hi-
I was a bit bored at work today so I started playing around with an air pressure/altitude formula. Formula works fine and gives me the correct result, but when I add in the units I get an error "Operation cannot be performed with units". Pretty sure it's most likely a user error, but can someone explainAir Pressure at Altitude.sm (8,88 KiB) scaricato 1177 volta(e). this to me?

thanks

Hi Sttrebo. You can't raise to this power:



I guess that's because the correct value for R is



That's because SMath is so useful doing engineering calculus: (mini) dimensional checking at fly of almost all formulas.

Best regards.

Wrote

can someone explain

You calculate pressure vs altitude from a theoretical invalid book formula.
Above your head [mine too !] is not homogeneous and can't be represented by a single formula.
The worst of that fact is that the discontinuity is sensibly around 11 km. That's why airplanes
tend to fly above or below that altitude in best agreement with the air-trafic assigned corridor.
In short: Atmosphere is only tabulated [OIAC, ISA, PDAS]
PDAS is from Ralph Charmichael, the most recognized.
We agree that that actual 'P' could be corrected vs 'km' altitude from estimate. and more parameters.
That correction, I have never seen any proposed correction.
For practical use, you may use some of the formulas-approximations in the attached.
Those things, you will find nowhere else ... Why ?
Advanced CAS like Smath, Mathcad ... are above the head of the old style Techocrates.
Something like Lagrange refused Fourier works up until Lagrange passed away.

Please, do not hesitate for more questions.
In those documents, you will find the link to PDAS Ralph web site.

Jean

Inst_ISA [kPa].sm (14,1 KiB) scaricato 969 volta(e).
Inst_ISA.sm (141,53 KiB) scaricato 1194 volta(e).
Inst_ISA Altitude vs Pressure.sm (77,3 KiB) scaricato 1169 volta(e).