Simple flow sensor wanted

[Einstein]

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 \(one of the many\)]

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?

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]

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.

 

[Chas]

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).

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)