42 CFR §84.83

1. (a)Elapsed time indicators shall be provided for apparatus with a chemical oxygen source, except:
   1. (1)Apparatus used for escape only; or
   2. (2)Liquefied breathing gas apparatus equipped with gages visible to the wearer which indicate the remaining liquid content in the container.
2. (b)The timer or other indicator shall be accurately calibrated in minutes of remaining service life.
3. (c)Timers shall be readable by sight and by touch during use by the wearer.
4. (d)Timers shall be equipped with automatically preset alarms which will warn the wearer for a period of 7 seconds or more after the preset time has elapsed.
5. (e)Remaining service-life indicators or warning devices shall be provided in addition to a pressure gage on compressed gas self-contained breathing apparatus, except apparatus used for escape only, and shall operate automatically without preadjustment by the wearer.
6. (f)Each remaining service-life indicator or warning device must give an alarm when the remaining service life is reduced to a minimum of 25 percent of its rated service time or any higher minimum percent value or values as specified in the approval. Open-circuit demand and pressure-demand respirators must alarm continuously until depletion of the breathing air supply. The percent value set for indicator activation must be identified by labels and/or markings on each respirator unit.