## Sunday, May 19, 2013

### HP 35S: Air Density & Density Altitude (Metric-US Conversion Factors included)

HP 35S: Air Density & Density Altitude (7 Digit Accuracy*)

*You can use 4 or 5 digits for the HP 15C and other RPN calculators on which each number takes a step. Slightly less accuracy but smaller program.

Source: Shelquist Engineering web page: http://wahiduddin.net/calc/density_altitude.htm

Formulas

Air Density:

D = P ÷ (R * T)
D = density in kg/m^3
P = pressure in Pascals (Pa)
T = temperature in Kelvins
R = specific gas constant = 287.05 J/(kg*°K)

Air Density Altitude: (where the plane/vehicle thinks it is)
H = 44.3307692 - 42.2665143 * D^0.2349695

H = air density in km

U.S.-SI Conversion Factors
Pressure: (convert and store in P)
1 inHg = 2275.5477799 lb/(ft*s^2) = 3386.3881579 Pa

Temperature Conversion Formula: (convert and store in T)
°K = 5/9 * (°F - 32) + 273.15

Length:
1 km = 3280.8398950 ft

Program
D001 LBL D
D002 INPUT P
D003 287.05
D004 INPUT T
D005 *
D006 ÷
D007 STO D
D008 VIEW D \\ display air density in kg/m^3
D009 0.2349696 \\ dimensionless constant
D010 y^x
D011 42.2665143
D012 *
D013 +/-
D014 44.3307692
D015 +
D016 STO H
D017 RTN \\ display density altitude

I chose to use SI units because the constants are much smaller than the formula would be in US units.

This blog is property of Edward Shore. 2013