pressure sensor?

I am looking to use a pressure sensor to activate a switch at a certain depth. I am going to contain the arduino inside an Otter 1000 series box, similar to the MIT underwater ROV:

https://seaperch.mit.edu/sensor_package/InstructionsV2_4_1.pdf

It is pretty cool and innovative how they figured this out ! :)

The only difference between this project and mine is that the use for the ROV is much more complicated, i.e. it uses the pressure transducer to convert the electrical signal to x and then uses a formula to convert x to an actual depth reading. My project need only close a switch at a constant depth (1 meter). I picked up the exact sensor, a MXP4250AP-ND:

http://www.freescale.com/files/sensors/doc/data_sheet/MPX4250.pdf

I was in the process of putting this together when I realized it can perhaps be much more easily accomplished by using a simple pressure switch. I know at a depth of 1m the pressure is about 1.2 PSI. The entire electronics box needs to be completely submersible, without a vent to atmosphere.

Does anyone have any suggestions? I am having trouble finding anything like this that has been used before with Arduino.

The pressure at 1m is about 1.45PSI above atmospheric if its freshwater, slightly more if saltwater.

You'll need to calibrate for surface pressure every so often as the air-pressure changes. The device has a sensitivity of 20mV/kPa so 1m of water is about 196mV (or 40 steps of the result from analogRead()).

MarkT: The pressure at 1m is about 1.45PSI above atmospheric if its freshwater, slightly more if saltwater.

You'll need to calibrate for surface pressure every so often as the air-pressure changes. The device has a sensitivity of 20mV/kPa so 1m of water is about 196mV (or 40 steps of the result from analogRead()).

The mix is confusing for me Pressure at 1M is 0.01 bar ? http://www.csgnetwork.com/pressconvtable.html So 1M=196mV(or 40 steps)?

I get 1PSI =6.894757 kPa So 1.45 PSI = 9.9974 kPa

Roughly 10kPa so thats times 20mV/kPa = 200mV on 1024= about 5 per step?

tytower:

MarkT: The pressure at 1m is about 1.45PSI above atmospheric if its freshwater, slightly more if saltwater.

You'll need to calibrate for surface pressure every so often as the air-pressure changes. The device has a sensitivity of 20mV/kPa so 1m of water is about 196mV (or 40 steps of the result from analogRead()).

The mix is confusing for me Pressure at 1M is 0.01 kPa ? http://www.csgnetwork.com/pressconvtable.html So 1M=196mV(or 40 steps)?

I get 1PSI =6.894757 kPa So 1.45 PSI = 9.9974 kPa

Roughly 10kPa so thats times 20mV/kPa = 200mV on 1024= about 5 per step?

That's right apart from the "pressure at 1M being 0.01kPa" which is way off. The symbol for metre is lower case m BTW.

Indeed, give up on PSI altogether and use SI units. Acceleration due to gravity is about 9.8m/s^2, density of water is about 1000 kg/m^3, so pressure per metre of water is 9.8 x 1000 = 9.8kPa (but a little more for saltwater)

Yeh that should have been bar , I corrected in the orig post

However from that link I posted I got
I get 1PSI =6.894757 kPa

1Meter=9.8 kPa so is that posted table right?

I’m wanting to use this in my own device so I’m trying to be sure