Simple flow sensor wanted

[John_H]

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?

 

[Mark Harriss]

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?

Is the liquid clear or coloured?, can you use an LED/phototransistor
combo?.

 

[John_H]

Is the liquid clear or coloured?, can you use an LED/phototransistor
combo?.

It varies between clear and relatively opaque.

The application is a crop sprayer where various chemical mixtures
(herbicides) are transferred from an auxiliary tank mounted on the
front of the tractor. The electric motor driven roller pump used
doesn't like being run dry for any length of time so I need to be able
to accurately detect when the tank is empty.

Conductivity is a possibility but past experience tells me that
keeping electrodes sufficiently clean is likely be a problem.

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.

I note that RS have an extensive range of sensors and switches (thanks
I.F.), which I haven't fully checked out as yet, but most of them are
more complex, and hence more expensive, than what's probably needed.
The simpler ones are also physically smaller than I want (mostly ¼"
diam) but it may be possible to use a bypass line for sensing. The
transfer hose is ¾" (19mm).

All ideas welcome.

 

[Geoff C]

It varies between clear and relatively opaque.

The application is a crop sprayer where various chemical mixtures
(herbicides) are transferred from an auxiliary tank mounted on the
front of the tractor. The electric motor driven roller pump used
doesn't like being run dry for any length of time so I need to be able
to accurately detect when the tank is empty.

Conductivity is a possibility but past experience tells me that
keeping electrodes sufficiently clean is likely be a problem.

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.

I note that RS have an extensive range of sensors and switches (thanks
I.F.), which I haven't fully checked out as yet, but most of them are
more complex, and hence more expensive, than what's probably needed.
The simpler ones are also physically smaller than I want (mostly ¼"
diam) but it may be possible to use a bypass line for sensing. The
transfer hose is ¾" (19mm).

All ideas welcome.

Flow is not easy to measure reliably. That is why sensors are a bit
expensive and each type has its drawbacks.

In your application, the most simple and rugged types in the RS catalog
are 256-562 and 256-578. Designed for water, metal bodies with
compression fittings, rugged, simple microswitch output. Price does not
appear unreasonable.

They work by a spring loaded vane in the flow, IIRC. I have used these
before and they are fine. Like most of these types, they may fail in
either state, unlike the rotary types (but more prone to wear and also
require more electronics in interface).

 

[K `Sleep]

http://www.discovercircuits.com/PDF-FILES/NewPDF/Capmeter1C.pdf

Hope this helps.
Kim

 

[GB]

The electric motor driven roller pump used
doesn't like being run dry for any length of time so I need to be able
to accurately detect when the tank is empty.

A simple mechanical float and microswitch arrangement at
the bottom of the tank perhaps?


G

 

[Dan H]

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?

Check out
http://qualitrolcorp.com/productdetail.asp?ID=36
This is a flow gage to verify oil flow in oil cooled power
transformers. Use to verify that oil is flowing when the pump is
running.

Dan

 

[Brenden]

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.

Regards,
Brenden Ede

 

[Ross Herbert]

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 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?SKU=1006766

They also have a cheaper version FS02
http://au.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=1006767

 

[Ross Herbert]

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?SKU=1006766

They also have a cheaper version FS02
http://au.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=1006767

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?cat=0005YF00021E&prid=000DBY&it=product

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

 

[Dan H]

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.

Regards,
Brenden Ede

This would only work if the liquid being pumped was at a different
temperature than the liquid that was stagnent because the pump wasn't
working. The fact that the liquid is moving doesn't mean it is cooler
when moving.

Dan

 

[John_H]

In your application, the most simple and rugged types in the RS catalog
are 256-562 and 256-578. Designed for water, metal bodies with
compression fittings, rugged, simple microswitch output. Price does not
appear unreasonable.

Either 256-578 ($76.60) or 257-082 ($38) look suitable for the job,
and don't cost an arm or a leg. The latter is a plastic body which
isn't necessarily a disadvantage.

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?

They work by a spring loaded vane in the flow, IIRC. I have used these
before and they are fine. Like most of these types, they may fail in
either state, unlike the rotary types (but more prone to wear and also
require more electronics in interface).

This sounds to be a very suitable system for my application as I don't
need to measure flow, only detect it's presence or lack of.
Reliability, or lack of, is something I'm used to living with.

 

[Brenden]

Dan,
Not quite, if you apply current through the sensor and the fluid
isn't moving, it heats the surrounding stationary fluid. When the fluid
is moving the heat is passed into the moving fluid which keeps the
sensor cooler. This technique can be used to detect movement in any
fluid or gas that is in contact with the sensor.

Brenden

 

[Geoff C]

Either 256-578 ($76.60) or 257-082 ($38) look suitable for the job,
and don't cost an arm or a leg. The latter is a plastic body which
isn't necessarily a disadvantage.

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?

Not sure, but I would not assume it. I'm no plumber so don't understand
their stupid pipe histories but you might have to be prepared to make an
adapter, preferably soldered copper tubings. I think somehow we had the
right size copper tube when we used one.

This sounds to be a very suitable system for my application as I don't
need to measure flow, only detect it's presence or lack of.
Reliability, or lack of, is something I'm used to living with.

I would recommend this type over complicated DIY electronic solutions
which will be unreliable and probably frustrating to calibrate.

If you are mechanically handy and have time, another quick and dirty
system is to have a section of your pipe made from a flexible pipe such
as poly, canvas or whatever. You can put a spring loaded welding clamp on
the hose and rig a microswitch so that as the pressure drops, the clamp
closes a little and the handle parts of the clamp close the switch. How
that is done is up to your clerness. Relies on a pressure drop on the
output of the pump in event of flow stopping though. Will not detect a
blockage on the outlet either.

 

[John_H]

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?SKU=1006766

They also have a cheaper version FS02
http://au.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=1006767

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]

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.

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.

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

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

 

[Ross Herbert]

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.

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

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

 

[John_H]

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]

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).

 

[Einstein]

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.