gauge pressure of water 1

[janne102]

My problem is to measure the water depth of ocean water, but only when the battery operated sensor unit is submerged. The sensor unit must operate on very little power and autonomously so I don't have information about when it is submerged, splashed on, or when it only reacts to barometric changes.
I have looked at pressure sensors but have a hard time finding sensor that operate in ocean water and nasty conditions, while recording a differential pressure. I have a good dsign for capacitive read-out electronics that is low on power so the best would be to find someone who can deliver just the pressure sensor chip, without electronics. Any ideas?

 

[MikeHalloran]

Look at the water level/ fill stop switch in a dishwasher. Chances are it's a cheap mechanical switch, operated by pressure on a diaphragm that is effectively part of the enclosure.

So put your pressure sensor in a box with a similar diaphragm in its exterior, and use a similar switch on the inside to control power to the sensor.

Use a syringe to fill the sensor's exposed cavity with silicone grease.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[felixc]

I guess that you don't expect this to be a cheap system. Have you googled for piezometers? I don't know about your requirements (pressure range, precision), but an example of proven technology is Geokon. You may ask them questions about saltwater compatibility with the stainless steel that they are using.

 

[OperaHouse]

So what is wrong with stainless? Many microprocessors have a low current sleap mode that they can go into. Turn on can be as simple as as two contacts on the body. Conduction tells the micro to turn on. You mentioned "differential." A small differential at high pressure may be harder to find. Engineers deal with numbers not fluff specifications. You will get more help if you are specific.

 

[felixc]

Not every flavor of stainless is ideal for saltwater. For materials compatibility issues you can post questions in the corrosion engineering forum.

 

[janne102]

The formal specifications are easy to satisfy: range of 0-10m of H2O with a resolution of about 100mm H2O. The hard part is to operate autonomously for 10 years, on a battery. Hence, the sensor must have a fairly stable offset and also not be affected by barometric pressure changes. Piezoresistive bridges tend to consume "a lot" of power and so I have so far focused on capacitive read-out. Thank you for helping me out here...

 

[OperaHouse]

That's the job I want! Design something and throw it in the ocean for 10 years. Then I can go on my next interview and say, "I designed a product and have not had a single reported problem in 9 1/2 years."

There is still a lot that has been left out like why if it has to run for 10 years on a battery for 10 years it even needs to be turned on and off. And, "not be affected by barometric pressure changes". If I remember right, that pressure adds to the water pressure. To eliminate that you need to run a tube to the surface. I'd be more concerned about aging over the years. All I know is if you put something in water, things start livinf in or on it. I would suggest a bulbous silicone effector that is filled with liquid. That would allow things to grow on it and you could use any presure sensor in the protected environment. If this were a stationary system that could always stay vertical (and size not a problem), I would use that fluid to fill a glass tube that has a gas charge on it. Capacitance would change as the dielectric fluid replaced the gas.

 

[BrianR]

Tha gage pressure of water will be influenced by atmospheric pressure so you must externally compensate for that. There are many sealed pressure transmitters having all stainless steel outer including the diaphram, or you could make your own by attaching a strain gauge to a suitable diaphram. You'd also need a temp sensor to correct for temp non-linearity, but that is also needed to correct for water density.

The low power consumption is do-able using a CMOS processor that spends most of it's life asleep and pulses on for perhaps 1s from time to time and takes a measurement. High quality lithium batteries should survive for 10 yrs.

At the end of it all you need to persuade all those little marine critters that this is not a good place to call home.

 

[Comcokid]

Just completed a battery operated design using a GE/Druck stainless submersible sensor which was custom made without any electronics (much of what Druck supplies is custom) - and -yes, they are expensive. Submersible sensors of this type have a vent tube inside the cable, but you will probably want just a sensor element without cable and enclosure.

Can't say what kind of gunk will build up on a sensor element in 10 years of salt-water exposure, but I do know that any stainless steel will corrode in saltwater in the proper circumstances. You may need a sacrifical anode in your design.

Ten years on batteries is similar to products made for the Automatic Meter Reading (AMR) market (desired battery life 10 to 15 years). Water and gas are interrogated by a RF link at a short distance (electric meter applications, of course, have a source of power). This market uses Photo-Lithium (LiMnO2) batteries for low temperature (-40C) and very low self-discharge charcteristics. This market also makes use of TI MPS430 micros which have superior sleep current requrements. This popular micro family was originally designed for the AMR market.

Write your software so the device is mostly in sleep mode. When it senses that pressure is present, it can assume that it has been deployed and revert to the desired operation schedule.