Pressure calculation & WTC's: 
The calculator below allows you to
input any one of the seven options, and will calculate the other six for
you. That enables you to find the pressure at any given depth, the depth at
any pressure, and to convert both depth and pressure measurements in
between.
The calculator gives you the pressure in reference to the surface
pressure. (Called
'PSI gauge') or in other words, the pressure with the surface
pressure as "zero". Your WTC's and modules are sealed at sea level,
which is at 1 atm. The pressure that the WTC / module has to endure is
only the difference between the surface pressure, and the
pressure at any given depth. It is that pressure that the calculator
helps you find.
Tip: If you were to pressurize your modules to say 1 or 2 atm. in reference to sea level, then they could endure going even deeper..
Download option: If you would
like to have this small HTMbased calculator to run from your own
computer, and in your Internet Browser, please feel free to download a 3
Kb zipped version here.
Watertight compartments:
A watertight compartment
holds electronics, valves or anything else that can not endure water.
Other WTC's are used for controlling buoyancy. This sub has four WTC's:
Two ballast tanks, and two compartments for electronics, valves and
pumps. When constructing your sub, you will have to decide if you're
going to use a cylinder shaped WTC for the technical stuff or a box
shaped one. Normally cylinder shaped objects can withstand higher
pressure than box shaped objects, but we will not dive so deep that we have to consider this. A box with
seethrough top offers the most usable space for circuit boards, several flat sides where you can put plugs
etc, and easy inspection. A cylinder shaped
compartment looks nice, but it takes more work to fit the components
inside.
When shopping for a WTC, look for one that is watertight, not
just "resistant". The IP rating must be 68. To read more about
IP Ratings, click here! You
can make them your selves, but many suppliers has both box and cylinder
shaped enclosures. Whether you buy or build, you should test the WTC
anyway. Place the WTC in a closed basket, bag or anything else that can
hold both the WTC and some weights. Put a piece of paper inside the WTC,
and submerge the hole thing down to a depth that you are sure that you
will never dive to with your sub. 20 > 25 ft. (6 > 7½ m.)
should be enough. Leave it there for an hour or two, and pull it back
up. Now take a look at the paper inside the WTC, it will tell if even
the smallest volume of water has entered.
Pressure calculation, relative pressure:
It might be interesting to calculate the pressure that our sub is
subjected to when designing the WTC's. The formulas for
relative pressure calculation for freshwater are as follows:
A) If the pressure inside the WTC is one atmosphere: Pressure_{(atm)} =
( Depth_{(Meters) }/ 10 )
The WTC's inside pressure will be 1 atm. as long as the WTC has been
sealed at sea level.
The relative pressure that the WTC needs to withstand at 10 ft. (3.05 m.) is thereby 0.305
Atm.
If you want that in psi, simply multiply the result in atm. by
14.69....... 0.305 x 14.69 = 4.48 psi
B) There is a few golden rules that you can use for estimates:
 The pressure will increase by 1 psi for every 27.68 US Inches (70.3 cm) you dive.
 The pressure will increase by 1 atm for every 33 ft. (10.03 m.) you dive.
 The pressure will increase by 0.4335147 psi for every ft. you dive.
 The easiest to remember is this: Pressure_{(psi)} = ( Depth_{(ft.) }/ 2 ).
 1 atm = 1.013 bar = 14.69 psi
 1 bar = 0.986 atm = 14.50 psi
 1 psi = 0.068 atm = 0.068 bar

The maximum depth is determined by
the smallest of the following five factors:
A) The depth where the radio signals no longer can reach the sub.
In salty waters this is only a few feet down, but in fresh waters this
is 6 > 8 ft. depending on your transmitter and antenna orientation /
location. It might be possible to go deeper by letting a towed sonar
ride close to the surface, in fact being the antenna, while the sub
seeks deeper waters. Please click here, scroll to item "I", and read more about this idea.
B) The depth where one of the watertight compartments (WTC) fold
as a result from the water pressure.
C) The depth where the ambient water pressure exceeds the
operating pressure range of the valves, and where they therefore fail to
remain closed. In my case this is app. at 100 psi. (6.9 bar) (Using the
valves described from McMasters.) and this pressure is present at 68
meters and below.
D) The depth where the ambient water pressure exceeds the onboard
regulated air pressure, so it's no longer possible to push the water out
of the tanks. In my case, with a system air
pressure of 1.6 bar / 23.2 psi, this is present at 53 ft. ( 16 m.) and
below.
Safe diving depth is therefore set to match the radio range of 68 ft / max. 2.5 m.
E) The depth where the ambient water pressure exceeds the
operating pressure range of the internal tubing etc. in the sub and WTC's
associated with air and water control. In my case this has been
tested to be greater than 83 ft. ( 25 m. )
It is common that model sub's never perform dives exceeding 6 > 8
ft, but as one guy said once when asked how deep it could go: "ALL the way to the bottom, if I'm not careful!" The
real Ohio class has a max. depth "greater than 1000 ft." If we
calculate the pressure on the HY80 steel hull of the Ohio at this depth,
we'll get (Relative pressure for freshwater):
Formula: 
Conversion: 
Calculation: 
Pressure_{(atm)} =
( Depth_{(Meters) }/ 10 ) 
1000
ft. = 304.8 meters 
304.8
/ 10 = 30.48 atm. 
1
atm = 1.013 bar = 14.69 psi 
1 atm.
= 14.69 psi 
30.48
x 14.69 = 447.75 psi. 
....enough to ruin your day if the
pressure hull fails.
