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
Try RS Components.
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.
-- John H
John_H wrote in news: snipped-for-privacy@4ax.com:
plastic
restriction.
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).
Perhaps a T piece in the pipe which feeds a pressure sensor? No restriction in flow but requires some pressure in the pipe.
Altenatively look at impeller driven units.
Alan
-- Sell your surplus electronic components at
Hope this helps. Kim
John_H wrote in news: snipped-for-privacy@4ax.com:
A simple mechanical float and microswitch arrangement at the bottom of the tank perhaps?
G-- "Men will never be free until the last king is strangled with the entrails of the last priest." (Diderot, paraphrasing Meslier)
Check out
Dan
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
John_H wrote:
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
They also have a cheaper version FS02
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
You will have to work out the connections to the pump but it fairly easy to work out.
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
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?
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. :)
-- John H
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
Dan H wrote:
John_H wrote in news: snipped-for-privacy@4ax.com:
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.
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.
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.
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
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.
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
Here's a table of chemical resistance properties for various "plastics".
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