about software and freediving

Using Subsurface for Freediving

Most freedivers I know analyse their dives only using the display of their dive computers or transfer the profiles using paper and pen. Subsurface is superior to all existing solutions out there I know of. This post will guide you how to use some features of Subsurface that are especially useful for freedivers.

Subsurface is a free, open source software solution (no cost at all) for divers of all kinds (Recreational, Tec-Diving, Teaching, Freediving…).
It runs on Windows, OS X and Linux and supports pretty much any dive computer out there.

In this post, I will use Subsurface version 4.0.3, which you can download (for free) from

Getting your dives into Subsurface

After installing Subsurface, you need to get your dives into the application.

With a data cable

The easiest method is to attach your dive computer to your workstation using a data cable. If you don’t have one, this shop sells cheap cables for Suunto and Mares computers. Please consult the official Subsurface manual on how to import dives using a data cable. (press F1 within the application or read it online)

Without a data cable

This is a bit more complicated as Subsurface currently does not allow entering sample points every second.

Download this file (you might need to right click and then select Save File) and open it using a text editor. Now change all VALUES according to your dive.
Then step through the log of your dive on your dive computer and transfer every sample (consisting of at least time and depth) into your text editor. Of course you can leave some samples out to save time. One sample every 5-10 seconds should be enough. The result should look something like this.

You can now open the file in Subsurface using File -> Open Logbook.

Our sample file in Subsurface 4.0.3

You can use Import -> Import Files to combine multiple dives created using the method described above into one single file.

Tip: Since this method takes time and is prone to errors, you should consider buying a data cable for your dive computer.

Some useful settings for freedivers

Click on File -> Preferences. In the dialog, click on Units and select Seconds for Ascent/Descent speeds. You can also enable interesting graphs like pO2 and pN2 in the Graphs section.

Adding tags to your dives

You can use the Tags field on the left to categorize your dives (different disciplines, lake or sea, shore or boat etc.). If you want to write something about the dive (e.g. conditions) you can use the Notes field for that.

Editing a dive in Subsurface 4.0.3

Once you are done, click Save.

Analysing your dive

First, click on the scale button in the lower right corner of the profile (red circle in the image).

Using the profile toolbar

While you hover along the profile, certain values like depth, temperature or ascent/descent speed are displayed in the tooltip. The ruler tool (blue circle in the image above) can be used to analyse certain parts of the dive like the decent or ascent. Simply drag the red circles along the profile.

Using the ruler tool

The ruler displays the time difference between the first and the second circle along with the maximum, minimum and average for depth and speed. The various speeds give information about your diving performance and can be used to optimize it (e.g. descent speed using propulsion vs. speed in freefall).

I hope you find this post useful. If you have questions about Subsurface, you can use the comment section below.