1. Let’s start

Let’s do a step-by-step example:

You want to simulate a normoxic dive, 17 minutes at 70 meters. Lets assumes you are using the Gradient Factor model, with all paremeters to OSTC’s default, a 18/30 trimix as bottom gas, and a 75% nitrox deco.

2. Setting the decompression model

First you need to setup the decompression model:

Select OC for open circuit mode.
Select the Gradient Factor model: this is Bühlmann decompression agmented with Erik Baker‘s extension to provide smooth deep stops.
Set the low gradient factor value, ie. the security margin used for the deepest stop. Lower values make deeper stops. Defaults to 30%.
Set the high gradient factur value, ie. the security margin used for the last stop (near surface). Lower values make longuer stops. Defaults to 90%.

3. Setting the gas

The next thing is to define is the gas you will have in your tanks. OSTC enables 5 gas, that can be defined in any order:

We will use two tanks (bottom and deco), so we check just two lines to enable them. The other one won’t be used by the simulator for computing the runtime.
first gas is our bottom mix. This is the gas the simulator will use until changing to a deco gas. Note that for the OSTC planner, there is no such thing as a descent or a travel gas.
Set the oxygen ratio in the mix.
Set the helium ratio, for Heliox or Trimix dives.
The interface is dynamically showing MOD (maximum operating depth, based on ppO2Max, CF18), EAD (equivalent air depth, for nitrox divers) and END (equivalent narcosis depth for trimix divers)

4. Running the simulator

Then you can launch the simulator:

Set the interval since the last simulation. Use Reset, because we do start a new dive.
Set bottom time. Note this do includes the descent time as well.
Set the dive depth.
You can change the ambient pressure at dive start. This is to adapt decompression for altitude divings. 1013 mbar is reference sea level.
Launch the computation.
You will see the simulation going on in the console. The OSTC planner uses a 2 secondes integration step for gas loading/off-gasing, and continuousy recompute a portion of the runtime. Every finished runtime (reaching surface) is displayed here.