Detecting moving air/water in a pipe

[cobra1]

i am trying to find a way of detecting moving water and air in a pipe.

should the water or air stop flowing then i need to activate an alarm, i am trying to find a way of sensing this movement without costing a small fortune for some sort of sensor.

i have 2 pipes that need monitoring, 1 pumps water (water is always in the pipe, i need to know when its moving)

the other pipe has water in it but also has air being pumped through it, i need to know if the air stops.

any help/ideas etc would be great

 

[canadaelk]

What are the diameters of your pipes and the flow-rates? If you mount a straingauge inside the pipe the "relaxed" state = no flow, when bent a change of resistance will indicate movement. The quicker the movement the mor R change. E

 

[Reloadron]

Does the pressure drop if either stops flowing?

Ron

 

[cobra1]

the water has a flow rate of 700lph or 11.6lpm. a strainguage in this case may do the trick. pipe diameter is 15mm. will have to see if a strainguage will fit inside this size pipe.
the air has a flow rate of 70lpm, pipe diameter 15mm also.

there will not be a difinative pressure drop if either stop flowing, pressure switches etc will not work.

i think i might have seen a device ages ago but cant find it now, it was a coil wrapped around the pipe, somehow it was able to tell if the water inside it was moving. can anyone elaborate on this??

 

[MikeMl]

For Air, you can put a counter-balanced, horizontally-pivoted vane that is pulled down by gravity, and lifted by air flow. A microswitch, magnetic or optical pick-up can detect the vane position.

For Water, a minimum pressure drop method would be to use a flow sensor like this one. If you can tolerate a higher pressure drop, use a venturi and two pressure sensors in a bridge.

 

[canadaelk]

Cobra: Thanks for reminding me! There is allways so much stuff, one forgets! The part is a Quantum Research QT114. Info on teir site Atmel Products - Touch Solutions. Atmel also makes such a part. There was also somthing here on general eclectronics chat/4140d1270902032, capacitive water sensor. I am not sure it will work on air though! E

 

[crutschow]

To detect air flow you could also use a thermistor. You put a voltage across the thermistor sufficient to heat it perhaps 20C above ambient. Air flow will reduce it's temperate, changing it's resistance and it's current, which you monitor. If the ambient air temperature changes significantly, then you may need a similar thermistor outside to differentially compare the two currents.

 

[Reloadron]

To detect air flow you could also use a thermistor. You put a voltage across the thermistor sufficient to heat it perhaps 20C above ambient. Air flow will reduce it's temperate, changing it's resistance and it's current, which you monitor. If the ambient air temperature changes significantly, then you may need a similar thermistor outside to differentially compare the two currents.

Years ago before we went to capacitance type vacuum gauges we used a gauge that worked on that principal. Thinking they were called thermocouple vacuum gauges. A voltage was placed across an open filament and in the center of that filament a tiny thermocouple junction was connected. Think tungsten filament like in a projector bulb less the glass bulb. As pressure in the line decreased the filament would glow brighter and the temperature at the junction increases.

I figure with airflow over the filament removing heat the output of a TC junction would remain low but if airflow stopped the temperature at the junction would increase quickly.

Anyway, I like the thermistor idea. Could be simple and inexpensive.

Ron

 

[3v0]

The original post said

the other pipe has water in it but also has air being pumped through it, i need to know if the air stops.

Is anyone taking this into account ?

 

[Vizier87]

I'd like to propose this design by **broken link removed** as well for your flowmeter part. It might help. It's a thermal anemometer, but since it's temperature compensated, it might be able to detect flowrate as well, giving a flowrate versus voltage reading. If you can use it and give us feedback about it's efficacy. I'll learn something as well.

 

[cobra1]

hi guys, the ideas/circuits you mention using a heated component (thermister/transistor) wont work, the pipe has water in it with air bubbling through it, this means at times the comonents could get flooded with water. i dont think this will work like i need it too.

 

[Vizier87]

the flowing water only needs to flood the twin transistors, it'll not affect anything. Just avoid the water getting to the rest of the circuitry, perhaps you can resin the gaps or opening or any channeling of water to avoid water from flooding the rest of the circuit. If you have been working around only 5V electronics you'll know that at that voltage no movie-styled "bang" can be produced. Just from experience.

 

[Reloadron]

hi guys, the ideas/circuits you mention using a heated component (thermister/transistor) wont work, the pipe has water in it with air bubbling through it, this means at times the comonents could get flooded with water. i dont think this will work like i need it too.

We are slowly working away from inexpensive. Other options include inline paddle wheel or turbine sensors. Your pipe diameter of 15mm is just over 1/2" US standard which is actually pretty small at o.59". There are also flow switches made by outfits like **broken link removed** I use their flow switches in RWS (Reclaimed Water Systems). Since you aren't concerned with flow rate but just the presence of flow a simple flow switch should work in both your cases, though I am unsure of the air & water mix line.

Ron

 

[crutschow]

hi guys, the ideas/circuits you mention using a heated component (thermister/transistor) wont work, the pipe has water in it with air bubbling through it, this means at times the comonents could get flooded with water. i dont think this will work like i need it too.

You can buy sealed thermistors that can tolerate water. The rest of the circuit would be outside the pipe.

The air in the water should cause fluctuations in the thermistor resistance which you should be able to detect. You would have to experiment to see how much variation there is. It may require a microprocessor program to automatically detect the presence or absence of this variation to signal an alarm.

Remember, cheap is not always easy. It often requires some ingenuity and experimentation to get the desired result.