That gauge doesn't seem to reflect percentage of CO2. It seems to represent percentage of CO2 until toxicity is reached. IE 100% on the gauge is toxic, whereas in real life it's something like below....
- 1,000,000 ppm of a gas = 100 % concentration of the gas, and 10,000 ppm of a gas in air = a 1% concentration.
- At 1% concentration of carbon dioxide CO2 (10,000 parts per million or ppm) and under continuous exposure at that level, such as in an auditorium filled with occupants and poor fresh air ventilation, some occupants are likely to feel drowsy.
- The concentration of carbon dioxide must be over about 2% (20,000 ppm) before most people are aware of its presence unless the odor of an associated material (auto exhaust or fermenting yeast, for instance) is present at lower concentrations.
- Above 2%, carbon dioxide may cause a feeling of heaviness in the chest and/or more frequent and deeper respirations.
- If exposure continues at that level for several hours, minimal "acidosis" (an acid condition of the blood) may occur but more frequently is absent.
- Breathing rate doubles at 3% CO2 and is four times the normal rate at 5% CO2.
- Toxic levels of carbon dioxide: at levels above 5%, concentration CO2 is directly toxic. [At lower levels we may be seeing effects of a reduction in the relative amount of oxygen rather than direct toxicity of CO2.]
I've run the gauge to nearly 100% before while slinking around in enemy harbors with no ill effects on the crew. As I recall, O2 levels return to 100% almost instantly upon surfacing too.