---
title: "Cylinder configurations"
description: "The nine cylinder configurations, which ones a plan offers, and why a twinset and a sidemount pair report the same gas differently."
---

> 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.

# Cylinder configurations

import { GasWizard } from "@/components/react/ModalScreens";

**Cylinder Configuration** is the last field on the wizard's **Tank
Configuration** step, and the rest of the gas follows from it: the role it takes,
whether it needs a switch depth, and how its pressure is reported.

## What the wizard offers

Nine configurations exist. Which of them appear depends on the plan and on what
is already in it:

- **A rebreather plan**: Onboard Diluent, Stage Tank, Drop Tank.
- **An open-circuit plan that already has a back gas**: Stage Tank and Drop Tank
  only, so a second back gas cannot be added.
- **An open-circuit plan with Quantity 2 or more**: Backmount Double, Sidemount.
- **An open-circuit plan with Quantity 1**: Single Tank, Stage Tank, Drop Tank.

On the gas card the configuration is printed beside the role, shortened to
Single, Backmount Double, Sidemount, Stage, Drop Tank, Onboard Diluent, Offboard
Diluent, OC Bailout or Dil/Bailout.

## Twinset and sidemount

Both hold two cylinders, and the planner counts both. A Backmount Double and a
Sidemount are always measured as two cylinders whatever **Quantity** says; every
other configuration takes Quantity as entered. Two 12 L cylinders therefore hold
the same gas either way, and the volume figures agree.

The pressure figures do not read the same, because the two are not the same
supply:

- **A Backmount Double is drawn as one bar.** The cylinders are manifolded, so
  there is one pressure and it falls on both together.
- **A Sidemount pair is drawn as two bars.** The cylinders are independent, each
  with its own valve, and each bar carries the same reading, because the plan
  spreads the gas evenly across the pair.

**Remaining** and **Left** are per cylinder in both cases, so a twinset and a
sidemount pair that used the same volume report the same number. On the twinset
that is a gauge reading you will see. On sidemount it is what each cylinder holds
only if you have switched evenly between them; the plan does not model the
switching, so an uneven pair is yours to manage.

## Next

[Gas names and labels](/gases/names-and-labels) covers what the gas is called
and the role the configuration sets for it.

Source: https://docs.divelogic.ai/gases/cylinder-configurations/index.mdx