Always Know Your PO2
Smithers Code is a system of blinking lights that communicate PO2 information to a diver in the Heads Up Display (HUD).
The “PPO2 Mode” as some rebreather systems call it, allows the diver and buddy to view actual PO2 at all times. Three color-states are used to indicate actual values of each individual sensor. If PO2 is 1.0, an LED will blink orange. At 1.1 an LED blinks green. At .9, an LED blinks red. The value for each sensor is displayed in succession followed by an extended pause. The lower the PO2, the more red flashes. The higher the PO2 is displayed with more green flashes.
Although this sounds complicated at first, it quickly wires in to the diver’s observations. Sensor values are rounded up and down to the nearest integer.
For example:
If all sensors are at .81, then the HUD will display:
RED, RED (pause) RED, RED (pause) RED, RED (extended pause)
If all sensors are 1.2, the DIVA will display:
GREEN, GREEN (pause) GREEN, GREEN (pause) GREEN, GREEN (extended pause)
If the sensors are 1.24, 1.31 and 1.27 respectively, the DIVA will display:
GREEN, GREEN (pause) GREEN, GREEN, GREEN (pause) GREEN, GREEN, GREEN (extended pause)
With this method, the diver not only tracks actual PO2, but also the behavior of each individual cell.
When oxygen levels are at a dangerous state of less than .25 or greater than 1.75, the HUD will display several rapid flashes consisting of green followed by a red with a pause between the current sensor and the next. Some different systems change to rapid red flashing at a higher PO2. Some units use three separate LEDs to simultaneously communicate PO2 using Smither’s Code.
On an average dive, a diver should always be seeing green LEDs. When a red LED flashes, it immediately grabs the attention of the diver, encouraging them to review their primary handset and take corrective action.
The HUD is very visible, even in extreme silt scenarios that overhead environment divers face. This terrific safety feature is one worth getting used to, if you haven’t tried it already. It helps us live up to one of the golden rules of rebreather diving – “always know your PO2!”
