The issue of air density
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The data in the figures are taken from the Handbook of Chemical and Physical Properties (Inorganic Volume), which contains the physical properties of air. The table above shows the density of air at standard pressure, while the table below shows the density of air at different pressures. According to the table above, at 0 degrees and standard pressure, the density of air is 1.293 kg/m3; whereas according to the table below, at a pressure of 0.1 MPa and a temperature of 120 K, the density is 0.2938 kg/m3. This density value is contradictory to the previous one (the density actually decreases under pressure and at low temperatures). I wonder if any fellow sailor can explain it.
Calculations are performed using 0 degrees as the reference temperature and an air density of 1.293 at an absolute pressure of 0.1 MPa; the density of air at any temperature and pressure is given by 1.293 * P, where P is the actual pressure in kPa and T is the temperature in degrees Celsius℃
This post was last edited by wanlirn on 2018-12-12 at 16:25. The pressure units in the table seem incorrect; the pressure should be in atm as absolute pressure, and the temperature in K, then it will be correct
Thank you for your reply. The values are somewhat similar when the pressure is low, but after doing some calculations, I found that the difference is quite large when the pressure is high.
In the table, for pressures in the range of 2.0–4.0, within the same pressure column, the density at 140K is higher than that at 120K, which is contrary to common sense.
I’m just guessing. The mixed gas might have partially liquefied, so it’s not counted as part of the gas phase. Low temperatures are encountered less often; I should check the air separation design manual – it contains all the information related to low-temperature air
It’s already quite good that you can think of pressure in terms of absolute ATM pressure.
I also think it’s a mistake on the part of the unit; MPa should be changed to atm