Water pressure sensors


Thread Starter


I am currently doing my final year report (FYP) entitled "intelligent children safety jacket". When the jacket goes overboard, the sensor will detect the water pressure, therefore the jacket will inflate automatically. So, what is the most suitable water pressure sensor used for this life-saving jacket sensing device? Thank u... best regards.
My opinion only, but consider re-thinking the idea of using a pressure sensor. I'm coming up blank for an easy and foolproof way to use one for your stated purpose ... guessing by the time you'd be able to get a high enough pressure to keep you of out of the 'false trip' range you'd already likely be under water.
Hmmm. Another minimal information post seeking design advice for a school project. OK, It's a lazy day Saturday and I can play academic advisor.

Let me get this straight, the life jacket, kept in storage for months, years, decades, perhaps, has to have a (presumably) non-powered sensor (or are there batteries involved?) that reacts to water pressure to inflate automatically? We don't know whether this is for small boats, aircraft, cruise ships, amphibious landing assault craft, 3rd world ferry boats that have a tendency to capsize, or bathtubs, but application is of little concern in academia. So . . .

The sensor of choice is a water pressure sensor, presumably because the life jacket wearer is sinking and the head pressure of the water over the sinking wearer's body is sufficient to activate a valve? Hmmmm.

Now let's see. Pressure is force divided by area and a life jacket that can provide sufficient bouyancy to float an overweight man of 140Kg surely has sufficient area to apply a force to pressure sensor. That will work.

Let's put a lightly filled bladder inside the life jacket, so that when the water head applies pressure to the bladder, the internal fill gas of the bladder will exert a pressure on an actuator mechanism which will either trigger the release valve directly or iniate some action to trigger the release valve (for which we have no details and you don't get your whole assignment done in one post on the internet).

Let's pick a bladder, like an aluminized ballon, similar to the ones party vendors fill with hydrogen for anniversaries, birthday parties and the like, because we all know plastic is porous and eventually diffuses the positive gas pressure that has to be maintained for at least a decade. Aluminized balloons can even hold hydrogen for a period of days before deflating. Your job is to research the porosity of such and find a fill gas sufficient to not leak for a decade (that's 10 years), minimum.

So we fill and seal this bladder with an actuator - a diaphragm and actuator pin - connected to a microswitch at an access port on the bladder so that the release valve triggers an "INFLATE" action, caused by the increase in pressure on the bladder which presses on the diaphragm which presses on the actuator pin. The actuator is just like the "Dry Bones" song: (guitar chords in [ ] parentheses):

Your [F] toe bone connected to your foot bone
Your [Gm7] foot bone [C7] connected to your [F] ankle bone
Your ankle bone connected to your leg bone
Your [Gb6] leg bone connected to your knee bone
Your [G6] knee bone connected to your thigh bone, etc. etc. etc.

Now, without considering application data upfront, I'll ask, as an academic advisor should, how does one deal with the ocean liner/cruise ship life boat drills where all passengers have to don their life vests and assemble at their appointed life boat? People milling about, crew members shouting into bull horns, general confusion, but no one bumps into one another, right? Because if they do, will the bump, which is a force, apply sufficient force over the area of the bladder to trigger an "INFLATE" ? Because the design is a pressure sensor, right? Force divided by area. Basic physics/chemistry.

When these life jackets are stowed away somewhere, they always folded and stacked and carefully placed in stowage by persons who take great pride in their work, and take all efforts to make sure that whatever method of stowage is needed to prevent accidental "INFLATION", they're the folks to make sure that these life jackets are properly stowed and secured. Sure.

Nothing will be stowed on top of these, the weight of which would be similar to the weight of of the head pressure of the water column above the sinking person? Because the pressure sensor doesn't know where the force that is applied to the area of the bladder comes from. It could be a bump in the night or the weight of a briefcase.

So there you are. A pressure activated life jacket design ready to go.

The assignment premise is flawed. If the children are intelligent, they don't NEED a sagety jacket. "They who would give up liberty for temporary security, deserve neither liberty nor security" -- Benjamin Franklin. ;-)

Michael Griffin

With regards to false triggering due to contact forces, you may wish to consider enclosing the bladder in a perforated box. As for the other aspects of the concept, I will leave those as an exercise for the student.
may i know which school are you from and perhaps we can discuss together as i am also a fyp student doing simliar project.

Bruce Thompson

I am curious why the efforts are seemingly being spent on active water pressure sensors.

When someone enters below the surface of the water, two things happen: The pressure on the life jacket (person) increases and the life jacket gets very wet.

What is wrong with designing a simple passive switch that opens up when it gets wet? The switch can be designed to last for years in high humidity environments and not actually trigger until it is 'wet'.

The remaining design of the gas bottle, etc. is left as an exercise for the student.