Not a substitute for formal training
This app is an unofficial study aid for practising HUD signal recognition only. It does not replace manufacturer-approved training, certification, or the official JJ-CCR user manual. Rebreather diving is hazardous: never dive a rebreather without formal instruction from a qualified instructor and the appropriate certification.
How the HUD works
The HUD has 3 columns of LEDs (one per O2 sensor) and 4 rows: top red, green, yellow, bottom red. Every 5 seconds a blink cycle begins. In each cycle, one LED per column blinks:
- Which LED blinks tells you the range (and direction from 1.0).
- How many blinks = how many 0.1 steps the ppO2 is from 1.0.
| ppO2 range | LED | Blink count | Example |
|---|---|---|---|
| Above 1.6 | Top red | 1 per 0.1 above 1.0 | 1.7 → 7 blinks |
| 1.1 – 1.6 | Green | 1 per 0.1 above 1.0 | 1.3 → 3 blinks |
| Exactly 1.0 | Green + yellow | 1 blink together | 1.0 → 1 double blink |
| 0.4 – 0.9 | Yellow | 1 per 0.1 below 1.0 | 0.7 → 3 blinks |
| Below 0.4 | Bottom red | 1 per 0.1 below 1.0 | 0.2 → 8 blinks |
Red = danger
Red is never used in the safe range (0.4–1.6). Any red means the loop is unsafe: top red = hyperoxic, bottom red = hypoxic. The separate buddy warning light also pulses whenever any sensor is outside 0.4–1.6.
Light density
Typical setpoints are 0.7 (low) and 1.3 (high), so at setpoint you expect 2–3 blinks per cycle. Seeing noticeably more or fewer blinks means the loop has drifted off setpoint, even if you’ve tuned out the blinking itself.
Try it yourself in the Simulator, learn the other displays under Signals, practise the button commands in How to, then test yourself with the Quiz.
Other HUD displays: start-up, calibration, alarms & errors
The HUD doesn’t only show ppO2. It also signals self-test, command entry, calibration and faults (manual §5.3.3–5.3.8). Each card below is a live animation of what you’d see. A useful rule of thumb: solid (non-blinking) red LEDs mean a sensor/calibration fault, while blinking red is a ppO2 warning. To practise actually putting the HUD into these modes - command selection, calibration, the options menu - head to the How to tab.
How to: drive the HUD with its one button
Everything on the HUD is operated with a single control - the rear wet contact, which doubles as a mechanical push button (§5.3.3, §5.3.5). Two moves cover it all: press (push and release) to advance the blinking selection, and hold for 3 seconds to execute it - the LEDs blink twice as acknowledgement. Button input is disabled while the wet contacts are wet, so commands can’t be entered while diving. Pick a procedure and drive the simulated HUD below with the PUSH button.
Click/tap = press · click & hold 3 s = execute
The simulator follows manual §5.3.3–§5.3.9. A few undocumented details (e.g. what happens when you press past the last command column) are approximated.
Quiz: read the HUD
You’ll be shown a live HUD. Watch the pattern and answer. 12 questions: a mix of “what is the ppO2?”, “is it safe?” and “what is the HUD telling you?” (alarms, errors and sequences from the Signals tab).