(1) Temperature. The temperature of the chamber in which the particulate filters are conditioned and weighed shall be maintained to within ±10 °F (6 °C) of a set point between 68 °F (20 °C) and 86 °F (30 °C) during all filter conditioning and filter weighing. A continuous recording of the temperature is required.

(2) Humidity. The relative humidity of the chamber in which the particulate filters are conditioned and weighed shall be maintained to within ±10 percent of a set point between 30 and 70 percent during all filter conditioning and filter weighing. A continuous recording of the temperature is required.

(3) The environment shall be free from any ambient contaminants (such as dust) that would settle on the particulate filters during their stabilization.

(4) It is required that two unused reference filters remain in the weighing room at all times in covered (to reduce dust contamination) but unsealed (to permit humidity exchange) petri dishes. These reference filters shall be placed in the same general area as the sample filters. These reference filters shall be weighed within 4 hours of, but preferably just prior to, the pre- and post-test sample filter weighings.

(5) If the weight of either of the reference filters changes between pre- and post-test sample filter weighings by more than ±2.0 percent of the test average primary filter loading (recommended minimum of 0.5 milligrams) or ±0.010 milligrams, whichever is greater, then the post-test sample filter weights are invalid. However, the post-test weighing procedure can be repeated to obtain valid weights within the time limits as specified in § 86.139.

(6) The reference filters shall be changed at least once per month, but never between pre- and post-test weighings of a given sample filter. The reference filters shall be the same size and material as the sample filters.