This applies to all systems.
Overview
Calibration of rebreather electronics is one of the most critical steps when setting up a rebreather for diving. Calibration ties each sensor’s mV output to a known PO₂ so that the controller reads oxygen accurately. Small pressure or gas input errors during calibration propagate to every dive, so it’s important to perform this procedure carefully. Always refer to the official rebreather manual provided by the manufacturer, as the procedure can vary by model.
How calibration is stored (Analog vs. DiveCAN)
It is important to note that analog systems and DiveCAN systems differ in how calibration works. In digital DiveCAN systems, the calibration process occurs in and is stored by the head electronics, only using the peripheral as an interface and, when possible, its pressure sensor to gather pressure data. In analog systems, the process occurs entirely in the handset/NERD. The calibration process is essentially an association between oxygen‑sensor mV readings and a known oxygen partial pressure (PO₂) determined using current ambient pressure (ATA) and the fraction of oxygen in the calibration gas (FO₂).
For handsets and NERDs, FO₂ is provided by the user in the O₂ Setup menu based on the calibration gas being used. Most commonly, pure oxygen is used for calibration. For LED HUDs, the calibration gas is always assumed to be 100 % O₂. It is important to follow impeccable gas‑analysis procedures to be sure of the oxygen content of the calibration gas.
Where pressure comes from
Ambient pressure is provided by the peripheral component (monitors and controllers) when possible. Peripherals such as Petrels and NERDs have their own pressure sensors and, provided System Setup > Display Setup > Altitude is set to Auto, they will use the current pressure in the calibration process. LED HUDs are not built with pressure sensors and will always use a standard SeaLvl pressure of 1013 mbar. Likewise, if a Petrel or NERD is set to SeaLvl in System Setup > Display Setup > Altitude, it will also use the standard pressure.
Why two devices disagree
In systems with multiple peripherals, surface pressure (mbar) values may not match exactly on different devices. This most often occurs because the diver is using a Petrel or NERD controller (with a pressure sensor and set to Auto) alongside an LED HUD monitor that has no pressure sensor. Disagreement may also occur with a combination of Petrel and NERD for the monitor and controller, both with pressure sensors set to Auto. In both scenarios, there is a chance that the surface pressure (mbar) between the two peripherals won’t match, producing a slightly different calibration PO₂ between the devices. A diver can always check the current surface pressure by scrolling through the bottom‑row information using the right button.
All of our pressure sensors have a drift specification of up to 20 mbar per year. This is equivalent to about 20 cm of seawater. The surface is referenced separately on each device at the beginning of each dive, so this offset drift will have no real impact on decompression calculations, but it can affect calibration and therefore the measured PO₂. These pressure sensors are specified to be accurate to 14 bar (14 000 mbar). Over such a large range, it is normal for there to be some small disagreement between sensors. However, large surface‑pressure deviations which result in major calibration differences between devices are not acceptable. Divers should contact Shearwater if they notice significant drift of their pressure sensors to discuss service options for their device.
Ways to align readings
Even small and acceptable differences in surface pressure between two devices can yield different PO₂ during calibration. If you want to make the PO₂ match, you can go to System Setup > Display Setup > Altitude and change Altitude to SeaLvl—but only if you are actually at sea level, of course. This will force the calibration pressure to 1013 mbar, so after calibration, both devices will match. Just remember, calibration may be slightly less accurate on one or both peripherals, so it is a trade‑off.
Another option for dual DiveCAN systems is to use the same peripheral to calibrate both controller and monitor sides. This is useful when at altitude and the monitor side is equipped with an LED HUD (no pressure sensor), or even if there is a slight difference among the peripherals’ surface‑pressure readings that yields a slightly different PO₂ during calibration. In these cases, after calibrating the controller side, temporarily unplug the controller and plug it into the monitor side. Go through the calibration procedures as set out in the rebreather manual provided by the manufacturer. Make sure to put the peripherals back into their correct positions.
Pressure limits for calibration
Finally, it is important to know that there are maximum surface‑pressure thresholds for calibration in place for DiveCAN systems. Maximum pressure is 1080 mbar for newer controller boards (SOLO v2) and 1050 mbar for older versions (SOLO v1). If the pressure reported by the depth sensor has drifted beyond these limits, contact Shearwater as your device may require service. For analog units such as Fischer and AK‑4pin units, there is no maximum pressure limitation.
Glossary
- FO₂ (fraction of oxygen): The fraction of oxygen in the calibration gas (e.g., 1.0 for pure oxygen).
- PO₂ (oxygen partial pressure): Partial pressure of oxygen in the loop; ≈ FO₂ × ambient pressure.
- ATA (atmospheres absolute): Unit of absolute pressure; ~ 1013 mbar at sea level.
- mbar (hPa): Pressure unit; standard sea level is 1013 mbar.
- SeaLvl: Setting that forces 1013 mbar for calculations/calibration.
- Auto: Setting that uses the device’s current ambient‑pressure sensor reading.
- LED HUD: LED‑based heads‑up PO₂ display; no pressure sensor.
- Petrel / NERD: Shearwater peripherals that can act as monitors/controllers and include pressure sensors.
- DiveCAN: Digital bus where head electronics store calibration; peripherals act as interfaces and may supply pressure data.
- Analog (Fischer / AK‑4pin): Systems where calibration occurs entirely in the handset.
- Head electronics: DiveCAN CCR electronics that live inside the rebreather.
- Peripheral: User interfaces like Petrels, NERDs and LED HUDs that plug into the rebreather.
