How to Make a Flow Switch

First let me say that this is the very first Instructable that I have made so please be easy on your critiques.

I have a very elaborate well water treatment system that had been plagued with problems.  One problem is I am using chlorine to kill bacteria and then removing the chlorine with charcoal before the water gets to my RO membrane.  Anyway that is more than enough background.

I decided to eliminate the chlorine and install a whole house UV filter.  The UV bulbs are good for 9000 hours and most people, I assume, just plug them in and leave them.  I fail to see any benefit to the light being on when there is no water movement so I figured I would control the light with a flow switch.  All of the flow meters that I found on ebay, Amazon or Grainger were either too expensive or did not give me the specifications that I was looking for.  So its do-it-yourself time.

The object of this project is to detect the movement of water in a 1" PVC pipe and turn a receptacle on and off. 

This is the finished project.  Most of the PVC was just stuff found in the workshop so please ignore the stains.

Step 1: Parts Layout and Parts List.

This project can be sized to fit the project you are working on so there is nothing special about 1" PVC.

Parts List:

PVC threaded adapter 
PVC threaded cap
PVC elbow
Plastic ( had nylon on hand) dowel with a diameter almost the same size as the inside of the size PVC that you choose
Some lead weight
high power magnets that will fit in the hole that you will drill in the plastic dowel
a rubber O-ring to fit the size of your dowel (see later steps)
A reed switch
Other PVC fittings and pipe to fit the application that you have
PVC cleaner
PVC glue

tools list:
Hacksaw or PVC cutter
A lathe or access to one is a real help.
27/64"  (for the 1" PVC size)
1/2" - 13 Thread per inch tap and wrench (also for the 1" PVC size)

Step 2: Making the Capsule

The capsule is the moving part that will signal the reed switch to turn on the UV light.
I helps greatly to have a lathe otherwise you are left to your own creativity.

For the 1" size PVC I made my capsule approximately 6" long.   Turn Its diameter of an 8" length to 1/16" smaller than
the nominal inside diameter of 1" PVC pipe.  Cut off 3"  and drill a 27/64" hole in the 5" section to withing 1" of the bottom. (I accidentally drilled too far and that is why there is an epoxy joint at the bottom)  This also happens to be a good size the 
magnets will fit in.

Next, tap the hole about an inch deep.  It is easier to do this while it is in the lathe chuck.

Now put the 3" piece in the lathe, trim about 1/2" length down to 1/2" diameter and cut a thread all the way.  If you don't know how to to do this then you may need to seek some help.  A die of the right style might work but it needs to be full threads all the way to the hilt and the hilt needs to be square.  

Trim the top of the capsule to about 1 - 1/2" and round the ends of the top and bottom so it won't hang up when doing its job.

While you are at the lathe take the threaded adapter   (the part screwed into the cap at the top above the capsule)  and increase the inside diameter to the same diameter as the PVC pipe.  This will allow you to service the capsule should you need to.

Step 3: Assemble the Switch

Now take the 5" and drop in the lead weight.  I put a spring in next but almost anything will do to keep the magnets as close to the top of the hole as you can without interfering with the threads.  Then drop in the magnets.  I used just 4 after experimenting with the capsule inside a test piece of PVC so that it would activate the reed switch.

Next put a 1/2" inside diameter O-ring over the threads of the top and screw it on tight enough to compress the O-ring about half way. You don't have to kill it!

Now the capsule is complete and it is time to calibrate it.  It must be negatively buoyant.  That means that it must sink in the water or what ever fluid you will be using.   So with it tightened put it in a container of water and make sure that it sinks.  Depending on the viscosity, flow and the sensitivity that you want this can be adjusted by adding or removing weight.  Now you are ready to assemble the whole switch.

With a little redesign you could use a spring instead of gravity like I did to return the capsule to the off position.

Lay all of the fittings out with the capsule so that you can measure how long the two pieces of 1" PVC need to be.  They can actually be longer than need but they cannot be too short.   Put the capsule below the Tee so that the top of the capsule is about even with the bottom of the left going hole in the Tee.  See the picture.  Now measure for the lower length of PVC.

The upper length of PVC need to be long enough so that when the water flows (from the bottom)  the capsule has plenty of room to get out of the way. 

Once the pieces are cut put them all together like the drawing.  Make sure to use teflon tape or goop on the threaded cap.

You are now read to install it.  In my case you can see the shinny housing of the UV lamp housing.  Also notice the position of the reed switch.

Step 4: NOTE on the Reed Switch.

I used a reed switch from an old window burglar alarm but you may have something different.  They do make them to take 120 volts AC but I chose to just use what I had which is good for maybe 5 to 12 volts with low current.  So with that I need to make an adapter box.  Simple enough circuit so I won't bore you with that. 

I also had a 12 volt DC controlled relay that can handle 120 volts AC.  So I used a 12 DC wall wart to control the relay through the reed switch which in turn powered a receptacle on the side of the box and a Hobbs meter on the front.  A Hobbs meter is a device that has a speedometer like display that just counts up the hours that what ever it is connected to is operating or powered up.  They are on things like generators, tractors, heavy equipment or anything that need maintenance by the hour.  Check ebay for some examples.  In my case I wanted to count up to 9000 hours so I know when to change out the bulb.  I had one on my hurricane generator which I have not used since 2004 so I just borrowed it.

The Position of the reed switch may need to be adjusted once you have water flowing so like I did have temporary wire holding it. Turns out it worked right off the bat without any adjustments.

Thanks for reading this far.  Let me know if you have any questions.  It was a pretty easy project and I have not seen anyone else tackle this problem.



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    15 Discussions


    1 year ago

    Why not just plug the UV lamp into a timer and just have it on during the time you would use the water? And just unplug it when you are away for long periods of time.

    Of course if you have well water that is seriously compromised most of the time, this is not a good idea due to "creapage" of critters past the inactive lamp, and the need for the lamp to warm up enough to produce the UV waves.


    3 years ago

    EricW76 I agree with your assessment. Matter of fact I have disabled the switch and am running the UV light 100% of the time. I thought that the flow switch design may inspire others to use it for other purposes so I left it listed. I have tried to delete this instructable so others may not go down my road but some how it is entered into a contest and will not allow me to delete it. When the contest is over I will try again.


    3 years ago

    This is an inventive design. But, turning off a UV lamp and back on will have two problems. One - bulb life is negatively impacted by number of power cycles. Two - and this is much more important - the UV bulb takes significant time to "warm up" when first turned on in order to provide adequate intensity to sterilize the water. The light wavelengths that actually sterilize bacteria is ultraviolet (you cannot see it with the naked eye). For the bulb to provide the necessary intensity, it can take several minutes to warm, depending on the bulb, and the age of the bulb. "Glowing" does not equal UV light. During that time, bacteria laden water will be flowing through your system and into your water system. This is why fluorescent UV systems typically are constant on. There are LED technologies that are coming that will offer that instant on ability you would need to safely do this. But cycling the bulb in fact does not provide safe water, and is not recommended.


    4 years ago on Introduction

    Nice work... simple yet very useful....
    Thanks for sharing


    4 years ago on Introduction

    Nice! I've seen a bunch of hacks for water flow sensors and I'd say this is the best of the lot.


    4 years ago

    With this setup, I would a little before getting a glass.


    4 years ago on Introduction

    I like the flow switch, what prevents the bacteria from migrating from the "dirty" side to the "clean" side withe the UV light off?

    1 reply

    Reply 4 years ago on Introduction

    Good question. The UV chamber is about 36" long and fortunately for us bacteria have very very short legs. I guess if you left the system go stagnant for a long time that might be a problem but my system gets used at least every couple of hours.


    4 years ago

    Have you thought of incorporating a redundancy switch powered by the pump circuit? Pump turns on flow circuit must turn on, or alarm sounds and the pump turns off. This keeps your system free of bacteria downstream of the light. Those reed switches are only rated for so many cycles, and are notorious for failing at the most critical times.

    1 reply

    Reply 4 years ago on Introduction

    The problem with the pump switch is that the pump maintains pressure in a tank that has a bladder in it. Once the tank and system are up to pressure and the pump turns off, any demand for water is satisfied by the air in the bladder until it reaches the bottom of the hysteresis curve and turns the pump back on for the next cycle. All that while the bladder is moving water past the UV filter and it won't be on because the pump is not on. Which defeats the purpose of the UV filter.


    4 years ago on Introduction

    very smart. was just wondering if you dont need a certain amount of flow to push up the weights, or in other words, can the water flow slowly without activating the switch?

    2 replies

    Reply 4 years ago on Introduction

    In my mind the pump and system has 40 to 60 PSI on it all the time. When the demand calls for the pump to come on there is a lot of volume of water moving through the system and in order to pass the switch it has to get the capsule out of the way in order to get to the output. In my research I have found commercially available switches that use this principal except that they use a spring instead of gravity. I wanted the least amount of friction and the most sensitivity so I made the tolerance close and buoyancy just a little negative. O' boy I did not talk about the buoyancy, did I? I hope I can edit it.

    Thanks for posting this ! I'm going to be installing a similar system at my cabin in the next couple of years. I was planning on using the auto pressure switch on the pump to turn on the UV light, but if that doesn't work out you've given me an option.