A breathing apparatus is necessary if you want to breathe underwater. How do you know that it easy enough to breathe on it? Does your rebreather actually give the right O2 level? Most of such answers can be had from test with breathing simulators, water and pressure chambers. This talk will explain how some of the tests are done and illustrate human tolerance to breathing resistance. How long does the rebreather's scrubber truly last (often far longer than you think)? Can you easily monitor the scrubber function during a dive? Can you trust such a gauge in another dive with the same absorbent? This talk will also show the influence of water temperature and diver workload on the endurance of the scrubber, and explain the method that lets a gauge tell a diver how much absorbent is left in real time.