How to detect that my device is underwater?

[tjvv]

Hi,

I have a waterproof smartphone with sensors like barometer, gyro, accelerometer, microphone). I want to do an app that
1) detects that the phone is underwater: I though in using some reading from touch panel as water is electro conductive and it may provide me some reading to indicate that. Another idea is to use barometer to detect is underwater due pressure variation

2) Detect how depth underwater my smartphone is: I am trying to do that using the barometer.

Questions I have:
1) What would be a good sensor to detect my device is underwater?

2) Does the barometer works underwater? Is there any kind of correction factor to add/subtract in order to read proper underwater pressure values hence calculate depth?

thanks a lot!

tj

 

[jedishrfu]

What about just using cell phone signal strength?

It seems it would drop off quite quickly. As an example, this article on sub comm mentions limited reception of 3-30KHz radio waves at upto 20 meters deep.

https://en.wikipedia.org/wiki/Communication_with_submarines

vs cell phone frequencies:

https://en.wikipedia.org/wiki/Cellular_frequencies

Also you might be able to use a light sensor to determine depth as the light diminishes as you go deeper.

Neither of these approaches are very accurate but the devices are already on your phone.

Also were you to enter a faraday cage then the cell phone signal strength would drop off as well and for the light sensor approach light intensity varies throughout the day and so it may not work very well either.

 

[CWatters]

I don't know for sure but...

The barometer in a smart phone is probably intended to measure air pressures over the range typically found on earth. It appears reasonable that you should be able to calibrate/set the sensor to zero on your desk at sea level and then travel to the dead sea where it will read -1400ft.

According to google the dead sea is at about 1065 mbar. The standard for sea level is about 1013 mbar. That's a positive pressure difference of around 1065-1013 = 52m bar

Now unfortunately water is about 800 times denser than air. 52 mbar of water pressure is achieved at a depth of about 20 inches of water. So it seems possible that the pressure sensor would quickly become overloaded unless it's designed for much higher pressures than found at the Dead Sea.

 

[David Lewis]

When a thermistor with current flowing throught it is submerged, its temperature goes down, and resistance will change.

 

[CWatters]

But it wouldn't change with depth?

 

[David Lewis]

No. When the current-carrying thermistor is surrounded only by air, it will be hotter than when it is submerged in a liquid.

 

[tjvv]

Thank you.
Light sensor and RF signal do not seem will work as they can fade away irrespective if I am underwater or not.
Barometer seems also will not work as it is designed for air measurements (not water)
I was wondering if the fact that the touch panel goes crazy underwater may provide a typical reading that I can assume with a good margin of precision that the device is in a liquid environment.

 

[Arqane]

I think the iPhones started putting a more detailed pressure sensor in, so they could determine light or heavier taps. Other phones might have that as well. If you can get the pressure data from the various inputs attempted, that would probably be enough. You can assume that if multiple inputs were all pressed with close to identical levels, then it's in a liquid, rather than just putting your hand on the whole thing.