Creating an Air Density Chart-math needed.

STEVEusaPA

Happy New Year gang. Does anyone know (or know where I can find) the math to re-create this chart? Been googling a long time, no luck.
Why? I want to put the calculation into an airflow app I'm trying to create. It's usually easier when programming to put in the math rather then create a whole table/chart to look up.
Thanks.
Steve

Tim_D

I would check the ASHRAE material. I would start with Fundamentals and then maybe Applications.

ewang

There is an Excel add-in called Psych that we've used in the past.
https://wcec.ucdavis.edu/resources/software-resource-applications/

You may have to input several assumed constants (wetbulb, etc) to get the outputs you want.

STEVEusaPA

@ewang Thanks! I'll take a look.

Jamie Hall

Good discussion here, with formulae :https://en.wikipedia.org/wiki/Density_of_air

STEVEusaPA

Jamie Hall said:

Good discussion here, with formulae :https://en.wikipedia.org/wiki/Density_of_air

Thanks. That just makes my brain hurt LOL. I'm just trying to find/figure out the formulae/calculations that will allow me to reproduce the chart I posted above.
Or I'll put the chart and all the numbers into the app and just look them up.
But the chart doesn't allow for the in between numbers for temperature and altitude, or maybe it doesn't have to be that exact.

Jamie Hall

Really it's not that bad. Since you aren't trying to correct for humidity, you only need the one equation (that first one). density = pressure divided by temperature, times a constant. I think the reason it looks hard (and isn't obvious) is that you need to use both the absolute pressure, not gauge pressure, and the absolute temperature -- not what's on the thermometer. In English units, the absolute pressure is the gauge pressure plus 14.7. The absolute temperature is that thermometer (Fahrenheit) plus 460.

If you want to work with altitude rather than pressure, it's a good deal messier (no surprise there! -- ask any pilot!). The density at any altitude can be found through a relatively simple but still somewhat confusing equation: it's equal to a constant times the value of 1-a constant times the altitude raised to a particular constant -- for the standard atmosphere. But that atmosphere is not standard -- some days are colder than others! -- so I' probably shoot for the first equation (first paragraph) and rather than concerning myself with altitude concern myself with barometric pressure corrected for altitude (or in ductwork absolute pressure)

STEVEusaPA

Now that you put it that way, so much simpler…lol
I am getting closer. Thanks again.

Derheatmeister

1.You Could get in touch with NCI and ask them what the multiplier is
2.You Could take the time and reverse engineer the formular ( It is somewere between 3% and 4 % for every 1000' ) This is going to be time consuming because they rounded the numbers up and down to the nearest solid number..
Good Luck.

Jamie Hall

Derheatmeister said:

1.You Could get in touch with NCI and ask them what the multiplier is
2.You Could take the time and reverse engineer the formular ( It is somewere between 3% and 4 % for every 1000' ) This is going to be time consuming because they rounded the numbers up and down to the nearest solid number..
Good Luck.

Not quite so fast. If you go by absolute pressure and absolute temperature, the relationship is linear and there is a nice single constant. It's a minor hassle to correct in a computer program to absolute pressure from gauge and absolute temperature from Fahrenheit.

However. If you go by altitude, as the chart does, and not pressure, the relationship is NOT linear and there is no simple constant involved. You can use a linear approximation within a two or three thousand foot range, but that's it. Pilots know this -- or should, but there's a lot of crumpled metal in the mountains to say otherwise.

delcrossv

This cuts to the chase. Equation 2 in the density section as you can ignore lapse rate. (they even give SAE units.)
https://en.wikipedia.org/wiki/Barometric_formula

STEVEusaPA

@delcrossv Thanks. I found the formulae and figured it out. The formula needed to create the factor is:


Here's what the calc looks like on a spreadsheet
=(70+460)/('Temperature Cell'+460) *(POWER(1-(6.8754*0.000001* 'Altitude Cell'),5.255588))



Thanks for everyone's help. If anyone wants the spread sheet, PM me with your email address. In about a week I'll have this in my app I made for iPhone.