Respirators often use filter cartridges to protect a user from breathing potentially hazardous vapors. When the respirator is in place on the user (typically attached to the face or head in a way to form a seal), air is drawn into the respirator through the filter cartridge whenever the user breathes (and air can typically only enter the respirator through the cartridge, so that the air may be filtered by the cartridge to ensure that air breathed in by the user while wearing the respirator is clean and safe). Such filter cartridges typically contain filtering material that can lock up one or more potentially hazardous vapors. As the filtering material is exposed to the vapor, it typically absorbs the vapor molecules through the pore structure of the material. Thus, by their very nature, the filter cartridges have a limited effective lifespan (after which the filtering material has absorbed all it can, and the cartridge cannot filter additional vapor). Once a filter cartridge has reached the end of its service life, it is no longer effective at protecting the user. Then the user should either remove themselves from the environment with hazardous vapors or else replace the filter cartridge on the respirator with a new cartridge. Thus, to effectively protect the user, it can be important to know when to change filters based on the service life of the cartridge.
Typically, the vapor concentration in the filters is low and reliable detection of the low concentrations by gas sensors (such as metal-oxide sensors or electrochemical sensors) may be affected by significant voltage drift in the sensor. The magnitude of the voltage drift in clean air is often greater than the voltage change between clean air and the contaminated air concentration that an end of service life indicator (ESLI) needs to measure. Therefore, direct measurement by the sensor may not be accurate enough to provide an effective end of service life indication.
Applicants have developed an improved end of service life indicator (ESLI) that may overcome the effect of voltage drift in the sensor. This may allow users to effectively replace filters in a safer way (reducing the risk of unintended exposure to potentially hazardous vapors).