ASTM D3631-99(2011)

1.1 These methods cover the measurement of atmospheric pressure with two types of barometers: the Fortin-type mercurial barometer and the aneroid barometer.

1.2 In the absence of abnormal perturbations, atmospheric pressure measured by these methods at a point is valid everywhere within a horizontal distance of 100 m and a vertical distance of 0.5 m of the point.

1.3 Atmospheric pressure decreases with increasing height and varies with horizontal distance by 1 Pa/100 m or less except in the event of catastrophic phenomena (for example, tornadoes). Therefore, extension of a known barometric pressure to another site beyond the spatial limits stated in 1.2 can be accomplished by correction for height difference if the following criteria are met:

1.3.1 The new site is within 2000 m laterally and 500 m vertically.

1.3.2 The change of pressure during the previous 10 min has been less than 20 Pa.

The pressure, P₂ at Site 2 is a function of the known pressure P₁ at Site 1, the algebraic difference in height above sea level, h₁ − h ₂, and the average absolute temperature in the space between. The functional relationship between P₁ and P₂ is shown in 10.2. The difference between P₁ and P₂ for each 1 m of difference between h₁ and h₂ is given in Table 1 and 10.4 for selected values of P₁ and average temperature.

1.4 Atmospheric pressure varies with time. These methods provide instantaneous values only.

1.5 The values stated in SI units are to be regarded as the standard.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific safety precautionary statements are given in Section 7.

TABLE 1 Selected Values

Average Tempera- ture,	Pressure P1, Pa
	110 000	100 000	90 000	80 000	70 000
	Correction to P1, Pa/m, positive if h1 > h, negative if h1 < h2
230	16	15	13	12	10
240	16	14	13	11	10
250	15	14	12	11	10
260	14	13	12	11	9
270	14	13	11	10	9
280	13	12	11	10	9
290	13	12	11	9	8
300	13	11	10	9	8
310	12	11	10	9	8