A Prototype Active End-of-Service-Life Indicator for Respirator Cartridges

Abstract

Abstract Respirator cartridges are used to allow workers to remain in an environment that contains gases which would be harmful to breathe. One of the problems users face is determining when the cartridge service life has ended. Air-purifying respirators for use with vapors are certified by the National Institute for Occupational Safety and Health (NIOSH). When the cartridge is no longer reducing the toxic gas concentration below the minimum permissible level, a ‘warning’ may take the form of an odor; an irritation to the eyes, nose, or throat; or perhaps dizziness. From the viewpoint of a user, the most desirable cartridge should have an indicator that actively and unambiguously notifies the operator when the cartridge's useful service life is almost ended. In 1984, NIOSH published standards for certification of Active-End-of-Service-Life-Indicators (AESLI) to encourage the development of AESLI systems. AESLI should provide advance warning to the user that the cartridge is 90 percent expended. To this date, development efforts have not been extensive, and no AESLI has been certified in an air-purifying respirator. This article presents a prototype AESLI designed to be used with organic vapors. The prototype device consists of a sensor that is to be located within the bed of the cartridge and is electrically connected to a signal-processing module located on the face mask. The small, low-power module causes a LED alarm to flash when the gas concentration reaches a preset threshold value. The LED is mounted on the mask, directly in front of the user, and is clearly visible when flashing; however, it does not obstruct vision. Locating the sensor within the filter protects the sensor from species in the ambient air that do not pass directly through the cartridge bed during use, thus reducing the demand for sensor selectivity and protecting the sensor from exposure to the breath of the user. The active element is a chemiresistor, a device whose resistance changes with the concentration of the toxic species. Important features of the sensor are 1) a generic response to many of the contaminants that the cartridge is designed to remove and 2) room-temperature operation which results in very low power consumption and safe operation even in a flammable or explosive atmosphere. The sensors were tested for response to cyclohexane in air mixtures (100–4000 ppm). Further, initial tests of the sensors in a compensating bridge circuit suggest that the effects of changing relative humidity and temperature may be minimized.