Air Density (rho) derived from temp, dew point, air pressure and elevation

[Whimlew]

I've just dug into learning about air density (http://bit.ly/2lJt4ev). I needed this knowledge in regards to doing some work with using the value of air density.

Apparently one can determine air density using a variety of parameters, but the most accurate may come from using atmospheric pressure, temperature, dewpoint and elevation.

I have create two calculators, a humid air density calculator (http://gliesians.com/air-density-calculator.faces) and a dry air density calculator (http://gliesians.com/air-density-calculator.faces). I did this using a Java-based utility class for air density that a wrote a couple of weeks ago; JAirDensity (https://github.com/robertjliguori/JAirDensity).

My question is for humid air density... I could only figure out determining the humid air density with three of the four parameters using Herman Wobus's algorithm (air pressure, temperature and dewpoint)...

However, I was not able to implement an equation that incorporates elevation... How significant is factoring in elevation for a more accurate representation of air density, and how closely do rotorcraft pilots pay attention to the value of air density, if you guys do at all?

[HeliComparator]

You only need elevation if you don't know the local air pressure, just the sea level pressure. In other words, you either need sea level pressure and elevation, or local pressure at the location in question. The latter is more accurate since the former relies on an approximate model of the atmosphere - in reality pressure decreases with height according to the complete temperature profile of the intervening air mass, something you probably don't know.

Helicopter flight manuals normally use graphs that allow entry of pressure and temperature. Normally dew point (humidity) isn't taken account of because it's affect is small compared to other errors / approximations.

[[email protected]]

If you look at the standard ICAO atmosphere, used as a baseline for most things aviation, you will see that, as HC says, humidity isn't much of a factor (except to the Meteorological types).

The standard atmosphere assumes sea level pressure of 1013.2 Hpa at plus 15 degrees C and, ISTR a water content of 1.225 kg/cubic metre.

Performance of aircraft very much depends on density altitude (DA) both for aerodynamic and engine performance so there are always graphs for calculating DA from the conditions of the day.

Heights based on the 1013.2 Hpa datum are know as pressure altitude (PA) so the first thing you need is to convert the actual pressure of the day to standard pressure - as a rule of thumb you can use 30 ft per Hpa but that varies with height.

Then to convert PA to DA (without a graph and quite approximately) you compare the actual temperature at your desired height (converted to PA) with what it should be based on the ICAO atmosphere (a lapse rate of 1.98 degrees C per 1000 ft).

The simple equation is DA = PA +/- 120T where T is the difference between the actual temperature and the ICAO temperature for a specific height/altitude.

EG, at sea level with a measured pressure of 993 Hpa and a temperature of plus 25 degrees C - difference from 1013 is 20 Hpa x 30 ft = 600 ft PA.

At 600 ft PA in ICAO, the temp should be plus 14 degrees (or close enough) giving a difference between measured T and ICAO T of 9 degrees x 120 = 1080 ft, added to the 600 ft PA = 1680 ft DA.

Higher local pressures than 1013.2 give lower PA and lower temperatures that ICAO give lower DA.

Apologies if I have just taught you to suck eggs but without knowing your background, I have to assume you might not be familiar with this.

[212man]

Crab, if you look at his links he appears to be interested in actual air density - not Hd. So, I think HC's comments apply in so far as elevation is not important if you know the local pressure.

[[email protected]]

212man - yes but he did ask about helicopter pilots and air density so DA is relevant. Perhaps he needs a relative humidity calculation, perhaps he needs to know about DALR and SALR - difficult to know but I'm sure he'll come back and ask.

However, he has also asked some other strange questions on other threads so we'll see what comes of them.

[5179]

set sub scale 1014.....read height, eg 2000
Guess temp best as......................eg 30C

now.....temp 30C less 15 ...........= 15 x 120 = 1800 ..now add alt height which was 2000 plus this 1800, gives you fairly accurate DA.....of 3800.

[[email protected]]

5179 - not quite - the temp at 2000 ft ICAO (ie PA on 1013) would be 11 degrees so your difference in temp is 19 degrees. 19 x 120 = 2,280 plus your 2000 PA = 4,280 DA.

[5179]

You are indeed correct....my error, thanks

[Gray 14]

Easy to determine appx DA using an E6-B.

[[email protected]]

5179 - no problem, your point is valid though, if you know the pressure and the temperature plus your elevation, you don't need graphs to work out DA:ok:

[HeliComparator]

You do if you want to know it accurately. And "rules of thumb" as mentioned are fine over a reasonably narrow "normal range" but less good at the extremes. I agree that with all the other sources of error, from a performance point of view it's unlikely to matter, but it is good to remember that that sort of "equation" is just a rule of thumb with lots of error. One day it might matter.

[5179]

Agreed......it's "a guide", and when the calculation is getting near your extremes, then out comes the whiz etc.

[[email protected]]

One day it might matter.Agreed - possibly in court if you have exceeded your RFM limits by 'fudging' the figures instead of using the graphs properly and it has all gone horribly wrong.

At least if you have a working knowledge of the rules of thumb, you know if you are close to the limits and can then use the graphs/whizzwheel for clarity.

[chuks]

Is there a slight math error here, or not?

"EG, at sea level with a measured pressure of 993 Hpa and a temperature of plus 25 degrees C - difference from 1013 is 20 Hpa x 30 ft = 600 ft PA.

"At 600 ft PA in ICAO, the temp should be plus 14 degrees (or close enough) giving a difference between measured T and ICAO T of 9 degrees x 120 = 1080 ft, added to the 600 ft PA = 1680 ft DA."

25 - 14 = 11

11 x 120 = 1320

1320 + 600 = 1920 ft DA

[[email protected]]

Chuks - Doh! that is embarrassing - don't know where I got 19 degrees from - you are entirely correct:{

[chuks]

No problem there, Crab. I call it a good day when I remember to put on both bars and get the hat on the right way around with the gold-braided bill at the front instead of off to the left or right.

[212man]

No problem there, Crab. I call it a good day when I remember to put on both bars and get the hat on the right way around with the gold-braided bill at the front instead of off to the left or right.
Don't you just wear it at a rakish jaunty angle?

[chuks]

Actually, just between us, when I did get such a wonderful thing, as a freebie from those losers at Oberpfaffenhofen to mark my transition to aviator status from being just a Twotter driver, I gave it to my son to wear to school here in Germany, when he promptly put in on "rapper" style, backwards.

I turned it the right way around; he turned it back around, saying "Jermaine wears his backwards!"

I told him, "Yeah, but Jermaine," product of a local girl and a black American GI, "is black! He looks cool. You just look like some doofus with his hat on backwards! Wear it the right way around, dammit!"

Getting that hat with the gold braid on its bill must have been the high point of my aviation career, now that I think of it. Oh well, but now I know how to figure out density altitude without a calculator ....

[Whimlew]

I was able to create dry air density and humid air density methods which is what my primary goal was. Thank you all for your input.