Not quite that simple my friend.
The relationship of density to temperature and pressure can be expressed as... P (pressure in mb) divided by [T (absolute temperature) x Density (Rho)] = Constant. Where Rho is mass per unit volume (usually expressed as kg per m3).
When air is compressed, a greater amount can occupy a given volume; i.e. the mass, and therefore, the density, has increased. Conversely, when air is expanded less mass occupies the original volume and the density decreases. From the above formula it can be seen that, provided the temperature remains constant, density is directly proportional to pressure, i.e. if the pressure is halved, so is the density, and vice versa.
Hope this helps.
Oooops... I should have read the question. Genghis is obviously on the ball.
Last edited by TheChitterneFlyer; 4th Jan 2008 at 10:25.