Step 1: Materials
You need one syringe, one zip tie, a marker pen and a knife.
Preferably the syringe should be large and have a scale with divisions to sixty.
Step 2: Mark Scale
The syringe should now be marked with the pen but unforunately the ink won't stick to the syringe. Therefore we will first scratch the plastic with the tip of a knife and then apply ink into the scratch.
A 60ml syringe is most suitable for making a logarithmic scale. The scale is practically readable to depths of 15-20m which I think is still comfortable. More extreme freedivers going deeper might want a more advanced instrument.
The depth marks should be as follows:
0m = 60ml
5m = 40ml
10m = 30ml
15m = 24ml
20m = 20ml
30m = 15ml
40m = 12ml
50m = 10ml
172m = 3.3ml (World record 2005/10, totally insane!)
Step 3: Make a Hole at the Surface Mark
Make a hole in the side of the syringe to always allow 60ml of air into it at surface level. You can make the hole by pushing a soldering iron through the plastic.
Step 4: Seal the Tip of the Syringe
Seal the tip of the syringe. You could fill it with glue, insert a screw in it or something. My preferred method is to melt the plastic tip with a gas lighter and then mold it together.
Step 5: Go Diving!
The depth gauge is now finished. Strap it to your hand with a rubber band and get under water.
Before you dive you insert the piston on and the syringe will contain 60ml of air (at surface pressure). When you dive and go deeper the air inside the syringe is compressed and the piston moves to equalize the pressure difference inside and outside the syringe. When you return to the surface the air in the depth gauge will expand and and the piston moves back.
Frederik-WillemF made it!