You strap on your tank, do your predive checks and jump in the water for a great dive. You and your buddy are Deep certified and the plan is to go to a wreck at 130 feet, no penetration.
You look at the dive table and see your NDL limit is 10 minutes so you set 8 minutes as your max time at that depth. OK great, but do you have enough air for the dive? Since it's a wreck you will be using the rule of thirds meaning you only have 2000 psi for the dive, the other 1000 is for reserve. This is where your sac rate comes in.
Your sac rate tells you how much gas you breathe per minute at the surface. This info is used to calculate how much gas you will consume per minute at any depth.There are 2 ways to calculate your sac rate
1- You will need a dive computer. Note your air at the beginning of the dive then note your air at the end of the dive. Use your computer to see your average depth. Then go over to Dive Buddies sac rate calculator https://www.divebuddy.com/calculator/sac.aspx and put that data in. You will see your sac rate in psi and your RMV, many recreational divers refer to their RMV as sac rate. RMV just adjusts your sac rate for depth.
2- No dive computer required, just a timing device and depth gauge. Note your air then swim normally at a constant depth for at least 5 minutes. Note your air and go to DIve Buddies site and put in your info.
If you are adventurous you can use this formula to figure it out yourself:
Now lets put it to use. You calculate your sac rate and it's 18.75 psi per minute at the surface. If you dive to 130 feet you are now at 4.9 ATM. Multiply your sac rate x ATM at depth:
18.75psi x 4.9 ATM= 91.875 rounded to 92 psi per minute at 130
So 8 minutes (8 x 92psi) at 130 uses 736 psi
Using the rule of thirds (2000 psi for the dive, 1000 for reserve) leaves you with 1264psi of bottom time gas. Now that you know how much gas you will consume on the wreck, you can calculate the rest of your dive. Perhaps you will go back up the line or do a slow ascent along a wall. Maybe you would consider bringing a pony in case of gas loss due to increased sac rate, free flow or equipment failure. Knowing your sac rate is the foundation for a Self Reliant or Tec diver