Diving Depth Gauge





Introduction: Diving Depth Gauge

Make a simple depth gauge for freediving out of a syringe. This is also an experiment with Boyle's law which states that the product of the pressure and the volume of an ideal gas is constant.

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.



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Thanks for the great instructable!

Hello, I would like your help... please !

I would like to know the following: If the 60ml = 0m and 40ml = 5m... where I can mark:

3m / 7m / 12m and 17m ?

Thanks a lot.

Your Ideia is very cool! Congratulations!

I chose a 60 ml syringe in part because it gives sensible values for
these different depths above and there are ready made markings to help
finding them.
Check Boyle's law for how to calculate volumes for
other depths. For the depths you mentioned, 60/(1+depth/10) would give
the values 46.15384615 / 35.29411765 / 27.27272727 and 22.22222222.

Wow! Thanks a lot!

this is so cool,but idont understand the point of making the little hole,isnt the plunger past the hole? and isnt it already wide open behind the plunger?

leaving the nozzle as it is will make the whole device more durable. just seal that with hot glue, silicone (or whatever airproofs it). also saves you time. and glue.

great idea and thanks for adding the formula!

I'm going mound this on my sea perch ROV and mount it so I can see it in the video camera. Thank for the idea.

How do you close the hole at the 60ml? with your finger?

Before you dive, you should push the piston in just past the hole. No need to close it.


Can you please tell me the formula to work out the depth marks.thanks