submerging a pressure sensor under water?

I'm trying to build a water level sensor using a differential pressure sensor, which you can see here:
https://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/partDetail.jsp?SKU=78H2274&N=0

I've seen a few designs on the web about how other people have used these sensors to measure water level. The sensor gets placed in a watertight container and submerged to the bottom of the volume of water you're trying to measure. Using some fittings and some tubing, one pressure port on the sensor is connected to the ambient air atmosphere via a long tube that rises up and out of the water. The other port simply gets ported to the surrounding water. However, I'm unsure about using this sensor in direct contact with water. The datasheets say "These sensors are intended for use with non-corrosive,non-ionic working fluids;such as air and dry gases." Would water mess it up? I've found a few other "wet/wet" sensors, but they don't have the right pressure ratings, temperature compensation, or voltage requirements for my circuit.

So can I use this sensor with one port in direct contact with the water? Or is there some kind of diaphram I could use to keep a little air in between the water and the actual sensor?

Thanks for any advice.

You don't have to place the sensor under water to measure the pressure and hence the water level.
Instead, use compressed air in a tube that Tee's from the sensor to an outlet at the bottom of the tank.
As soon as the air pressure in the tube overcomes the water pressure at the outlet it will escape in a stream of bubbles. The air pressure in the tube at this stage has a direct relationship with the water level above the tube outlet.
All you need to do is calibrate it.
Where to get the air pressure from? It can be as simple as blowing into the tube until the bubbles escape or you could try one of these squeeze bulbs like used on blood pressure monitors.
The pressure sensor can be anywhere in the line, preferably above water level so there is no chance of flooding it due siphoning.
Klaus

Thanks for the reply, but this is for a river level data recorder that will be deployed for months at a time, so a bubbler method isn't really feasable.

Providing you can really make the enclosure watertight it should work. You'll have to provide some sort of screening to keep the crud out of the inlet, what are you going to use for power/data lines? The pinouts are different on some of the packages (1 where 8 usually is) or the picture is transposed..Is the water current strong? That may influence the readings..

The sensor will be in a stilling well (with screended holes) installed on a bridge abutment, so crud and current won't be a problem. The sensor only needs 3 wires: Vref, +Vout and ground. They'll come up to the surface inside the tubing that connects the inside of the container to the atmospheric pressure above.

My main concern is whether the actual strain sensing device inside the sensor can handle direct contact with water.

Hadn't thought of the water contact aspect You might have to go with some sort of diaphram, thin rubber (ballon) to isolate it. As the diaphram is pushed inwards from the water pressure it will change the pressure to the sensor. It will have to be flat accross the inlet , but not stretched so tight it wil take a lot of pressure to deflect it..

In one approach you have a submerged sensor with a tube that goes to the surface to sense the air pressure. Any water leaks into the tube will foul things up.

In the other approach the sensor is above the water and the pressure sensing tube is submerged AND the sensing tube must be full of air (or other gas) in order to correctly sense the pressure at depth. A minor leak in the pressure sensor or tube would let an initial charge of air leak out.

In either case preventing leaks can be a problem and this doesn't settle the issue of whether or not the sensor can be exposed to clean water.

I wouldn't abandon the bubbler concept. It adds some complexity but allows some simplicity in terms of sensors and minor leakage or dirt is less of a problem. They do make small air compressors (like the digital blood pressure monitors use) or larger ones such as those used in my truck for the auto levelling system. Batteries with solar charging might be one solution with a small storage tank. An alternative, possibly more reliable, is compressed air or nitrogen. A bottle would last quite a long time. Propane or some other liquid-to-gas fluid might work as long as combustibility is addressed.

1. You should simply use a part intended for water contact. There are plenty out there. Check with Freescale semiconductor. The spec sheet for one I use says use with other than dry air can affect the reliability, but it doesn't say it's not suited. In fact, under the applications, it does say "washing machine water level". It would be wet for that.
2. How are you going to be sure water won't get into that vent tube?

How about coating both ports of the sensor with silicone grease? This will transmit pressure while keeping water away from the sensor.

What about using PVC pipe? Cap one end with the pressure sensor inside, make the seal air tight. Holding upright so no air (or very little) air escapes hammer it into the river bed. Make sure there is a hole or some way for the water to get into the base of the pipe. The deeper the water the higher the air pressure will be inside the pipe.

Dry option

Air displacement could be the best method ?
As dingo suggests this keeps the sensor dry and should be maintenance free?

If the idea is taken one step further ...
A displacement chamber is placed close to the river bed, open at the bottom to allow water in, has a flexible pipe which leads up to the surface. any pressure inside the vessel will be seen by the sensor which can now be above the water.

Things to consider here are ...

Air is compressible - as the river level increases so too will the water level inside the vessel, this will give a small range error. If the vessel has a surface area which is massive compared to the volume of the flexible pipe then this error will be minimised - or calculated and allowed for? (chemical engineering books show calculations for this sort of thing).

The above situation would be aided by using small bore pipe (capilliary tube?).

The system must be full of air - maybe a bike tyre pump could be used to fill the system after installing.

The vessel will be bouyant :!: - weights or a rigid mounting will be needed to keep it in place.

Pascals law...

j.

Re: Dry option

mechie said:

Air displacement could be the best method ?
As dingo suggests this keeps the sensor dry and should be maintenance free?

If the idea is taken one step further ...
A displacement chamber is placed close to the river bed, open at the bottom to allow water in, has a flexible pipe which leads up to the surface. any pressure inside the vessel will be seen by the sensor which can now be above the water.

Things to consider here are ...

Air is compressible - as the river level increases so too will the water level inside the vessel, this will give a small range error. If the vessel has a surface area which is massive compared to the volume of the flexible pipe then this error will be minimised - or calculated and allowed for? (chemical engineering books show calculations for this sort of thing).

Click to expand...

The fluid will always require some form of purging. It might not need it that often. An air reservoir at the top will reduce its need, but hardly eliminate it. One, thermal variations will cause the air to expand, bubble out, and contract sucking water up the hose. Also, some air does dissolve in water. Adding air to the reservoir isn't a problem but dissolving it away is.

A small air pump would do it. A microcontroller could do a great job by taking an initial pressure reading, then run the pump and see if the pressure is rising. If it doesn't, it's safe to assume the pipe is now empty of water and it's just bubbling air out the bottom. It could even record the history and come to its own conclusions about how often a purge is needed to minimize power consumption while staying within specified tolerances.

Re: Dry option

Oznog said:

mechie said:

Air displacement could be the best method ?
As dingo suggests this keeps the sensor dry and should be maintenance free?

If the idea is taken one step further ...
A displacement chamber is placed close to the river bed, open at the bottom to allow water in, has a flexible pipe which leads up to the surface. any pressure inside the vessel will be seen by the sensor which can now be above the water.

Things to consider here are ...

Air is compressible - as the river level increases so too will the water level inside the vessel, this will give a small range error. If the vessel has a surface area which is massive compared to the volume of the flexible pipe then this error will be minimised - or calculated and allowed for? (chemical engineering books show calculations for this sort of thing).

Click to expand...

The fluid will always require some form of purging. It might not need it that often. An air reservoir at the top will reduce its need, but hardly eliminate it. One, thermal variations will cause the air to expand, bubble out, and contract [bleep] water up the hose. Also, some air does dissolve in water. Adding air to the reservoir isn't a problem but dissolving it away is.

A small air pump would do it. A microcontroller could do a great job by taking an initial pressure reading, then run the pump and see if the pressure is rising. If it doesn't, it's safe to assume the pipe is now empty of water and it's just bubbling air out the bottom. It could even record the history and come to its own conclusions about how often a purge is needed to minimize power consumption while staying within specified tolerances.

Click to expand...

What about keeping the water and air apart completely?
A plastic container with the sensor and air inside. The deeper it goes the more the water pressure will bow in the sides of the container increasing the air pressure. You would need to have a container that will return to its initial shape as the pressure decreases. The water current or the river might cause the container to bow, so housing it in a second container with a hole or two in it might solve that.

circuit diagram for pressure sensor...

hey Andy, wondering how far you went with the water level measurement project!
i'm very much intrested in doing a similar project, will be glad if u could show me some circuit diagram on this.
Thanks.