Water Pressure Sensor
- Thread starter LizzieB
- Start date
Rickharris
Senior Member
Do you want to measure absolute pressure or just some spot level.
As an aside I would warn you against using picaxe in a safety critical situation - Underwater may just be such a situation!.
As an aside I would warn you against using picaxe in a safety critical situation - Underwater may just be such a situation!.
Freescale.com may have free samples for these as well. I recently got 2 of the MPXM2010GS version free from them.
premelec
Senior Member
If you find you need some isolation of the water to the transducer and can stand some damping [e.g. you are not using the transducer as a microphone...] you can put a few inches of silicone oil in the tubing by the trnsdsucer. Another method of isolation is to use an immersed pipe which you blow air through bubbling out at the level you want to measure and measure the pressure of the _air_ pressure it takes to make bubbles... Be careful of water 'slam' if you do a direct fluid into the transducer connection as stopping flowing water can produce much higher than static pressures if turned off suddenly. Have fun...
Most transducers can handle water directly. Some have an internal sealed cavity which acts as the reference, and this might be calibrated at sea level or a vacuum. Some have two ports and measure the difference, so you could have the transducer under water and the reference port could be a pipe up to the air. Parts vary from $25 to $1000's. Farnell and Radio Spares have a number of sensors though you have to really search to find the reasonably priced ones. Sensortechnics http://www.sensortechnics.com/ are great with very reasonable prices but the minimum shipping is expensive. Sometimes transducers come up on ebay for good prices. Is the water fresh or salt water?
demonicpicaxeguy
Senior Member
if you need it for water level monitoring how about an ultrasonic distance measurement module? all you do is mount it facing down inside the tank all you need to know then is litres/cm if the tank is circular then thats easy but if it's an odd shape then ... have fun
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demonicpicaxeguy
Senior Member
so just waterproof it and stick the ultrasonic module on the bottom
pointing up i dare say with a little adjustment it should probably work
pointing up i dare say with a little adjustment it should probably work
Yes, that's exactly what it needs.
demonicpicaxeguy
Senior Member
Dippy, fisrtly you need the correct transducers for it to actaully be submerged or glued in place up against hard surfaces
eg,http://www.jaycar.com.au/productView.asp?ID=AU5550&CATID=&keywords=ultrasonic&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=
isn't a bad start
as for water proofing it stuff like casting resin,polymorph,liquidnails(don't laugh), (don't try superglue some type are corrisive) is one way
i'd personally use a section of pvc pipe maybe 40mm with two endcaps, on one endcap i'd glue the transducers to the inside of the cap
on the other endcap i'd fix some kind of heavy weight that is just heavy enough to keep it sunk but also the transducer end point towards the surface
all thats needed than is the required length of cable and you're set... just make sure you sealup the two endcaps nice and tightly and where ever the cable comes out preferably near the bottom should also be nicely sealed up
eg,http://www.jaycar.com.au/productView.asp?ID=AU5550&CATID=&keywords=ultrasonic&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=
isn't a bad start
as for water proofing it stuff like casting resin,polymorph,liquidnails(don't laugh), (don't try superglue some type are corrisive) is one way
i'd personally use a section of pvc pipe maybe 40mm with two endcaps, on one endcap i'd glue the transducers to the inside of the cap
on the other endcap i'd fix some kind of heavy weight that is just heavy enough to keep it sunk but also the transducer end point towards the surface
all thats needed than is the required length of cable and you're set... just make sure you sealup the two endcaps nice and tightly and where ever the cable comes out preferably near the bottom should also be nicely sealed up
http://www.sensortechnics.com/index.php?fid=300&fpar=YToxOntzOjQ6InBjaWQiO3M6MzoiMTcyIjt9&isSSL=0&aps=0&blub=427034b7f02c92e64c378cfd5bee1755
And look for the ones that are absolute pressure with 0-1bar, eg BDSX1000A2R
These ones are amplified so they can go straight into a picaxe. Datasheet at http://www.sensortechnics.com/download/bsdx-631.pdf Unamplified ones are cheaper but need an op amp with a gain of 30 or so.
And look for the ones that are absolute pressure with 0-1bar, eg BDSX1000A2R
These ones are amplified so they can go straight into a picaxe. Datasheet at http://www.sensortechnics.com/download/bsdx-631.pdf Unamplified ones are cheaper but need an op amp with a gain of 30 or so.
DPG: Very interesting.
So the coupling between this cheap little air-design transducer and a plastic end-cap/water will be good enough to provide sensitivity over a good distance? I take it that it won't resonate and mess up the returned signal. Has it really got enough ooomph?
Talking of the returned signal, how 'strong' is the echo from an interface compared to a solid object? And by how much will the plastic end-cap attenuate/upset the signal?
We don't know the depth range of this lake so you may need something quite powerful maybe?
I take it that unless you have a radio-bouy that you will also need a cable to the shore?
Have you tried this transducer with your electronics, you sound confident it'll work 'straight out of the box'. Have you any example electronics to drive and recieve as this is interesting?
So the coupling between this cheap little air-design transducer and a plastic end-cap/water will be good enough to provide sensitivity over a good distance? I take it that it won't resonate and mess up the returned signal. Has it really got enough ooomph?
Talking of the returned signal, how 'strong' is the echo from an interface compared to a solid object? And by how much will the plastic end-cap attenuate/upset the signal?
We don't know the depth range of this lake so you may need something quite powerful maybe?
I take it that unless you have a radio-bouy that you will also need a cable to the shore?
Have you tried this transducer with your electronics, you sound confident it'll work 'straight out of the box'. Have you any example electronics to drive and recieve as this is interesting?
demonicpicaxeguy
Senior Member
the only place i've ever seen this particular setup was actaully used in a sydney water storage tank, it was somthing that was aparently trialled a few years ago as a means of allowing sydney water management to see and compile details of water storage levels on demand
the ultrasonic modules that sat at the bottom were linked to a 56k modem which sat in a shed long storey short like good ideas it worked, but as usaul there was a "cheaper alternative"
i certainly do imagine there would be somthing not nessarily benificial caused by the endcap, the reason i suggest the trasducers from jaycar is because it is not open at the front like the srf005, it's solid stainless steel flat surface which you could safely glue to another flat surface with that kind of setup i dare say the interference may either be non existant or not worth worrying about
as for distance, i've never done anything with ultrasonic equipment underwater, but i dare say compared with air, water is much much better when it comes to sound travelling through it you'll probably find it won't take much "oopomph" for the wave to travel up to several meters underwater,
i've got one of the transducers from jaycar, so i might try a few things out once my scope arrives
demonicpicaxeguy
Senior Member
i bet they don't, although with my luck they probably cost several hundread dollars and will also make a coffee on demand.Yes, give it a go. I bet the pukka ones don't use a 5 buck transducer though
demonicpicaxeguy
Senior Member
the uw glider method is a little more attractiveAnyway, stick with the pressure method as I don't think dpg has finished his trans-oceanic gps controlled canoe yet
Peter M
Senior Member
LizzieB, you say a lake... how far out are you trying to measure ie shore to sensor position.?
what time period are you measuring the depth over?
if you use a cable under water for long periods it will eventually get water inside.you could use a long tube to your sensor but this is more susceptible to temperature variations (although this too would eventually drown, just a matter of blowing the water out and reconecting)
Also check this site http://www.smartec.nl/index.htm
have a look at the data sheet page under pressure sensors
what time period are you measuring the depth over?
if you use a cable under water for long periods it will eventually get water inside.you could use a long tube to your sensor but this is more susceptible to temperature variations (although this too would eventually drown, just a matter of blowing the water out and reconecting)
Also check this site http://www.smartec.nl/index.htm
have a look at the data sheet page under pressure sensors
Peter M
Senior Member
No, the tube must be open at the bottom end for the water to enter thereby pressurizing the air in the tube.
It wouldnt last longer than a cable (as far as water ingress), but when its full of water who cares, empty it and go again. cable on the other hand is... well... stuffed
It wouldnt last longer than a cable (as far as water ingress), but when its full of water who cares, empty it and go again. cable on the other hand is... well... stuffed
I was thinking of running the wire inside nylon tubing. That's available in various sizes here along with bulkhead fittings to ensure a waterproof entry to the sensor enclosure. I did think of just running the tube to an onshore sensor as you describe, and I didn't run the numbers but I think the temp compensation might be a challenge.
premelec
Senior Member
As I mentioned a long time ago you can use a bubbler type of structure then the only temperature sensitivity should be from the density of the water - the pressure transducer is on shore with the electronics - and some sort of low volume compressor to blow bubbles out under water at a very slow rate - in short you are measuring water head through pressure and that will vary with water temperature - but what accuracy are you expecting to measure the head / depth to?
Just an idea.
It seems to me that you could do this: Use a tube/pipe full of air, sealed at the TOP, open at the bottom and sufficiently weighted down. The water will try to rush into the bottom, compressing the air in the tube. An air-pressure sensor placed at the top of the tube will tell you the water pressure at the bottom of the tube. No wires or electronics are required to be submerged.
It seems to me that you could do this: Use a tube/pipe full of air, sealed at the TOP, open at the bottom and sufficiently weighted down. The water will try to rush into the bottom, compressing the air in the tube. An air-pressure sensor placed at the top of the tube will tell you the water pressure at the bottom of the tube. No wires or electronics are required to be submerged.
Range approx 5 to 30 feet, accuracy isn't critical but I would hope for maybe +/- 1 or 2 feet.
If I understand this method correctly it would be something like start the pump and then keep sampling the pressure until it stops increasing, is that right?
premelec
Senior Member
Yes basically it will depend how often you need to take a reading... you could have the PICAXE start a compressor [low volume!] and then take readings until reading is stable +/- and accept that as a correct value. Or you could have the compressor run continuously keeping the submerged line clear of water all the time... an easy opening check valve can be used to keep water from backing up the line but it must be low pressure to open to prevent decreasing accuracy...
It depends how covert this needs to be - a line of bubbles might give away our secret submariner! What exactly is the application?
My dam is almost empty and I've had an idea of putting a sensor at the bottom while I can get there easily and then measure the pressure as it fills up. Thinking about how to solve this problem, I'd probably go for a sensor that reads absolute pressure so it only has one port. Making things waterproof is another problem - maybe pot the sensor connections in fibreglass resin.
My dam is almost empty and I've had an idea of putting a sensor at the bottom while I can get there easily and then measure the pressure as it fills up. Thinking about how to solve this problem, I'd probably go for a sensor that reads absolute pressure so it only has one port. Making things waterproof is another problem - maybe pot the sensor connections in fibreglass resin.
Water pressure
Lizzie:
Suggest you look at this->
http://www.akcontrols.com/pages/General PDF/Manual AK70.pdf
Works very well. You would need a 1 bar pressure transducer, this will give you 0-10 mts (4-20 Ma). Easily read by readadc by wiring the output across a 250 ohm resistance to read 5 volts.
jaka
Lizzie:
Suggest you look at this->
http://www.akcontrols.com/pages/General PDF/Manual AK70.pdf
Works very well. You would need a 1 bar pressure transducer, this will give you 0-10 mts (4-20 Ma). Easily read by readadc by wiring the output across a 250 ohm resistance to read 5 volts.
jaka
inglewoodpete
Senior Member
premelec's idea of using a low volume air compressor is a good one.
Stagnant air trapped in a tube will gradually disolve in the water. This would result in a progressively smaller volume of air, and consequently the surface level of the water in the tube would rise, giving incorrectly lower pressure readings.
Stagnant air trapped in a tube will gradually disolve in the water. This would result in a progressively smaller volume of air, and consequently the surface level of the water in the tube would rise, giving incorrectly lower pressure readings.
Would think the most accurate way of doing this would be to;
1. turn on the pump
2. run for say 1 minute (long enough to be sure all the water is pushed out of the tube)
2. close the pipe with a valve between the pump and the pressure sensor.
3. turn off the pump.
4. wait for pressure to stabilise (or sample pressure reading until it does).
5. record pressure reading
6. convert pressure reading to depth
7. log.
This process will eliminate the variations that would otherwise occur with the pump pushing air down the pipe and any variations in that air flow. Backpressure on the pump with it running will not be as stable as static air in the pipe compressed by the water pressure.
This scheme combines the bubbler with the static column eliminating/reducing the problem areas of both.
If there is concern about keeping the tube clear, then the pump could run all the time except when the reading is taken.
The valve could be anything simple - like a solenoid squashing the pipe, or a peristaltic pump could be used as it won't leak air when stopped. These are actually quite easy to make - a slow geared motor from almost anywhere, a disc with two to four rollers and a track with soft flexible hose - plenty of info on the web e.g. http://en.wikipedia.org/wiki/Peristaltic_pump.
The hardest part is getting the right tubing - needs to be flexible and survive being squashed flat - silicone rubber is a frequent choice.
1. turn on the pump
2. run for say 1 minute (long enough to be sure all the water is pushed out of the tube)
2. close the pipe with a valve between the pump and the pressure sensor.
3. turn off the pump.
4. wait for pressure to stabilise (or sample pressure reading until it does).
5. record pressure reading
6. convert pressure reading to depth
7. log.
This process will eliminate the variations that would otherwise occur with the pump pushing air down the pipe and any variations in that air flow. Backpressure on the pump with it running will not be as stable as static air in the pipe compressed by the water pressure.
This scheme combines the bubbler with the static column eliminating/reducing the problem areas of both.
If there is concern about keeping the tube clear, then the pump could run all the time except when the reading is taken.
The valve could be anything simple - like a solenoid squashing the pipe, or a peristaltic pump could be used as it won't leak air when stopped. These are actually quite easy to make - a slow geared motor from almost anywhere, a disc with two to four rollers and a track with soft flexible hose - plenty of info on the web e.g. http://en.wikipedia.org/wiki/Peristaltic_pump.
The hardest part is getting the right tubing - needs to be flexible and survive being squashed flat - silicone rubber is a frequent choice.
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I saw a commercial bubbler that had two air lines. One on the bottom and one a known distance (verticle) above it. This allowed for scaling to correct for water density. It also had a port to absolute atmosphere, and temperature sensing. Probably a bit much if not for scientific or commercial use, but I liked the two bubbler lines.
Ken
Ken