow.

enter image description here

References:

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  • $\begingroup$ This is a very helpfull, but it might be a littlebit overkill. I will try using a cheap ultra sonic range finder first. $\endgroup$
    – Tim
    Jan 15 '17 at 11:02
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So what you're hoping is to keep circulating after the thermostat has shut down the furnace, in the hope of extracting as much heat as possible from the water? Your question only asked about flow, not temperature.

The problem, I fear, is that you're not looking at the entire system's gains/losses. The best thing you can do, by far, to minimize waste heat, is to insulate the pipes everywhere except the radiators themselves. The next largest waste heat source is the furnace & exhaust system, and unless you want to put in some passive air feed to send furnace-room air into the house proper, there's not much you can do about that.
Compared with those two items, the proposed heat recovery from warm water in the pipes is miniscule at best. Yes, it is sensible to set the circulator to continue for a few minutes after the thermostat triggers, but you get diminishing returns in a hurry. For one thing, the rate of heat transfer in the radiators is a strong function of the temperature difference. You're going to end up burning more electric power to run the pump than you'll ever get in room warmth.

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  • $\begingroup$ No, i'm only analyzing if water is flowing into the "system" to analyze when water is used. I hope to get results when warm water is used the most and will adjust the runtime of the circulating pump accordingly. I'm only interested in getting warm water faster when opening the faucet, i`m not interested in optimizing anything regarding temperature loss. $\endgroup$
    – Tim
    Jan 16 '17 at 18:47
  • $\begingroup$ Then what you're asking makes no sense. The hot water system for radiators is not connected to the hot water feed for sinks, tubs, etc. What do you really want? $\endgroup$ Jan 16 '17 at 18:54
  • $\begingroup$ The initial question was how to measure water flowing trough a copper pipe. I added that the pipe belongs to my heating system to clarify that it is always under pressure. I never said that i wanted to optimize anything regarding the furnace. I´m sorry if i didn't clarify this, but the pipe i want to measure is feeding cold water into the heating circuit, so using the temperature for measurements is not an option. $\endgroup$
    – Tim
    Jan 16 '17 at 20:07
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Mount two surface temperature sensors (or the hot and cold junctions of a thermocouple arrangement) to the pipe say 100mm (4") apart. Place a resistive heater between the two temperature sensors. Add 5-20W of heat to the pipe with the heater. Compare the temperatures.

  • When water is stationary you will measure the same temperature at both locations.
  • When water at above or below ambient flows (and stops) you will initially see large change in both sensor values and small differential due to additional heat indicating flow direction.
  • Water flowing at ambient temperature will only result in small change in one sensor indicating flow direction.
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