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Re: Simple flow sensor wanted



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Something similar is used as an air flow monitor in the intake of fuel
injected engines, some types use a hot wire sensor the hot wire resistance
is dependant on it's temperature and the ECU calculates the power required
to keep the wire at a temperature relative to ambient, and therefore the
amount of airflow cooling it down.



Re: Simple flow sensor wanted


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Davey use a similar idea to control their constant pressure household
pumps (XP models from memory) -- they have a metal plate in contact
with the liquid which presumably has a thermistor behind it.

Only problem is they're very sensitive to scaling from hard water.
The slighest scale buildup stops them in their tracks.

Scale buildup in spraying equipment is also a problem which makes me
reluctant to pursue the idea.

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Not looking to control the pump... merely want a warning device for
the operator.  The simpler the better when it comes to things
agricultural.  :)

--
John H

Re: Simple flow sensor wanted


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how many watts are you going to need to push through tat thermistor to get a
result in under minute?

that setup works great with air but water based solutions take much much
more energy to warm up.

Bye.
   Jasen

Re: Simple flow sensor wanted



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I have used the Gentech FS01 as a flow sensor to switch on a pump for
the rainwater tank supply to my house and it works at extremely low
flow rates.

available from Farnell
http://au.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU10%06766

They also have a cheaper version FS02
http://au.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU10%06767

Re: Simple flow sensor wanted


On Thu, 27 Apr 2006 16:29:00 GMT, Ross Herbert

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In addition:

If your pump is working as a on-demand pump for water supply see if
you can fit an Onga Presscontrol unit to the pump. This unit takes
care of the dry-running problem automatically. I fitted one in place
of the old air pressure bulb switch on my Davey Dynajet and it works
perfectly.

I bought mine from
http://centre.net.au/index.html?cat00%05YF00021E&prid00%0DBY&it=product

You will have to work out the connections to the pump but it fairly
easy to work out.

Re: Simple flow sensor wanted


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The diffrence between the two seems to be the switching.  The FS01 has
a triac, the FS02 has contacts.

On the face of it they're perfect for the job, especially the 3/4" BSP
connections.  The problem might be the material -- apparently Noryl
has rather poor resistance to chemicals in general.

--
John H

Re: Simple flow sensor wanted



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Here's a table of chemical resistance properties for various
"plastics".

http://www.wps.on.ca/technical/tables/chemical_resistance.htm

Re: Simple flow sensor wanted


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flow switches are available (try a commercial plumbing outlet)


otherwise the can be made... here are two ideas.

stick two stainles steel wire electrodes along opposite sides (of the inside)
of the tubing and put a magnetic field through the gap. (not across)
measure the voltage with a high impedance voltmeter.

may not work well if mounted too close to the pump or if the pump develops
an electrical fault. running the water through a metre or so of metal pipe
(copper/stainless/galv) pipe before the sensor may ba all that's needed,

This one may collect rust particles if they are present in the water and the
flow is too slow.

some boats use a setup like this for their speedometers.

you can use copper wire electrodes to test-drive this one, but stainless
lasts much longer in service.



  another option is this, (cross section)

  if garbled view in a fixed-pitch font (like courier)
  or paste into notepad

                  |     |
                  |     |
                  |     |<--- 19mm pipe with 4x12mm holes
                  |     |     drilled near the end
                  |     |
                  |     |
                  |     |  .--- pipe joiner  
                  |     | /
               |XX|     |XX|
               |XX|     |XX|
               |  |  _  |  |<-- 40 mm pipe
               |  \ / \ /  |
               |  / \_/ \  |      
             $$|  |_____|  |$$ -- 100 to 1000 turns of magnet wire        
             $$|           |$$      
               |           |      
               |           |      
               |           |      
               |   00000   |      
               |  0000000<----- stainless - steel (or brass etc) ball      
               |  0000000  |    a mouse ball may even work  
               |  0000000  |    - they have a metal core      
               |XX|00000|XX|
               |XX|     |XX|          
                  |     | \  
                  |     |  `--- pipe joiner
                  |     |
                  |     |
                  |     |<--- 19mm pipe    
                    /|\
                     |
                     | ---- flow in this end.


 reading this one's a bit harder it needs to be fed high frequency AC (eg
 from a 555) the "response of the coil to this signal" of the coil will
 change as the ball aproaches pushed by the flow from the pump...

 less prone to leaks or electrical interferance, can possibly double
 as a check valve, but harder to make.


Bye.
   Jasen

Re: Simple flow sensor wanted



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Look at placing a plastic encapsulated bar magnet in a flaring high pressure
plastic pipe with a magnetic reed switch above the magnet and against the
pipe. Flow causes the magnet to rise, actuating the reed switch. Put the
reed switch in series with a relay actuation coil. No issues with corrosion
and very simple. As with any pumping application, size a foot strainer to
ensure particles will pass by the magnet in the flaring tube. The flaring
tube can be an adaptor from 3/4" to 1",  the magnet diameter is sized to
have sufficient restriction in 3/4" tube to have the liquid flow cause the
magnet to rise into the bigger cross section tube above with flow.



Re: Simple flow sensor wanted



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PS, if you don't want to use an electrical circuit, use a magnet on the
outside of the pipe, located on a pivoting arm, so that the outside magnet
follows the movement of the inner magnet. The position of the outside magnet
will indicate whether you have fluid flow or not.



Re: Simple flow sensor wanted


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Sounds neat... same principle as the flow meters used on gas cylinders
I presume, but is it commercially available as a switch?

Whilst it's not too difficult to fabricate something for a special
purpose, ease of service and readily available replacements is a major
consideration in this particular application, where downtime can be
very costly.

Thus whatever I decide on needs to be serviceable, or readily
replaceable, by the machinery operator -- which does tend to eliminate
some of the cleverer solutions.  Any potential problems that might be
associated with the device also need to easily diagnosed by the
operator -- such as a blockage or a failure to work as it should.

--
John H

Re: Simple flow sensor wanted



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I was looking through our Omega catalog for other bits and pieces and this
caught my eye after having read your post.

http://www.omega.com/ppt/pptsc.asp?ref=PSW-141&Nav=preh04

If you insert a small tube into the flow, and it doesn't have to be very
far... like the dripper take-off in small irrigation systems... you will
develop a negative pressure in the tube (venturi effect).  Cut the end of
the tube to a 45 - 60 degree angle and by placing it facing towards the flow
you get a positive pressure, facing away you get a negative pressure. A
sensitive low-pressure switch can then be used, similar to the one above.




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