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'[EE]: Flow sensing'
2004\08\19@142500 by dr. Imre Bartfai

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Hi PIClisters,

I need to check whether there is a water flow in a 1" rubber pipe. I
thought about a so-called rotameter at first. It is made by Krohne. I got
an offer for $840 which is hardly affordable for that project. Any
alternative suggestions are welcome - I do not need the amount of the
flow, only whether it is there or not.

Thank you in advance.

Imre

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2004\08\19@152147 by Matt Pobursky

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This is a somewhat self-serving plug, since I designed the electronics
for it...

I have a client that sells a product that would work for you, I think.

http://smartflow-usa.com/switching_flowmeter.html

The DDS-3B model would most likely be the model you'd want but it will
depend of the flow rate you have in your system. Last I checked, they
sell for about $250-$300 USD in single unit quantities.

It used to have a PIC in it, but has an MSP430 in the the current
models... ;-)

Matt Pobursky
Maximum Performance Systems

On Thu, 19 Aug 2004 20:40:51 +0200, dr. Imre Bartfai wrote:
{Quote hidden}

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2004\08\19@154431 by Matt Redmond

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I have an uncle in the water systems equipment business.  He asks these questions:

Is the water being pumped?
How small a flow are you trying to detect? (in gal/min)?
Horizontal or vertical flow?

With that he can probably tell me to tell you where to look for an appropriate device.

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2004\08\19@172236 by dr. Imre Bartfai

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Hi,

On Thu, 19 Aug 2004, Matt Redmond wrote:

> I have an uncle in the water systems equipment business.  He asks these questions:
>
> Is the water being pumped?

Yes and not so. The water is pumped from a kind of refrigerator to a laser
gun (2kW nominal power) and delivers the heat from it to the refrigerating
device. The real problem is that the laser gun may stop the flow if shuts
down. I need to detect this stop and cancel the pump. The test of the
inflow pressure seems not to be appropriate as the pump has some
integrated electronics which regulates the pressure accordingly so - as
the customer told me - there is only a little pressure increase, if any.
There is a huge amount of pressure drop between the inflow and outflow
side (the gun has a significant resistance). An idea would be to check the
differential pressure but it seems me a bit complicated (I would need to
tap both pipes which should be avoided).

> How small a flow are you trying to detect? (in gal/min)?
Approximately 2000 l/h.
> Horizontal or vertical flow?
As you want because of rubber pipe.
>
> With that he can probably tell me to tell you where to look for an appropriate device.
>
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2004\08\19@173105 by dr. Imre Bartfai

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Hi,

this seems to be promising. Only the bureacracy can disturb the nice
impression (transport+fees, customs, &c.). I store this as a 2nd best
resort for the case. Thank you for the information. If I could persuade
them to use DHL for transport... (U*S is a plague...)

Imre

On Thu, 19 Aug 2004, Matt Pobursky wrote:

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2004\08\19@175941 by Russell McMahon

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> ... the pump has some
> integrated electronics which regulates the pressure accordingly so - as
> the customer told me - there is only a little pressure increase, if any.

If you can access these electronics they may well provide a signal that
tells you what you ant to know.


       RM

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2004\08\19@190205 by steve

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> I need to check whether there is a water flow in a 1" rubber pipe.

Are you looking to make something as a one of ?

If so, how about a centrifugal pump of about the same capacity, pull out
the motor magnets and replace with a simple opto device.
That way you get an impellor in a sealed housing that is designed to
have the right sized hoses. You'd find those types of pumps in boat
bilges, ponds, washing machines, etc for a lot less than $840.

Steve.

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2004\08\19@190413 by Matt Redmond

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Hmm.  Still need to know what flow is.  My uncle mentioned this flow switch:  http://www.pottersignal.com/industrial/products/flowandpressure/pdf/IFS.pdf  which requires 4.5 gal/min of flow to trip.

Another possibility is a pressure transducer like this:  http://www.omega.com/pptst/PX40.html - the 0-15psi model might work for you.  You said little if any pressure in the line - but that's not possible if you are getting flow (like current without voltage).

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2004\08\19@221308 by M. Adam Davis

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If you can afford a DIY solution then make a section of the hose
translucent, affix a cage with a ball in the hose, and put the assembly
vertical - use an optosensor to see if the ball is up during flow, down
when no flow.  Make sure the ball is about the same specific gravity as
the solution so it doesn't float naturally, or sink even during flow.

Can also use impeller blades, or other objects.

-Adam

dr. Imre Bartfai wrote:

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2004\08\19@232514 by Ken Pergola

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dr. Imre Bartfai wrote:

> I need to check whether there is a water flow in a 1" rubber pipe.

Hi Imre,

Are you able to use or have access to a section of pipe that has no water in
it when water is not flowing?
If so, I'm wondering if a Motorola/Freescale E-field sensor might be able to
be used in your application.

Best regards,

Ken Pergola

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2004\08\19@233723 by Roger, in Bangkok

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www.dwyer-inst.com

-----Original Message-----

... I thought about a so-called rotameter ...

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2004\08\20@003357 by A.J. Tufgar

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Try an ultrasonic flow meter.  Basically you aim a 40 KHz tx/rx at the
pipe and if there is a phase shift, you have flow.  It gets a lot more
fancy if you want to know the exact flow, but for your application it
should work well.  For more information, google ultrasonic flow meter.
I love the idea since it's non-invasive.

This was the reason why I was searching for ultrasonic pairs about a
week ago.  :)

Hope this helps,
Aaron Tufgar

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2004\08\20@015554 by Shawn Wilton

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You know, an alternative to that would be to use the refraction
properties of water to his advantage.  Water does bend light.


Shawn Wilton
Junior in CpE
MicroBiologist


http://black9.net


M. Adam Davis wrote:
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2004\08\20@043327 by Peter L. Peres

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> I need to check whether there is a water flow in a 1" rubber pipe.

You have about 10 cm/sec liquid speed (2000l/h 25.4mm diameter) which may
be too slow for a wheel type sensor. Could you use a heat transport sensor
? Would it have to stop and start very quickly (the heat transport sensor
cannot do that) ?

Peter

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2004\08\20@050101 by Alan B. Pearce

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>If you can afford a DIY solution then make a section
>of the hose translucent, affix a cage with a ball in
>the hose, and put the assembly vertical - use an
>optosensor to see if the ball is up during flow, down
>when no flow.  Make sure the ball is about the same
>specific gravity as the solution so it doesn't float
>naturally, or sink even during flow.

You can get flow-meters that work exactly this way. They are a conical clear
plastic with an opaque ball in them. The ball is pushed up towards the wide
end of the conical shape by the liquid flow. All you would need to do is
mount an opto sensor outside it at a suitable height for the flow rate you
have. You could probably put two sensors so that both are blocked at the
correct flow rate, but you then have "low", "correct", and "high"
indications for very little extra expense.

You may need to enclose the whole thing in a box to stop extraneous external
light from upsetting the sensing.

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2004\08\20@061443 by dr. Imre Bartfai

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Of course. But it is a resin-filled black box...

Imre

On Fri, 20 Aug 2004, Russell McMahon wrote:

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2004\08\20@062858 by dr. Imre Bartfai

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Sorry, it is not the case as it is a closed system like human blood
circulation system.

I thought all the suggestions, by the way. I will carefully investigate
all the options, and thank you and all PIClisters for the valuable
contribution. An important point is: a bad decision may be catastrophic as
I would falsely detect a stop of the flow and therefore the logic halts
the pump but in the reality the flow is not stopped (and the laser gun
runs). It would blow up. Of course, the customer's request was to stop the
pump if the flow really stopped because of it shortens the life span of
the pump.

Regards,
Imre

On Thu, 19 Aug 2004, Ken Pergola wrote:

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2004\08\20@083324 by Olin Lathrop

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dr. Imre Bartfai wrote:
> An important point is: a bad decision may be
> catastrophic as
> I would falsely detect a stop of the flow and therefore the logic
> halts
> the pump but in the reality the flow is not stopped (and the laser gun
> runs).

It sounds like that is more important to address.  If the flow stops - for
whatever reason - the lazer should be turned off quickly.  Or perhaps a
thermal sensor on the lazer shuts it off before damage occurs.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2004\08\20@084112 by Mauricio Jancic

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Ok, can I ask you something? If, you make a small hole to the pipe and put a
pressure sensor, it must work. There are sensors with precision down to 10
Pa, (that I used, there must be lower). 10 Pa is close to a blow if I recall
that sensor would detect a blow directed to its input from a distance of 2
meters.... So, I must detect the pressure increasing when the flow is
stopped but the pump still runs.

       When you detect the flow increasing, you shutdown the pump. And the
"high" pressure will remain stable (you might need a non return valve after
the pump) until the flow is allowed again by the laser...

       Now, why this wouldn't work? (just wondering...) :)


Mauricio Jancic

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2004\08\20@105153 by hael Rigby-Jones

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{Quote hidden}

A better way is to use two pressure sensors, one either side of a fixed
restriction in the pipe.  You can now measure the difference between the two
sensors to get pressure drop (and hence flow) without having to worry about
what the static pressure is (which may vary).

Regards

Mike

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2004\08\20@124734 by Joe Jansen

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How about a simple pool / hot tub style flow switch, able to read at >
1 gpm, in the return line of the hose.  Since you said that the
concern was that the laser could interrupt the flow, then by
monitoring the line coming out of the laser back to the pump, you
could tell quite easily whether the laser has closed it's valve or
not.

These flow switches are usually in the area of $50.

Google for water flow switches, or go down to the local pool dealership.

--Joe Jansen

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2004\08\20@140623 by Mauricio Jancic

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Actually, the sensor I used was a differential sensor. One side was the
outside (of the room) pressure and the other was the room pressure, which
has to be 100 Pa below the outside one.

Mauricio Jancic
Janso Desarrollos
Microchip Consultant Program Member
(54) 11-4542-3519
infospamKILLspamjanso.com.ar
http://www.janso.com.ar


>>{Original Message removed}

2004\08\21@073840 by Peter L. Peres

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> Sorry, it is not the case as it is a closed system like human blood
> circulation system.

a) the heat transfer method also works applied to the outside of the pipe
(no holes) as long as the pipe is thin and an insulator (thermal)

b) why do you not tap into the power supply of the laser (at the power
cord) to sense the mains current ? I'd bet a large virtual sum there is a
huge difference between running/not running. After all the power that
could 'blow up' the tube comes from the mains, so, no power, no blow up,
yes power, run the pump and cross your fingers.

Peter

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2004\08\21@080333 by Gerhard Fiedler

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> Ok, can I ask you something? If, you make a small hole to the pipe and put a
> pressure sensor, it must work. There are sensors with precision down to 10
> Pa, (that I used, there must be lower).
...
>         Now, why this wouldn't work? (just wondering...) :)

If the pump has a pressure regulator, that effect may be within the noise.
The pressure is probably not a nice steady signal where a minimal pressure
change would be significant.

Gerhard

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2004\08\21@090509 by Gerhard Fiedler

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> b) why do you not tap into the power supply of the laser (at the power
> cord) to sense the mains current ? I'd bet a large virtual sum there is a
> huge difference between running/not running. After all the power that
> could 'blow up' the tube comes from the mains, so, no power, no blow up,
> yes power, run the pump and cross your fingers.

Or, similarly to what someone else suggested (tapping into the pump's
pressure controller), maybe you can tap the signal in the laser controller
that controls the water flow?

ge

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2004\08\21@104827 by dr. Imre Bartfai

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Excellent! If LPM fits, then it is the best buy! Thank you!

Imre

On Fri, 20 Aug 2004, Roger, in Bangkok wrote:

> http://www.dwyer-inst.com
>
> {Original Message removed}

2004\08\21@110111 by dr. Imre Bartfai

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You are right. That's why I plan the supervising circuitry (actually a
Parallax BS2SX based box) gives an alarm which should be confirmed
manually.

There was also a suggestion of the customer to obtain the signal from the
valve which is located in the laser gun housing and is controlled
electrically. But I dislike this one as there is a lot of HV (20kV+) and
if something nasty happens, then it may lead to endless discussion (at the
court, maybe).

So the strategy:

if flow stops, then

       stop pump
       give an alarm until confirmed

There is an option also to re-start pump forcibly in this case.

On the other hand, if the whole system boots, there is no flow at the
beginning, and it is NOT the signal to stop the pump. I guess an initial
delay would help. Or the inflow pressure may also be monitored (beneath
output flow) and the dp/dt could be also monitored.

Imre


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On Fri, 20 Aug 2004, Olin Lathrop wrote:

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2004\08\21@110524 by dr. Imre Bartfai

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Hi,

I consider this option. However, the pressure must be looked at the
forward pipe, and I wanted to check the flow in the return pipe. The
problem is (as I mentioned before) the pump has a built-in pressure
regulation with unknown characteristics, and I do not know whether and how
the pressure varies under normal conditions. I am pretty sure it happens
otherwise the pressure regulator would be superfluous.

Imre

On Fri, 20 Aug 2004, Mauricio Jancic wrote:

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2004\08\22@132315 by Gerhard Fiedler

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>> http://www.dwyer-inst.com

> Excellent! If LPM fits, then it is the best buy! Thank you!

You can also check out http://www.captor.com/ -- click on the "flow captor"
link. They have various flow switches. Their advantage is that they don't
have any moving parts in the liquid. (I worked on some of them... :)

ge

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2004\08\23@034639 by Omer YALHI

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>You can also check out http://www.captor.com/ -- click on the "flow captor"
>link. They have various flow switches. Their advantage is that they don't
>have any moving parts in the liquid. (I worked on some of them... :)

Do they use pressure change to calculate the flow rate?

Omer

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2004\08\23@073032 by Gerhard Fiedler

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>>You can also check out http://www.captor.com/ -- click on the "flow captor"
>>link.

> Do they use pressure change to calculate the flow rate?

No. They use the calorimetric method -- reading a (self)heated temperature
sensor, in various variations.

Gerhard

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