# I Why does the Dry Air Gas Constant vary?

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1. Mar 25, 2017

### Robert James Liguori

I'm confused on why the dry air gas constant varies:
• Dry Air Gas Constant of 286.9 from The Engineering Toolbox
• Dry Air Gas Constant of 287.22 from https://www.physicsforums.com/ttp://www.peacesoftware.de/einigewerte/luft_e.html [Broken]
• Dry Air Gas Constant of 287.05 from Brisbane Hot Air Ballooning
• Dry Air Gas Constant of 287.058 from Wikipedia
Can someone please explain why it differs and how it is constructed. Thanks, Robert

Reference:

Last edited by a moderator: May 8, 2017
2. Mar 25, 2017

### lychette

to within 3 significant figures these are all the same !!!
It would be worth checking how each source obtained the value, especially the date when the values were measured.
Methods vary and improve over the years

3. Mar 25, 2017

### Robert James Liguori

I am not a physicist... so you are implying that the differences in the values is insignificant? To me, it should be exact... or a means should be in place to get the exact value at each given location of sampling... if this was indeed possible, which it may not be. Please advise.

4. Mar 25, 2017

### Robert James Liguori

universal gas constant = 8314.32 (in 1976 Standard Atmosphere)
molecular weight of dry air = 28.964

8.3143 / 28.964 = 0.28705634580859 (e.g., 287.056)

5. Mar 26, 2017

### Robert James Liguori

I figured it out... thanks for your help.

Last edited by a moderator: Mar 28, 2017
6. Mar 26, 2017

### Staff: Mentor

Can you measure the length of your kitchen table exactly? Why not? Are you going to let that keep you from buying a tablecloth?

Even if the value of the gas constant is not exact, does that bring your calculations to a screeching halt (until you find the exact value, which doesn't exist)? Isn't 3 significant figures accurate enough for you?

7. Mar 28, 2017

### Staff: Mentor

On these words of wisdom, the OP's question has been answered. Thread closed.