Standard Atmosphere
A properly installed barometer can be used with the hydrostatic equation to quantify the pressure of the atmosphere.
With the barometer, according to Torricelli's discovery, the pressure in the void above mercury is zero (the first quantified pressure). We let the hydrostatic principle start "in the void" and be applied along a path downward through liquid mercury to the atmosphere (at zero elevation and sea level). Thus the magnitude of term (1) (previous page) is zero and the result of the calculation will be the pressure of the atmosphere.
pvacuum(z* = 0.76m) - ρmergurygo(0.0 - 0.76m) = patmosphere
To proceed, the acceleration of gravity and the density of mercury must be known and applied.
0 + 13,595(kg/m3)(9.81m/s2) (0.76m) = 101,300 N/m2 = 101.3kPa
| Standard Atmosphere |
| 25°C 101.3 kPa ρ = 1.2 kg/m3 |
| 80°F 14.7 lbf/ft2 ρ = 0.075 lbm/ft3 |
Next to the assumed "zero pressure" of the void in a barometer, the most commonly known pressure is the called Standard Atmosphere. Measurements of atmospheric pressure have been recorded at sea level all over Earth and found to vary only slightly from the average 101.3kPa (or 14.7 psi). In the absence of a specified pressure, engineers assume those values to be standard.