---
title: "Consumption"
description: "Setting working and deco SAC rates, and reading the per-level gas ledger, reserve and turn pressure."
---

> Documentation Index
> Fetch the complete documentation index at: https://docs.divelogic.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Consumption

import { GasLedger } from "@/components/react/ReviewScreens";
import { deepTrimix } from "@/demo/plans";

Consumption comes from two breathing rates you set and the ambient pressure the
water sets.

## The SAC rates

All live in the **Breathing Rates** card at the top of the **Gas Mixes**
section. See [Breathing rates](/planning/breathing-rates).

- **Working SAC (L/min)**: consumption during descent, bottom time and ascent.
  Starts at 15 L/min.
- **Deco SAC (L/min)**: applies on mandatory decompression stops, where exertion
  is lower. Starts at 12 L/min.
- **Bailout SAC Rates**: a second pair on a rebreather plan, starting at 45 and
  25 L/min, for a diver who has just had a problem.

Ascending between stops is charged at the working rate, not the deco rate. Each
segment is charged to the gas breathed at the end of it.

## From litres to bar

A SAC rate is a surface figure. At depth each breath holds more gas in
proportion to the ambient pressure, so the planner multiplies the rate by that
pressure and the minutes spent there. Ambient pressure follows the plan's water
type and altitude, so a fresh-water or altitude plan consumes a different amount
for the same profile.

Litres become bar through the cylinder: the volume consumed, spread across the
cylinder capacity, subtracted from the working pressure you filled to. Both the
tank size and the working pressure must be set, or every pressure figure for
that gas stays blank.

## The per-level gas ledger

Under **Gas ledger by level** the Review step gives one card per planned level.

- **Start pressure**: the working pressure of the cylinder being measured.
- **Leaving at**: pressure remaining in it as you leave this level.
- **To next switch**: bar needed to reach the next gas switch, or to the surface
  when no switch follows.
- **Reserve**: a fixed third of the starting pressure, which you cannot change.
- **Turn pressure**: to next switch plus reserve, the gauge reading you must not
  go below.
- **Runtime**: elapsed dive time at the end of the level.

The cylinder measured is your back gas, or on a rebreather plan the first
bailout-capable cylinder at the bailout rates. Every stretch of the dive is
charged to that one cylinder, even a stretch actually breathed off a deco gas,
so it reads as more consumed here than in the per-gas figures.

**Below turn** appears when the pressure you leave a level at is lower than that
level's turn pressure. **Supply exceeded** appears when remaining pressure has
gone negative, and it is deliberately not clamped at zero.

## Next

- [Best mix](/gases/best-mix) works a composition out from a target depth.
- [Bailout planning](/ccr/bailout) shows where the bailout rates are spent.

Source: https://docs.divelogic.ai/gases/consumption/index.mdx