Corrected Altitude

[Bob Drury]

Does anyone know if there is a standard mathmatical formula for correcting altitude?  I know you can buy  the software, but I was wondering if you could do it by hand with a hygrometer, barometer and thermometer?

[hitz]

Bob, I think if you buy/beg a working aircraft altimiter and set the pressure at 29.92 "/HG it will give you the corrected altitude.
   The better the altimeter the closer the results.If I'm wrong on this I am sure we'll hear about it.
 Harv

[John Burk]

Bob
   Atmospheric pressure drops about 3.78% for every 1000 feet above sea level .

[Bob Drury]

I think I should have said  Corrected Density Altitude.

[fwillyj]

-I use a formula that I got from a Holley book years ago.
 -it goes like this:
 -29.92"HG (ABSOLUTE air pressure) times 1.326 equals 39.67392
 
 -Then 59.9 degrees F plus 459.6 degrees F which equals 519.5
 
 -Then 39.67392 is divided by 519.5 equals .0763694.
 --------------------------------------
 -This .0763694 is considered 100% air density at sea level on a 59.9 degree F day considering dry air.
 --------------------------------
 -Now if you run at Bonneville two major numbers in the formula change, like the barometer at the starting line in august may be 26.20"HG instead of 29.92"HG and the temperature may be 105 degrees F.
 -so the results of this are:
 -26.2 x 1.326 = 34.714, divided by (105 + 459.6 = 564.6) 564.6 = .0614842
 -----------------------------
 -So the percentage difference in the basic air density considering dry air in both cases is:
 .0614842 divided by .0763694 = 80.5%
 --------------------------------------
 -This .0763694 and the .0614842 are the weights of ONE cubic foot of air at either altitude.
 -This formula doesn't relate to us anything about altitude, just the density of the air.
 ----------------------------
 -The next small problem is you should really include the humidity but that is a HUGE problem because you have to go thru a lot of hassle to get the vapor pressure which this vapor pressure needs to be subtracted from inches of mercury to get dead nuts.
 -The vapor pressures vary from .025"HG at 40 degrees F and 10% humidity to .580"HG at 100 degrees F and 30% humidity which is the close to the range you'd likely see at Bonneville.
 --------------------------------
 -One major problem I see with this is having a way to actually measure ABSOLUTE air pressure in inches of mercury.
 -Most household barometers aren't capable of doing baraomtric pressures much lower than about 28".
 
  <small>[ April 23, 2005, 12:24 PM: Message edited by: fwillyj ]</small>

[Bob Drury]

Willy, Thanks, I think I need to buy the program, I can't even remember my phone number..............

[hitz]

Bob,
   And if that isn't enough:try
  www.wahiduddin.net/calc/density_altitude.htm
  Plenty of formulas and online calulators!
 
 Harv

[Glen]

Racers, The DA is on the time slips at Bonneville and the timing group will reply while you are setting in line waiting to run and want to make an adjustment. Just call the timing trailer on your CB radio.

[Bob Drury]

Glen, is there a normal channel to the timing tower other than the main channel?  Thanx....

[Glen]

Just Channei #1 We can change to another to give info just ask the person timing at the time.
 We aim to please.

[promachine]

Also , the corrected alt is usually posted at the gas trailor on the way to the starting line.
 (if you run gas}.
 Actually, if you look in the National Dragster you will see some weather stations advertised that really arnt bad price wise.
 Or if you dont want to spend any money at all, you could just walk from pit to pit and sure enough sombody will be able to tell you how bad the air is. I have heard that the land speed racer group are a pretty freindly bunch and are more than happy to help their fellow racer.

[dwarner]

John is right - look at the chalk board at the gas trailer on your way to the starting line, wait 4 hours and do the old 10-10-10(riching 10, add 10 lead and if fuel add ten) or EFI.

[ddahlgren]

Ah EFI tunes while running down track.. In a 5 mile run what makes you think it is the same at the 5 as it is at 0?
 Dave

[Jack Hester]

Bob -

Here is a small program that I'm working on, for my use when I start running my Flathead Harley at Maxton, NC.  It's a freebie to anyone who wants it.  No installation necessary, as it runs just like it is.  However, it's still in Beta.  No corrections for non-numerical characters entered where numbers should be.  Save it to a folder on your machine.  It can save your data in an Excel spreadsheet, if you like.  Or, in just a text format.

http://www.terra-glide.net/pgms/LSRutil.exe

I work on it when time permits, as I no long program for anything other than as a hobby.  The calculations are correct, and tested against published tables.  Right-click on the link and save it to your computer.  Give it a try and let me know if it serves your purpose.

Jack

[desotoman]

Bob,
   I bought a Kestrel 4000 Pocket Weather Tracker. If I remember it was a little over $200 to my house. I think it is great. It comes with a certificate of Conformity. It tracks:
Wind speed,
Current Temp,
Chill Factor,
Humidity,
Heat Index,
Dew Point,
Wet Bulb,
Barometer,
Altitude,
Density Altitude,
and has 3 screens you can put what ever you want on. It also will track the weather during the day if you want. Neat unit and you will never find one like this any cheaper. Well worth the investment. If you don't want to spend that much, I think you can still get a Moon type density gage for around $100.
I hope this helps,
Tom Gerardi