Simple flow sensor wanted

[Jasen Betts]

On 2006-04-26, John_H <john4271@hotmail.com> wrote:

Looking for a flow sensor that will detect liquid flow in 19mm plastic
tubing. Don't need to know the flow rate, merely the presence or
absence of flow. Preferably without any significant line restriction.

It's purpose would be to switch a warning device when flow stops so
that a pump can be manually switched off to prevent it working dry.

Flow rate when liquid is present is around 20 litre/minute.

Is any such device commercially available?

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

 

[Jasen Betts]

On 2006-04-27, John_H <john4271@hotmail.com> wrote:

Restricting the flow and sensing the resulting pressure is also a
possibility but isn't ideal as it will both reduce the flow rate and
increase the motor's current draw.

Just put a pressure switch after the pump. when the tank is empty the
pressure will drop.

Bye.
Jasen

 

[Jasen Betts]

On 2006-04-27, John_H <john4271@hotmail.com> wrote:

Geoff C wrote:

I'm a bit confused with the plumbing fittings though. 22mm is the
approximate OD of 3/4" galv pipe, whereas I'd have expected them to
compatible with copper tube... or am I just being confused by the
terminology used?

Presumably the compression fittings will be a match for 3/4" copper?the

the threads beneath those compression nuts should match the threaded adaptors
used for 19mm plastic pipe too...
19mm is just the metric name for 3/4" pipe. it's still 3/4".

Bye.
Jasen

 

[Jasen Betts]

On 2006-04-27, Brenden <brendenede@hotmail.com> wrote:

John,
A nice simple solution would be to place a thermistor in the liquid
flow, pass some current through it and it will heat up. When the liquid
flows it will cool the sensor, no flow, no cooling. A simple comparitor
circuit can then turn off your pump and the circuit itself, so not too
much heating.

The whole circuit with a Relay to drive your pump should be under $20.

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

 

[John_H]

Jasen Betts wrote:

On 2006-04-27, John_H <john4271@hotmail.com> wrote:

I'm a bit confused with the plumbing fittings though. 22mm is the
approximate OD of 3/4" galv pipe, whereas I'd have expected them to
compatible with copper tube... or am I just being confused by the
terminology used?

Presumably the compression fittings will be a match for 3/4" copper?the

the threads beneath those compression nuts should match the threaded adaptors
used for 19mm plastic pipe too...
19mm is just the metric name for 3/4" pipe. it's still 3/4".

I've got problem with that, but WTF is 22mm -- which is the size these
come in.

The 1/2" equivalent appears to be called 15mm, and the 3/4" equivalent
is called 22mm!

--
John H

 

[John_H]

Jasen Betts wrote:

On 2006-04-27, John_H <john4271@hotmail.com> wrote:

Restricting the flow and sensing the resulting pressure is also a
possibility but isn't ideal as it will both reduce the flow rate and
increase the motor's current draw.

Just put a pressure switch after the pump. when the tank is empty the
pressure will drop.

What sort of pressure drop do you have in mind?

The frictional loss in 19mm tubing flowing 20 litre/min is something
less 3 metre of head per 30 metre. (A google on Hazen-Williams should
find the various tables for frictional losses)

I'm pumping through 6 metre of hose, which will give around 600mm head
at the pump. This works out to be around 0.1 bar (1.5 psi) pressure.

I'd also suspect that the problem would be compounded by the fact that
I'm feeding to the bottom of the receiving tank (to minimise the head
pressure, and hence the pump load). This will have the effect of
significantly varying the static head at the pump outlet, depending on
the level of the main tank and the slope the machine is working on
(which can be relatively steep on occasions -- 30° or so).

--
John H

 

[Einstein]

"John_H" <john4271@hotmail.com> wrote in message
news:cabu421dubb4nlj5d6hlenqjkvpmru5uva@4ax.com...

Looking for a flow sensor that will detect liquid flow in 19mm plastic
tubing. Don't need to know the flow rate, merely the presence or
absence of flow. Preferably without any significant line restriction.

It's purpose would be to switch a warning device when flow stops so
that a pump can be manually switched off to prevent it working dry.

Flow rate when liquid is present is around 20 litre/minute.

Is any such device commercially available?

--
John H
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.

 

[Einstein]

"Einstein" <Einstein@relativityresearch.com> wrote in message
news:4452bc26@dnews.tpgi.com.au...

"John_H" <john4271@hotmail.com> wrote in message
news:cabu421dubb4nlj5d6hlenqjkvpmru5uva@4ax.com...
Looking for a flow sensor that will detect liquid flow in 19mm plastic
tubing. Don't need to know the flow rate, merely the presence or
absence of flow. Preferably without any significant line restriction.

It's purpose would be to switch a warning device when flow stops so
that a pump can be manually switched off to prevent it working dry.

Flow rate when liquid is present is around 20 litre/minute.

Is any such device commercially available?

--
John H
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.
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.

 

[David (one of the many)]

"John_H" <john4271@hotmail.com> wrote in message
news:cabu421dubb4nlj5d6hlenqjkvpmru5uva@4ax.com...

Looking for a flow sensor that will detect liquid flow in 19mm plastic
tubing. Don't need to know the flow rate, merely the presence or
absence of flow. Preferably without any significant line restriction.

It's purpose would be to switch a warning device when flow stops so
that a pump can be manually switched off to prevent it working dry.

Flow rate when liquid is present is around 20 litre/minute.

Is any such device commercially available?

--
John H

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.

 

[John_H]

Einstein wrote:

"John_H" <john4271@hotmail.com> wrote in message
news:cabu421dubb4nlj5d6hlenqjkvpmru5uva@4ax.com...
Looking for a flow sensor that will detect liquid flow in 19mm plastic
tubing. Don't need to know the flow rate, merely the presence or
absence of flow. Preferably without any significant line restriction.

It's purpose would be to switch a warning device when flow stops so
that a pump can be manually switched off to prevent it working dry.

Flow rate when liquid is present is around 20 litre/minute.

Is any such device commercially available?

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.

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

 

[Jasen Betts]

On 2006-04-28, John_H <john4271@hotmail.com> wrote:

Jasen Betts wrote:

On 2006-04-27, John_H <john4271@hotmail.com> wrote:

I'm a bit confused with the plumbing fittings though. 22mm is the
approximate OD of 3/4" galv pipe, whereas I'd have expected them to
compatible with copper tube... or am I just being confused by the
terminology used?

Presumably the compression fittings will be a match for 3/4" copper?the

the threads beneath those compression nuts should match the threaded adaptors
used for 19mm plastic pipe too...
19mm is just the metric name for 3/4" pipe. it's still 3/4".

I've got problem with that, but WTF is 22mm -- which is the size these
come in.

It's the size of the threaded part. pipe is sized by the diameter of the hole
(as that's the most important part) but the couplers are sized by the thread
(as that's where the action is)

The 1/2" equivalent appears to be called 15mm, and the 3/4" equivalent
is called 22mm!

Yeah that sounds about right I haven't dealt with that stuff for over a
year now.

Bye.
Jasen

 

[Chas]

"John_H" <john4271@hotmail.com> wrote in message
news:704552d0t8rl2i8a97h0aii1p9aosice4p@4ax.com...

Jasen Betts wrote:

On 2006-04-27, John_H <john4271@hotmail.com> wrote:

Restricting the flow and sensing the resulting pressure is also a
possibility but isn't ideal as it will both reduce the flow rate and
increase the motor's current draw.

Just put a pressure switch after the pump. when the tank is empty the
pressure will drop.

What sort of pressure drop do you have in mind?

The frictional loss in 19mm tubing flowing 20 litre/min is something
less 3 metre of head per 30 metre. (A google on Hazen-Williams should
find the various tables for frictional losses)

I'm pumping through 6 metre of hose, which will give around 600mm head
at the pump. This works out to be around 0.1 bar (1.5 psi) pressure.

I'd also suspect that the problem would be compounded by the fact that
I'm feeding to the bottom of the receiving tank (to minimise the head
pressure, and hence the pump load). This will have the effect of
significantly varying the static head at the pump outlet, depending on
the level of the main tank and the slope the machine is working on
(which can be relatively steep on occasions -- 30° or so).

--
John H

If you want a DIY solution, can I suggest a plastic ball or slug with an
embedded magnet operating a reed switch on the outside of a short piece of
tubing next size larger than your delivery tubing. The ball or slug will
need to be slightly larger than the ID of the delivery tube, and have some
kind of restraint to limit its travel. If you cannot arrange the flow
vertically up, you would also need a light ss spring ahead of the ball/slug.
Food for thought.
--
Regards,

Chas.

(To email me replace 'xxx' with tango papa golf)