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A respirometer is a device which is used to measure respiration, where organisms exchange carbon dioxide and oxygen as part of their metabolism.
In-facepiece dust concentration is changed dozens of times in a matter of minutes due to changes of the size of the gaps between the mask and face. A respirator is a device designed to protect the wearer from inhaling particulate matter, including airborne microrganisms, fumes, vapors, i.e. gases.