Author Topic: Pressure Sensor  (Read 3218 times)

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rahulisola

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Pressure Sensor
« on: May 28, 2008, 08:10:20 AM »
Hi guys....

i'v just finished making a submersible ( an underwater robot ).....

Now, I need to measure the density of the water it is in and send it to a computer wirelessly.....

My Idea: (Addressing the wireless part later) It has a chamber which fills up with water that lets the 'sub' to sink. Now, if the volume of this chamber was known, is there any way to add a few pressure sensors in it and calliberate it to give density, since the volume of the chamber is constant, and the water in it will also have uniform temperature? ( Using Pressure proportional to density, Boyle's Law or something? )

PS: I am new to microcontrollers though I can do coding in C/C++/Java very well. So please help me in this aspect as well...
!\$õ

AndrewM

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Re: Pressure Sensor
« Reply #1 on: May 28, 2008, 09:27:19 AM »
You can use an air pressure sensor contained in a small flexible bubble of air on the outer wall of the sub.  As you go deeper, the water pressure increases, thus increasing the atsmospheric pressure on the sensor.  If I remember correctly from my diving lessons years ago, every 30ft down in salt water equals one atmospheric pressure level.

Depending on how deep your submersible goes, you will need a pressure sensor that can handle well above 101kPa/14.7psi (sea level pressure).
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Saving the world from humanity one robot at a time.

rahulisola

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Re: Pressure Sensor
« Reply #2 on: May 28, 2008, 09:39:35 AM »
thanx AndrewM....

you are right in saying that i can use air pressure sensor.....
but I need to measure pressures with really high presition......
To explain better, i need to measure the % of presence of salt in water.
With more salt concentrarion, density will increase......(so density sensor).
I think an atmospheric sensor won't give such precise readings (will it?)?
The sub needs to measure only the salt content near the water surface, n doesn't need to go in deep...
So, will the atm. sensor give me desired readings? If yes, how much will it cost, and what should i ask for @ shops (ratings etc)?
!\$õ

Brandon121233

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Re: Pressure Sensor
« Reply #3 on: May 28, 2008, 01:39:47 PM »
If your trying to measure the concentration of salt in the water just use a resistance sensor, as the amount of ions (electrolytes) in water increases the resistance decreases. Read these http://en.wikipedia.org/wiki/Electrolyte and http://en.wikipedia.org/wiki/Ion_selective_electrode
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SixRingz

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Re: Pressure Sensors
« Reply #4 on: May 29, 2008, 02:57:55 PM »
Are you talking about measuring the density of the water in the tank? Water has a very low compressibility and if I'm reading the graphs I found on the topic right, water density is changing much more with temperature than with pressure. Salt level is also a factor for density if you are building a sub. Maybe a thermal sensor and measuring electric resistance (salt level) would suffice?
You got me really curious though... why do you need the pressure in the tank?  What are you building?!?  Super-curious...
Grounding things properly means burying them in the backyard...

awally88

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Re: Pressure Sensors
« Reply #5 on: May 30, 2008, 09:50:43 AM »
density of water is a constant 998kg/m^3. The pressure water will change however to the formula pressure=density*gravity*height from surface.

If you have a pressure sensor you can just divide by the constant density*gravity and you will get height.

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Re: Pressure Sensor
« Reply #6 on: May 30, 2008, 07:36:51 PM »
Quote
Now, I need to measure the density of the water it is in

Quote
I need to measure pressures with really high presition

Wait, density or pressure? Precise to how many decimal places?

I use Honeywell pressure sensors:
http://sensing.honeywell.com/index.cfm?ci_id=16046&la_id=1&catId=79830

Brandon121233

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Re: Pressure Sensor
« Reply #7 on: May 30, 2008, 08:33:10 PM »
Quote
To explain better, i need to measure the % of presence of salt in water.
I think he is going about that goal in a very round-about fashion, its a lot easier to take the resistance of that water and match that up to tested values.
Hell, there are no rules here—we're
trying to accomplish something.

—Thomas Edison

AndrewM

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Re: Pressure Sensor
« Reply #8 on: May 30, 2008, 10:16:34 PM »
I think I am lost on what you are trying to accomplish.  If you are measuring the salt level of the water, then pressure would be a bad source of measure.  As Brandon said, resistance might be a better gauge.  Air pressure can change minute by minute practically (i live in Florida where barometric readings can change that fast), and the air pressure will definitely effect the readings gained from a pressure sensor anywhere near the surface of the water.  Once you get down 30-45 feet, it doesn't matter so much.

The only problem with resistance would be that you would not just be measuring salt (Sodium Chloride) content, but salt in the chemist sense of the word (meaning all the chemicals).  But it would still be much more relative than pressure.
blog: www.iamwhen.com
Saving the world from humanity one robot at a time.

kd5kfl

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Re: Pressure Sensor
« Reply #9 on: May 31, 2008, 06:19:36 PM »
Kulite brand pressure sensors are accurate to + - .5%. 10 volts in, .100 V out at max rated pressure. So, 4 mV / PSI for a 25 PSI gage. ( don't waste your time; that's the correct spelling )

Sea water increases in pressure 1 Atmosphere - 14.7 PSI - per 33 feet. If you don't plan on going below ~30 feet, a 25 PSI gage should be fine.

You need a very linear amplifier to get that into an easily readable range.

SixRingz

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Re: Pressure Sensors
« Reply #10 on: June 01, 2008, 12:34:44 PM »
Quote
density of water is a constant 998kg/m^3.
Density of water is NOT constant. It is depending on temperature and pressure! Just google and you'll see I'm right.
Grounding things properly means burying them in the backyard...

paulstreats

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Re: Pressure Sensors
« Reply #11 on: June 01, 2008, 05:52:07 PM »
It also changes due to impurities such as salt (sea). Chlorine/bromide/calcium(tap water). Though it could be argued that things like chlorine arent fully bonded in water you still have to accept that it is present and that it has a different weight and also its own presence in density to water

awally88

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Re: Pressure Sensors
« Reply #12 on: June 01, 2008, 07:20:27 PM »
Hey guys,

Sorry about that, I read through my notes again and I saw that we do just use the constant as an assumption of pure, incompressible water at 300K however as you said it does change as these parameters change.

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