Dive Computer Setup Guide

Interactive walkthrough for gas programming, gradient factors, and closed-circuit configuration

Your dive computer is the single most important piece of safety equipment you carry underwater. Getting the settings right is not optional. This interactive guide walks you through the setup process step by step, showing you exactly what your screen should look like at each stage.

Important: This guide is a reference aid. Always consult your computer's owner manual and follow the training from your certifying agency. Menu paths and options may vary with firmware versions.

Step 1 of 8 Shearwater Support

Enter the Main Menu

Press the right button (labeled SEL on the device) to enter the main menu from the dive screen. You will see five top-level categories: Dive, Deco, Gases, Mode, and System.

Use the left button (NAV) to scroll through menu items. The selected item is highlighted in cyan.

Why this matters

The Perdix menu structure is hierarchical. All configuration lives under these five categories. Understanding the menu tree means faster adjustments on the boat between dives, when you need to change gas plans or GF settings quickly.

Navigate to GF Low

Scroll down to "Deco" using the NAV button, then press SEL to enter. Select "GF Low" from the deco settings list. The current value is displayed to the right of each item.

Common mistake: Not updating GF values after changing dive plans. Always verify your GFs match the profile you are about to dive.

Why this matters

GF Low controls when your first decompression ceiling appears. A lower GF Low value (e.g., 20) creates earlier, deeper stops. A higher value (e.g., 40) delays the first stop. This setting directly shapes your ascent profile.

Set Your GF Low Value

The screen now shows the GF Low value in large text. Use the NAV button to scroll the value up or down. The valid range is 10 to 100.

For technical diving, common values are 20-40. Press SEL to confirm your selection.

Why this matters

GF Low = 30 is a popular choice for CCR technical dives. It provides a buffer against fast tissue supersaturation during deep phases. More conservative than 40, but less extreme than 20. Your training agency may recommend a specific value.

Set Your GF High Value

After confirming GF Low, scroll down to GF High and select it. Adjust the value using NAV. The valid range is 50 to 100.

For technical diving, common values are 70-85. A GF High of 85 is typical for moderate conservatism. Press SEL to confirm.

Common mistake: Setting GF High too high (e.g., 95-100) for tech dives. This reduces shallow stop time and increases the risk of DCS on aggressive profiles.

Why this matters

GF High determines how close to the Buhlmann M-value the algorithm lets you surface. A GF High of 85 means the computer clears you at 85% of the maximum theoretical supersaturation limit. Lower values produce longer, safer shallow stops.

Navigate to Gas Programming

Return to the main menu and navigate to "Gases." You will see up to five gas slots. Gas 1 is your primary gas (or diluent in CC mode). Gases 2-5 are for bailout or deco mixes.

Each gas shows its O2/He ratio and mode designation. Select Gas 1 to begin editing.

Why this matters

Incorrect gas programming is one of the most dangerous setup errors. The computer uses these values to calculate decompression, PO2, and END. If your gas mixes do not match what is actually in your cylinders, every calculation will be wrong.

Set Your Diluent Composition

On the Gas 1 edit screen, set your O2 percentage first (21% is common for normoxic trimix diluent). Then set the He percentage (35% is shown here for a 21/35 trimix). The N2 balance is calculated automatically.

Set the gas mode to "CC" for closed-circuit. The MOD at 1.4 PO2 is displayed for reference.

Common mistake: Programming the diluent as an OC gas instead of CC. If Gas 1 is set to OC mode, the computer will not calculate CC PO2 correctly and deco obligations will be wrong.

Why this matters

In CC mode, the computer uses the diluent gas composition with the setpoint PO2 to calculate your actual breathing mix at depth. Helium reduces narcosis and gas density. A 21/35 trimix gives you clean thinking at depths where air would produce severe narcosis.

Set CC/BO Mode

Return to the main menu and navigate to "Mode." Select CC/BO (Closed Circuit with Open Circuit Bailout). This configures Gas 1 as your diluent and remaining gases as OC bailout options.

CC/BO mode enables setpoint management, cell monitoring, and automatic gas switching for bailout scenarios.

Common mistake: Forgetting to switch from OC to CC/BO mode before the dive. If you splash in OC mode, the computer ignores your setpoints and calculates decompression based on the gas fraction at depth, not the controlled loop PO2.

Why this matters

CC/BO is the standard mode for rebreather diving with the Perdix. It tells the computer to use the setpoint PO2 (not the diluent fraction) for all decompression calculations during closed-circuit operation. The "BO" part means the computer also tracks OC bailout gases and can switch calculations instantly if you go off the loop.

Configure Your Setpoints

Navigate to Deco and find the Setpoints section. Set SP High (typically 1.3 for most CCR diving) and SP Low (typically 0.7 for descent and ascent phases).

SP High is your working setpoint at depth. SP Low is used during descent and shallow phases to prevent hyperoxia on the surface. Confirm that these values match your primary controller settings.

Why this matters

Setpoints are the core of rebreather diving. SP 1.3 provides excellent decompression efficiency while staying safely below oxygen toxicity limits for most dives. SP 0.7 during descent prevents the PO2 from exceeding safe levels before you reach depth and switch to high setpoint. These values must match your controller, or your backup computer will disagree on deco.

Your computer is ready to dive

All core settings are configured. Before entering the water, do a final cross-check: verify GF values, gas compositions, mode, and setpoints all match your dive plan and primary controller.

Pre-dive checklist:

GF Low / GF High confirmed Gas compositions verified CC/BO mode active Setpoints match controller O2 cells calibrated

Step 1 of 7 Garmin Support

Open Dive Settings

On the watch face, hold the middle-left button to open the main menu. Scroll down to "Dive Settings" and select it. You will see categories for conservatism, gases, dive mode, air integration, alerts, and display configuration.

Why this matters

The Garmin Descent organizes dive settings separately from smartwatch settings. All diving configuration lives under this single menu tree. Familiarize yourself with the layout before your first dive on this computer.

Select Custom Conservatism

Select "Conservatism" from the dive settings menu. Choose "Custom" to enter specific GF values instead of using the Low/Medium/High presets.

Why this matters

The preset options are fine for recreational diving, but for technical dives you need precise control over gradient factors. Custom mode unlocks separate GF Low and GF High fields.

Set GF Low

Use the up/down buttons to set your GF Low value. Press the START button to confirm. A value of 30 is shown here for a typical CCR tech profile.

Why this matters

Same principle as the Shearwater. GF Low controls where your first deco obligation appears. Ensure this value matches your other computers and your dive plan.

Set GF High

After confirming GF Low, set the GF High value. Press START to confirm. A value of 85 provides moderate conservatism for most profiles.

Why this matters

GF High determines your surfacing limit. Both the Garmin and your backup Shearwater should use identical values to avoid conflicting deco schedules.

Program Your Gases

Navigate to "Gases" and select each gas slot. Set O2%, He%, and gas status (On, Off, or Deco). The Mk2i/Mk3i supports trimix across all 5 gas slots.

Common mistake: Not pairing the SubWave transmitter before getting in the water. Pair on the surface and verify pressure readings before splashing.

Why this matters

Your gas programming drives every calculation the computer makes. Verify each gas slot against your physical cylinders. The MOD displayed should match your dive plan.

Set CCR Mode

Navigate to "Dive Mode" and select "CCR" from the mode list. Verify the CCR icon appears on the dive screen. Program your diluent as Gas 1 and bailout gases in the remaining slots.

Common mistake: Leaving CCR mode active when switching back to OC dives. Always check your dive mode before entering the water.

Why this matters

The Garmin uses your dive mode to determine how it calculates PO2 and decompression. In CCR mode it uses setpoint values. In OC mode it uses gas fractions. Wrong mode means wrong calculations.

Configure Setpoints

Set SP High to 1.3 and SP Low to 0.7 (or per your dive plan). Confirm that these values match your primary controller exactly. Mismatched setpoints between computers will produce different deco schedules.

Why this matters

Setpoints define the PO2 the computer assumes you are breathing. If your Garmin says 1.3 and your controller says 1.2, each will compute a different ascent schedule. One computer thinking you have more O2 than you actually do is a recipe for trouble.

Your Garmin is ready to dive

Core dive settings are configured. Remember to disable smartwatch notifications before long dive days to preserve battery. Use the Garmin Dive app for detailed log review after each dive.

Step 1 of 5 Shearwater Support

Open Dive Setup

From the dive screen, press and hold the right button (SEL) to open Dive Setup. The Peregrine 2 is an OC nitrox computer. It does not support trimix or closed-circuit modes.

Why this matters

The Peregrine is designed for simplicity. Its streamlined menu gets you configured and in the water fast, which is exactly what you want on a recreational dive boat.

Set Conservatism

Select "Conservatism" and choose a preset: Low (GF 45/95), Medium (GF 40/85), or High (GF 35/75). For custom GF values, you must first enable Advanced Mode in System settings.

Common mistake: Trying to set custom GFs without enabling Advanced Mode first. The option will not appear until you toggle it on under System settings.

Why this matters

The preset conservatism levels cover most recreational profiles. Medium (40/85) is a solid default for nitrox diving on reefs and shallow wrecks.

View Gas Settings

Navigate to "Gases." The Peregrine supports up to 2 nitrox mixes. Gas 1 is your primary. Gas 2 can be used as a deco or travel gas. The computer automatically calculates MOD at PO2 1.4.

Why this matters

Two gas slots is enough for most recreational nitrox diving: one primary gas and one deco mix. If you need more, it may be time to upgrade to a Perdix.

Set Your Primary Gas

Select Gas 1 and set the O2 percentage. For EAN32, set to 32%. For EAN36, set to 36%. The computer will calculate and display the MOD for your selected mix.

Common mistake: Forgetting to switch Gas 2 back to "Off" after a nitrox dive when returning to air diving. Leftover gas programming from a previous dive can cause confusion.

Why this matters

Always verify the O2% on your computer matches the analyzer reading on your tank. A mismatch means your computer will calculate the wrong MOD and the wrong deco obligations.

Configure Safety Stop

Navigate to "Safety Stop" and choose your preferred duration: 3 minutes (standard), 5 minutes (extra conservative), or Off. The stop depth is fixed at 15 feet / 5 meters.

Why this matters

Safety stops are recommended but not mandatory for no-deco dives. The 3-minute stop at 15 feet is industry standard. Use 5 minutes for deeper or repetitive profiles where you want extra conservatism.

Your Peregrine is ready to dive

The Peregrine 2 is a great recreational computer. Remember: it supports nitrox only, no trimix or CC mode. The battery is user-replaceable (AA-size). Carry a spare.

Shearwater NERD 2

Shearwater Support

The NERD 2 (Near Eye Remote Display) is a CCR-specific heads-up display that mounts to your BOV or DSV. It reads O2 cells directly via the Fischer connector, providing an independent PO2 reading. The NERD 2 shares the same menu structure as the Perdix. Use the Perdix walkthrough above for navigation guidance. Focus on the critical NERD-specific items below.

Match All Settings to Your Controller

Set GF Low and GF High to the same values as your primary controller.
Program identical diluent gas in Gas 1 (O2%, He%).
Program identical bailout gases in Gas 2 through Gas 5.
Set SP High and SP Low to the same setpoints as the controller.
Verify PO2 alarm thresholds match (PO2 High 1.6, PO2 Low per plan).
Double-check all values side by side before every dive.

Fischer Connector Care

Inspect connector pins before each dive for corrosion or bent pins.
Apply a thin layer of silicone grease to the connector O-ring only (not the pins).
Align the connector key before pushing in. Never force the connection.
Rinse thoroughly in fresh water after every dive.
Store with the protective cap on when disconnected.

Three-Way Cross-Check (Before Every Dive)

Compare PO2 readings across all three sources on a stable loop:

Primary controller (e.g., Hollis Prism 2 controller display).
NERD 2 (reading cells directly via Fischer connector).
Backup wrist computer (e.g., Shearwater Petrel).

All three should read within 0.02 PO2 of each other. If any source disagrees by more than 0.05, troubleshoot before diving. Common causes: bad cell, loose Fischer connector, different calibration gas.

Common Mistakes

Setting different GFs or setpoints on the NERD vs. the controller. They must match.
Forcing the Fischer connector and bending pins.
Not rinsing the Fischer connector after saltwater dives. Corrosion builds quickly.
Ignoring a PO2 disagreement between NERD and controller. Always investigate before diving.
Forgetting to calibrate the NERD independently.

NERD 2 Tips

The NERD 2 shares the same menu structure as the Perdix. If you know one, you know the other.
Mount position matters. Adjust the angle so you can read it without tilting your head excessively.
Battery life is shorter than the Perdix due to the smaller case. Check charge before every dive.
The Fischer connector is the most common failure point. Carry a spare if traveling.

Which Computer Should You Buy?

It depends on how you dive. Recreational nitrox divers will be well served by the Shearwater Peregrine 2 or Garmin Descent G1 Solar. If you want a watch-style computer that doubles as a daily wearable, the Garmin Descent Mk2i or Mk3i is hard to beat. For technical diving, trimix, or rebreather use, the Shearwater Perdix 2 or Petrel 3 remains the industry standard. And if you are diving CCR, the NERD 2 is an essential heads-up display.

Computer	Best For	Trimix	CC Mode	Air Integration
Perdix 2 / Petrel 3	Tech, CCR, advanced rec	Yes	Yes	Yes (Perdix 2 only)
Peregrine 2	Recreational, nitrox	No	No	No
Descent Mk2i / Mk3i	All-rounder, daily wear	Yes	Yes	Yes (SubWave)
Descent G1 Solar	Budget rec, nitrox	No	No	Yes (SubWave)
NERD 2	CCR heads-up display	Yes	Yes (always)	No

For detailed reviews and purchase links, visit the gear page.

Understanding Gradient Factor Settings

Gradient factors (GFs) control how aggressively your dive computer calculates decompression. GF Low governs the deep stops and first deco ceiling. GF High controls the surfacing value, determining how close to the M-value the algorithm allows you to ascend before completing all stops.

Common GF pairings:

GF 40/85 . Conservative recreational default on many computers.
GF 30/70 . Common for technical OC diving. Produces deeper stops and longer deco.
GF 20/70 . Aggressive deep stop philosophy. Used by some CCR divers on deep dives.
GF 45/95 . Minimal conservatism. Matches older Haldanean models closely.

Lower GF Low values produce earlier and deeper deco stops. Lower GF High values require longer shallow stops before surfacing. There is no universally "correct" setting. Your GFs should reflect your training, dive profile, personal risk tolerance, and fitness level.

Use the deco planner to see how different GF values affect your deco schedule.

When to Upgrade Your Dive Computer

Your first dive computer will serve you well through Open Water, Advanced, and nitrox certifications. But as you progress into technical diving, the demands on your computer change significantly. Here are the signs it is time to upgrade.

1. You need more than 2 gas mixes. Multi-gas deco diving requires 3 to 5 programmable gases. If your computer only supports 2, you have outgrown it.
2. You want custom gradient factors. Computers with only Low/Medium/High conservatism presets do not give you the control that technical diving requires.
3. You are starting trimix training. Not all computers support helium. If your next course involves trimix, your computer needs a He% field.
4. You are transitioning to CCR. Closed-circuit mode, setpoint management, and cell monitoring are specialized features. You will need a Perdix 2, Petrel 3, or comparable CCR-capable computer.
5. Your computer no longer receives firmware updates. Manufacturers periodically drop support for older models. An unsupported computer is a liability.

Dive Computer Setup Guide

Enter the Main Menu

Navigate to GF Low

Set Your GF Low Value

Set Your GF High Value

Navigate to Gas Programming

Set Your Diluent Composition

Set CC/BO Mode

Configure Your Setpoints

Your computer is ready to dive

Open Dive Settings

Select Custom Conservatism

Set GF Low

Set GF High

Program Your Gases

Set CCR Mode

Configure Setpoints

Your Garmin is ready to dive

Open Dive Setup

Set Conservatism

View Gas Settings

Set Your Primary Gas

Configure Safety Stop

Your Peregrine is ready to dive

Garmin Descent G1 Solar

Setting Conservatism

Programming Gases

Air Integration (SubWave)

G1 Solar Tips

Shearwater NERD 2

Match All Settings to Your Controller

Fischer Connector Care

Three-Way Cross-Check (Before Every Dive)

Common Mistakes

NERD 2 Tips

Which Computer Should You Buy?

Understanding Gradient Factor Settings

When to Upgrade Your Dive Computer