I'm looking to build a water level sensor using a pressure sensor. They're quite pricey so I don't want to just buy a bunch and try it out.
Some research made me settle down on either a MS5540 or M5541 or related type. The problem is that I don't fully understand the workings of these sensors, and what they really measure.
This are "absolute" sensors, so with some waterproofing you can dunk them in the water and take a reading, they don't need a reference. That's how I want to use them. The MS5540 is rated to measure up to 1200 mbar, which is slightly more than the ambient air pressure. The 5541 can do 30 bar (i.e. 30 meters of water).
When exposed to the air, do they read 0 or around 1000 mbar - which is the typical air pressure at sea level? I just can't figure that one out. I expect it returns the current air pressure, which would make it all but useless for any serious water level sensing.
With 1,200 mbar tops, if it starts with the air pressure, this would mean this sensor could measure up to 150-180 mbar of water pressure (after deducting the extreme high), or 150-180 mm worth of water level, making it more of a vacuum sensor.
So then I'd have to go for the 5541, which can do up to 30 meters of water pressure. That's a bit overkill as I'll be measuring 150-1000 mm levels, so I'd be below 3% of the sensor range, often below 1%. That usually means you're losing accuracy, fast.
So what do you guys suggest? Which of these sensors would be suitable? I like them for their inherent waterproofing and ability to work under water without any serious modification (some hot melt adhesive or epoxy or polyester resin should do the job), on top of the really small size of the unit. It's just that these two types are quite pricey, and they seem to be the lower priced MS5xxx sensor already with the others like double the price.
Please be more specific about what you want to measure.
If you are measuring water level in a tank, and the sensor is at or near the outlet and can be exposed to air, then you want a gauge (or gage) sensor, not absolute.
These measure the difference between the current atmospheric pressure and the water pressure, which gives a very accurate measurement of the water level.
For many years I have been using the rather pricey Omega PX309 series, which are extremely reliable and accurate.
Sorry forgot the levels. It's a water reservoir for hydroponic applications. Basically clean water with ionic content (so it's pretty conductive), water depth from 0-100 cm (or a little more) would be a good range. Current air pressure I will know as well (using a BMP180) so can compensate for that.
Currently measuring it using an HC-SR04. Works quite well but sensor is not waterproof and needs rather careful installation, also needs to be calibrated with the actual depth of whatever container is being measured. A pressure sensor in contrast can be dunked in the water, sink to the bottom, and gives the real pressure. Waterproof versions of this same sensor cost about 10x as much and have a minimum distance of 20 cm which is way too much for my application (highest water level can be as little as 1-2 cm under the sensor, which is already pushing the limits of the HC-SR04).
I've looked into capacitive measuring - not suitable as it also requires careful installation, is not portable between different sized tanks, and will be influenced by the medium: as said it's water with nutrient solution, and this nutrient solution can vary greatly over time.
based on the project, a simple, open air, water based tank. consider two arrangements of a magnetic float wtih multiple reed switches.
have one that sits on the bottom with two points. empty and almost empty.
another that sits on the top, and hangs into the tank like an aquarium heater.
simple installation into a PVC tube, you can 'calibrate' with altering wire lengths before sealing it up.
I use a JSN-SR04T with the circuit board, in a 3 inch PVC pipe, it is taller than the water level, but the tube reduced all noise and the readings are smooth. cut to length.
Pressure would be a nice single point, but getting a water proof, immersible sensors for ultra-low cost is difficult.
The MS5540, MS5541 and (most of) their siblings are water proof (the top part at least - the PCB part has to be waterproofed, which shouldn't be too hard: epoxy or putty or hot melt glue come to mind), and designed for just this kind of applications. That's why I was looking so closely at those exact sensors. They're unfortunately rather pricey at about USD 6-12 a piece, which is still not too bad compared to other such sensors. They also use a weird SPI type communication - though some more expensive types also have an I2C option which would make life much easier.
The final contraption is to be installed in various tanks, dissimilar in size and shape and material. Installation should be dead easy - the sensors are to be sold to end users. For those reasons a PVC pipe or sensors at multiple levels are out, far too complex. Simplicity in use is much more important than cost. The whole PCB for this sensor, which should also include an EC probe, would be as small as 13x17 mm, or even less, which is also something I really like.
I know I'm a bit late to this, did you find a solution?
I noticed something that no-one else had mentioned - your depth/Bar calculations are incorrect -
30 Bar is actually a depth of 290meters
Sea water has a relationship of roughly 1 Bar per 10metres depth.
Because you say that the maximum depth your water will be is 100cm, the 1200mbar rated sensor should be absolutely fine, even if it reads as 1Bar before submerging:
Total rated = 1.2 bar
1bar for ambient air pressure. = 200mbar
this allows you up to 2metres of depth which this logger should be able to record.
Well a few days ago I received my prototype boards. In the end I decided to go for a MS5837-02BA sensor, the successor of the 5541. Even smaller (those things are tiny!) and 24-bit resolution.
Unfortunately not much testing done as the factory attached a far too short cable (barely 10 cm, while I need at least a meter. So ordered some of suitable cable and when that's in can fix it, pot it in the housing with epoxy, and will be able to finally really test it out.
I'm not doing scuba, and normally don't manage to go deeper than about 4 meters (and that's not a factor 10 off :-)) - which is deep enough to noticeably lose buoyancy.
My latest projects were with an MPXV5004. The MS5837 in combination with BMP180 (or the 280) works fine as well. As does the HC-SR04 measuring from above.
