40 CFR §86.139-90

1. (a)At least 8 hours, but not more than 56 hours before the test, place each filter in an open, but protected, petri dish and place in the weighing chamber which meets the humidity and temperature specifications of § 86.112.
2. (b)At the end of the 8 to 56 hour stabilization period, weigh the filter on a balance having a precision of one microgram. Record this weight. This reading is the tare weight.
3. (c)The filter shall then be stored in a covered petri dish which shall remain in the weighing chamber until needed for testing.
4. (d)
   1. (1)If the filter is not used within one hour of its removal from the weighing chamber, it shall be reweighed.
   2. (2)The one hour limit may be replaced by an eight-hour limit if one or both of the following conditions are met:
      1. (i)A stabilized filter is placed and kept in a sealed filter holder assembly with the ends plugged, or
      2. (ii)A stabilized filter is placed in a sealed filter holder assembly, which is then immediately placed in a sample line through which there is no flow.
5. (e)After the test, and after the sample filter is returned to the weighing room, condition it for at least 1 hour but not more than 56 hours. Then weigh a second time. This latter reading is the gross weight of the filter. Record this weight.
6. (f)The net weight (Pe) is the gross weight minus the tare weight.