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.
<p>Just finished mine. About to test it with a free-diving crowd in Simon's Town, South Africa. Thanks for the Instructable!</p>
Thanks for the great instructable!
<p>Hello, I would like your help... please !</p><p>I would like to know the following: If the 60ml = 0m and 40ml = 5m... where I can mark:</p><p>3m / 7m / 12m and 17m ?</p><p>Thanks a lot. </p><p>Your Ideia is very cool! Congratulations!</p>
<p>I chose a 60 ml syringe in part because it gives sensible values for <br>these different depths above and there are ready made markings to help <br>finding them.<br>Check Boyle's law for how to calculate volumes for <br>other depths. For the depths you mentioned, 60/(1+depth/10) would give <br>the values 46.15384615 / 35.29411765 / 27.27272727 and 22.22222222.</p>
<p>Wow! Thanks a lot!</p>
<p>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?</p>
<p>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.</p><p>great idea and thanks for adding the formula!</p>
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
Thanks . Ill build one for my under water video camera. Its only goes down to 16 feet for snorkeling. Hopefully the pet store will have a syringe for sale , or Ill just have to find a dead junky in some back alley . lol
I have a 20 ml syringe. How many feet is 5 ml? 10 ml? How could I figure it out without testing it? Thanks for any help.
Pressure p = volume at surface / compressed volume.<br>Depth in meters = (p-1)*10<br>For 20ml at surface pressure you get 15ml at 10 feet and 10ml at 33 feet. I guess you don't need a mark at 5ml. The 20ml syringe will not be very accurate.
wont water go into the hole?
No, because the plunger is below the hole. The hole is opposite the sealed end and you push the plunger just past the hole into the body of the syringe.
Thanks Aleksi, we had great fun taking this baby to depth! Spot on at 20m, and think it was pretty much there at 30m as well (but to be honest i was thinking more about my chest squeeze than the gauge at 30m!) Version 2 to have some kind of max depth indicator to check once back on the surface? Maybe a movable O-ring inside?
Nice! I only went to 25m and then my ears were telling me to stop. The chest squeeze gets uncomfortable too. To record the maximum depth, you could add a valve which lets the air out when you ascend. The piston would then hopefully stay at the maximum reading. Have you got any ideas for making this instrument more wearable?
Don<em>t you equalize your ear tubes whilst diving by pinching nostrils and blowing air to &quot;clear &quot; your ears? Saved my ears for years, specially whilst scuba diving.</em><br/>
The technique you described only works if you have an excess of air (which you have when scuba diving). For a freediver it is more difficult because at -25m the air in his lungs has been compressed to one third of its surface volume. This is what rbotes means by chest squeeze. It occurs at about -30m and beyond this depth it is practically impossible to blow out any more air from the lungs. I would need to learn a more advanced technique, like e.g. the frenzel-fattah equalisation technique if I wanted to go deeper.
hmm, good idea on the valve, will try and think of a nice way to do this. In terms of "wearability" I was thinking of removing the side wings and cutting off as much of the plunger arm as possible, then inserting into a simple neoprene cover that can be slipped over the forearm. A smaller syringe would be good too but I guess you'd loose too much accuracy?
I think you must do a video to show us how to perform it ! Thank You :)
Have you tried this out at depth? How well does it perform? Any problems from the friction on the plunger?
Interesting. Is any temperature drop noticable? Buoyancy experiments would be fun to try.
I haven't noticed any. Next time I'll check for condensation.
this is really cool i have to try to build this to use when i go snorkeling

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