# Title 40

## SECTION 1065.690

### 1065.690 Buoyancy correction for PM sample media.

§ 1065.690 Buoyancy correction for PM sample media.(a) *General.* Correct PM sample media for their buoyancy
in air if you weigh them on a balance. The buoyancy correction
depends on the sample media density, the density of air, and the
density of the calibration weight used to calibrate the balance.
The buoyancy correction does not account for the buoyancy of the PM
itself, because the mass of PM typically accounts for only (0.01 to
0.10)% of the total weight. A correction to this small fraction of
mass would be at the most 0.010%.

(b) *PM sample media density.* Different PM sample media
have different densities. Use the known density of your sample
media, or use one of the densities for some common sampling media,
as follows:

(1) For PTFE-coated borosilicate glass, use a sample media density of 2300 kg/m 3.

(2) For PTFE membrane (film) media with an integral support ring of polymethylpentene that accounts for 95% of the media mass, use a sample media density of 920 kg/m 3.

(3) For PTFE membrane (film) media with an integral support ring of PTFE, use a sample media density of 2144 kg/m 3.

(c) *Air density.* Because a PM balance environment must be
tightly controlled to an ambient temperature of (22 ±1) °C and
humidity has an insignificant effect on buoyancy correction, air
density is primarily a function of atmospheric pressure. Therefore
you may use nominal constant values for temperature and humidity
when determining the air density of the balance environment in Eq.
1065.690-2.

(d) *Calibration weight density.* Use the stated density of
the material of your metal calibration weight. The example
calculation in this section uses a density of 8000 kg/m 3, but you
should know the density of your weight from the calibration weight
supplier or the balance manufacturer if it is an internal
weight.

(e) *Correction calculation.* Correct the PM sample media
for buoyancy using the following equations:

*m*cor = PM mass corrected for buoyancy.

*m*uncor = PM mass uncorrected for buoyancy. rair = density of air in balance environment. rweight = density of calibration weight used to span balance. rmedia = density of PM sample media, such as a filter. Where:

*p*abs = absolute pressure in balance environment.

*M*mix = molar mass of air in balance environment.

*R*= molar gas constant.

*T*amb = absolute ambient temperature of balance environment. [70 FR 40516, July 13, 2005, as amended at 73 FR 37339, June 30, 2008; 75 FR 23056, Apr. 30, 2010; 79 FR 23805, Apr. 28, 2014; 81 FR 74191, Oct. 25, 2016]