# Thread: Density Altitude Gauge-- How hard would it be to make one of these?

1. ## Density Altitude Gauge-- How hard would it be to make one of these?

This is probably a bit of a reach, but I figured it's worth a try:

Is it possible to make a Density Altitude Gauge, using the parameter of "Ambient Density" in units of Slugs/cubic feet, and then converting that to density altitude in linear feet?

Since ambient density varies with sea level pressure, temperature, and the plane's actual altitude (and I suppose relative humidity would also be a factor in real life-- though in the simulator changing the dew point temp seems to have no effect on the density reading), I thought there might be a way to get the simulator program to calculate the equivalent density altitude if the above factors were taken into consideration, by entering the correct formula into the .xml file. But again, I'm not 100% sure.

Thanks,

-- John

2. Member
Join Date
Jul 2009
Posts
735
You can use pressure altitude and outside airtemperature.
http://en.wikipedia.org/wiki/Density...de#Calculation

from a gauge I made once:

Code:
`<Macro Name="DensAlt"> (A:PRESSURE ALTITUDE, feet) (A:AMBIENT TEMPERATURE, celsius) (A:PRESSURE ALTITUDE, feet) 1000 / -1.98 * 15 + - 120 * + </Macro>`
Last edited by jeroen79; 06-30-2012 at 03:43 PM.

3. This is my formula for calculating density altitude (using a simplified model that assumes dry air). It is 95% accurate to the more robust formulas that take dew-point into account.

Code:
```<!--Calculate Density Altitude -->
(A:Ambient temperature, celsius) (A:STANDARD ATM TEMPERATURE, celsius) -
118.6 * (A:Pressure Altitude, feet) + (>L:DensityAlt,enum)
```

4. Thank you to both 'jeroen79' and 'n4gix'. I will try both and run them side by side just to see how close they approximate one another in practice.

It's been my experience that when manually setting my own weather conditions in the "user weather" section, that changing the dew point temp (relative to the chosen actual temp) makes no difference in the "ambient density" as far as the simulator is concerned. Therefore any formula for density altitude that is dependent upon a specific %'age of relative humidity doesn't appear to have any bearing.

5. Bill; Thanks for pointing out the existence of A:STANDARD ATM TEMPERATURE. Is there A:STANDARD ATM PRESSURE - I cann't find it in the SDK?

6. Originally Posted by mgh
Bill; Thanks for pointing out the existence of A:STANDARD ATM TEMPERATURE. Is there A:STANDARD ATM PRESSURE - I cann't find it in the SDK?
Not that I've seen either. Even this one seems to be new to FSX, since there's no such variable listed in the FS9 SDK.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•