I've seen the following equation for calculating density altitude on a dozen websites:
[ (OAT - ISA) * 120 ] + Pressure Altitude.
But it was never very clear what the units of measurement are supposed to be.
Question 1: Are OAT and ISA in Celsius?
Question 2: So for example, if current true altitude is 6000 feet above sea level then: ISA = 15 + (-2C * 6) because for each 1000 feet we lose 2 Celsius. Does this mean to make this calculation I just substitute field elevation for true altitude? So ISA is equal to 3.
If the outside air temp is 10C, then we have 10 - 3 = 7 multiplied by 120 = 840.
Question 3: Pressure altitude confuses me a little bit, I read that it is the altimeter reading when the Kollsman window is set to 29.92" hg. So let's say I'm sitting in my airplane on the runway, I set my altimeter to 29.92 and it reads 6200 feet. In this case my final equation would be: 840 + 6200 = 7040 feet density altitude.
What happens if I don't have access to my plane's barometer, is there another way to calculate pressure altitude?
Question 4: How does this all change if I'm trying to figure out density altitude while flying the plane? OAT is the air around the plane. What about ISA? Do I calculate pressure altitude by resetting my Kollsman window in the middle of the flight to 29.92" hg? Is it normal to try to figure out density altitude while up in the air?
Question 5: Does this entire equation change or become meaningless above a certain altitude threshold, such as 10,000 feet above sea level?