Ohio Class, Ballistic Missile Nuclear Powered Submarine, USS Kentucky SSBN737

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..

Pressure / Depth   

  Depth in feet
  Depth in inches
  Depth in meters

Please use dot as decimal point.
Reset by enter new number any where.

Download option: If you would like to have this small HTM-based 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 see-through 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 --> 7 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 6-8 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 HY-80 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.

Webmaster: Robert Holsting