It calculates air time remaining by determining your current consumption rate. It updates itself frequently, I forget just how frequently. But adding air to your BCD, purging your reg through the purge button, free flowing the reg, and etc all have an effect on the air time remaining calculation. So if your purging your reg the air time remaining calculation will be incorrect until you stop purging and the cobra updates its calculations. Just remember that the calculation is always based your current air consumption/conditions and if those conditions change, then so will your air time remaining.