Stepping on a scale with all your gear will give a very accurate starting point for this calculation. You just need to make sure to do it dry, and compensate for the mass of the gas in your cylinder if taking the measurement with anything but empty tanks. This has worked well for me with a drysuit too, since we can assume drysuit squeeze/gas volume will be the same or very similar between diving situations.
@@DoktorBEN It will only apply to neutrally buoyant things. For example: If you dive in fresh water (1g/ml) and dial your weight in, such that an end of dive weight check finds you neutrally buoyant with no air in your wing. If you weigh let's say 150kg with all your gear on in that state, Archimedes principal says you displace 150L of water. Now if you take that same setup and go to salt water (1.03g/ml), you would need to be 3% more negatively buoyant, or 4.5kg. This means you would need to add 4.9kg of lead for example, since lead is 11.3g/cm3 outside of water, but has a buoyancy of 10.27g/cm3 inside salt water.
loved this video and the website you used to do math for you! I understood the concept before watching video but you made math easy with the website. thank-you.
Your displacement cancels out, leaving only the water density to consider. Salt density is about 2.4% higher than fresh, so *add 2.4% of total dry weight* (including you, gear, tank, everything). 200 lb diver, an AL80 tank is about 30 lbs empty, so call it 240 lbs total. Therefore, add about 6 lbs to your fresh-water weight (240*0.024). If your total is 110 kilos, add 2.5 or 3 kg (110 * 0.024). I've heard some places like the Red Sea are more salty, so perhaps add 3%.
I read somewhere in the fundamentals book that generally, you add 1-2kg of weight to compensate for sea water. I'm not sure if this refers to single cylinder.
Hi Ben, I am really struggling to do my gas calculation for my fundies pass, thinkgs like SAC rate, tank volume etc. is there a site that I can do drills on this ? like a online test? or a book i can order? HELP, thanks
Stepping on a scale with all your gear will give a very accurate starting point for this calculation.
You just need to make sure to do it dry, and compensate for the mass of the gas in your cylinder if taking the measurement with anything but empty tanks.
This has worked well for me with a drysuit too, since we can assume drysuit squeeze/gas volume will be the same or very similar between diving situations.
It depends…BPs for instance have a much higher density than body tissue. So if you have a lot of dense stuff on you, it might not work.
@@DoktorBEN It will only apply to neutrally buoyant things.
For example: If you dive in fresh water (1g/ml) and dial your weight in, such that an end of dive weight check finds you neutrally buoyant with no air in your wing. If you weigh let's say 150kg with all your gear on in that state, Archimedes principal says you displace 150L of water.
Now if you take that same setup and go to salt water (1.03g/ml), you would need to be 3% more negatively buoyant, or 4.5kg. This means you would need to add 4.9kg of lead for example, since lead is 11.3g/cm3 outside of water, but has a buoyancy of 10.27g/cm3 inside salt water.
Yes in that case it works
loved this video and the website you used to do math for you! I understood the concept before watching video but you made math easy with the website. thank-you.
Thank you! That was my intention!
Your displacement cancels out, leaving only the water density to consider. Salt density is about 2.4% higher than fresh, so *add 2.4% of total dry weight* (including you, gear, tank, everything). 200 lb diver, an AL80 tank is about 30 lbs empty, so call it 240 lbs total. Therefore, add about 6 lbs to your fresh-water weight (240*0.024). If your total is 110 kilos, add 2.5 or 3 kg (110 * 0.024). I've heard some places like the Red Sea are more salty, so perhaps add 3%.
I guess North Sea is around 3.5%. Baltic has less…you can anyway look it up.
I read somewhere in the fundamentals book that generally, you add 1-2kg of weight to compensate for sea water. I'm not sure if this refers to single cylinder.
Well…that’s a very rough approach, maybe for a standard person of 70kg
@@DoktorBEN what about a rough approach for a 100+kg person? 😅
It’s about what I said in the video…just use the formula in the comments and punch it in WolframAlpha
Hi Ben, I am really struggling to do my gas calculation for my fundies pass, thinkgs like SAC rate, tank volume etc. is there a site that I can do drills on this ? like a online test? or a book i can order? HELP, thanks
Good question…you should find a video on my channel explaining this. Who’s your instructor?