Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D3631-99 – Standard Test Method Technical Guide
🔬 Instrumentation and Scope of D3631
ASTM D3631-99 (Reapproved 2017) defines standard test methods for measuring surface atmospheric pressure using a Fortin-type mercurial barometer or an aneroid barometer. These methods deliver instantaneous pressure values, with the mercurial barometer requiring specific safety precautions as outlined in Section 7 of the standard. Measurements are conducted exclusively in SI units.
📏 SI Unit Definitions and Conversion Factors
Per the standard, the fundamental unit of pressure is the pascal (Pa). To assist users with instrumentation calibrated in legacy units, D3631 provides exact conversion factors derived from standard gravity (9.80665 m/s²).
| 🟦 Unit | 📏 Equivalent to 1 Standard Atmosphere | 🎯 Value of 1 Pascal (Pa) |
|---|---|---|
| Pascal (Pa) | 101,325 Pa | 1.00000 Pa |
| Millibar (mbar) | 1,013.25 mbar | 0.0100000 mbar |
| Inches of Mercury (inHg) at 273.15 K | 29.9213 inHg | 0.000295300 inHg |
| Millimeters of Mercury (mmHg) at 273.15 K | 760.000 mmHg | 0.00750062 mmHg |
| Pound-force per square inch (psi) | 14.6959 psi | 0.000145037 psi |
📌 Technical Reference: The standard explicitly defines a standard atmosphere as exactly 101,325 Pa. This value is fundamental for meteorological calculations and calibrations performed under the jurisdiction of ASTM Committee D22. All other units are derived from this SI base.
🌡️ Validity Range and Spatial Data Extension
A pressure measurement is statistically valid within a horizontal distance of 100 m and a vertical distance of 0.5 m of the instrument, assuming no catastrophic atmospheric events (e.g., tornadoes). For applications requiring pressure data beyond this zone, D3631 outlines a strict set of criteria for calculation and extension.
| ⚡ Extension Parameter | 📐 Requirement |
|---|---|
| Maximum Lateral Extension | 2000 m |
| Maximum Vertical Extension | 500 m |
| Maximum Barometric Drift (10 minutes) | Less than 20 Pa |
| Horizontal Pressure Gradient (Typical) | 1 Pa/100 m or less |
The calculation for extending a known pressure P₁ at altitude h₁ to a new site P₂ at altitude h₂ is heavily dependent on the mean absolute temperature of the intervening air column. Specific functional relationships and tabulated pressure differences per meter of height are provided in Sections 10.2 and 10.4 of the standard.
⚠️ Critical Limitation: The spatial extension criteria defined in Section 1.3 are strictly void in the presence of catastrophic phenomena. Furthermore, the temporal stability requirement (less than 20 Pa change in 10 minutes) must be rigorously validated before applying any height correction formula.
❓ Frequently Asked Questions
🔍 What is the standard horizontal distance of validity for a direct pressure measurement?
In the absence of abnormal perturbations, atmospheric pressure measured by these test methods at a point is valid everywhere within a horizontal distance of 100 m and a vertical distance of 0.5 m of that point.
💡 Which two types of barometers are specified in D3631-99?
The standard specifically covers test methods for the Fortin-type mercurial barometer and the aneroid barometer.
⚡ What criteria must be met to extend a pressure reading to a distant site?
The new site must be within 2000 m laterally and 500 m vertically of the measurement point, and the atmospheric pressure must have changed by less than 20 Pa over the 10 minutes preceding the measurement.
📌 What is the value of one standard atmosphere in Pascals?
One standard atmosphere is exactly equivalent to 101,325 Pa (101.325 kPa) at a standard gravity of 9.80665 m/s².