Air density is calculated from your environmental readings and it applies several corrects to the flow calculated from your flow pressure readings.
How is air density calculated?
It’s best to simplify this by looking at how to calculate air density without factoring in that it may contain water. Air density for dry air is calculated as below:
So from this we can see that both the barometric pressure and temperature are being used in the calculation. Both and internal (ρi) and external (ρe) air density are calculated.
| Location | Temperature | Barometric Pressure | Air Density (Dry Air) |
|---|---|---|---|
| Internal (ρi) | 20°C | 1000 hPa | 1.1884 |
| External (ρe) | 10°C | 1000 hPa | 1.2303 |
The equations for the internal and external air density are as shown below:
What happens when we consider water vapour?
Water as a gas is called water vapour, which occurs when water as a liquid evaporates. Water vapour has less weight to it than both Nitrogen and Oxygen which together form 99% of air. The difference in the molecular mass of water vapour (18.016 g/mol) and dry air (28.966 g/mol) is that water vapour has a molecular mass 37.802% less than dry air. Therefore the more water vapour in air the lighter it is.
This is why considering water vapour present in air applies a further adjustment to the air density calculation. This adjustment is applied to the absolute pressure by modifying the previous calculation as follows:
The equation to calculate the water vapour pressure is shown below.
The other 3 numbers are constants and we also assume 50% relative humidity in the ATTMA test standards rather than measuring it.
The equations for the internal and external air density are as shown below:
The comparison with and without the water vapour adjustment are shown below.
| Location | Air Density (Dry Air) | Air Density (Water Vapour) |
|---|---|---|
| Internal (ρi) | 1.1884 | 1.1823 |
| External (ρe) | 1.2303 | 1.2274 |
Water vapour therefore makes a minor adjustment to the air density but as these calculations are handled in the background within your software there’s no reason to not do these adjustments.
