mechmon
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calculating Volume using US units

Mon Feb 05, 2024 10:03 pm

Hello all,
I am building a circuit involving a bmp230. As a check I wanted to verify the results I was getting. For "reasons" I am using us customary units.
I know at stp the volume of 1 mol of dry air occupies 22.414L. At least I assume NASA is right. L->ft3 is 0.7915429ft3.
V=nRT/p

n=1, R=1545.329(ft lbf)/(lbm mol Β°R), T=59F=518.67Β°R, p=14.696lbf/in2=2116.224lbf/ft2.

Just plugging the values in the above leaves me with 378.75ft3/lbm.
I know this isn't correct.
Even if I use n = 28.966lb/mol or 28.966g/mol*lb/453.59g, the units do not come out correct.

Can someone point out where I'm wrong?
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W. H. Heydt
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Re: calculating Volume using US units

Mon Feb 05, 2024 10:16 pm

You could do the calculation in metric and just convert the end result.

As it is...I'm not sure what you are calculating the volume *of*.

terribleted
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Re: calculating Volume using US units

Mon Feb 05, 2024 10:23 pm

also, shouldn't temp be Rankine, and pressure absolute? (for english units)
agree with @whHeydt, calc using what you are good at, then convert the result
The only people that don't make mistakes are people that don't do anything.

mechmon
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Re: calculating Volume using US units

Mon Feb 05, 2024 10:24 pm

I "could" but that won't scratch my brain itch.....

Dry air.
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drgeoff
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Re: calculating Volume using US units

Mon Feb 05, 2024 10:54 pm

What has this to do with Raspberry Pi?
Quis custodiet ipsos custodes?

mechmon
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Re: calculating Volume using US units

Mon Feb 05, 2024 11:06 pm

It's being developed for a picow.

Why here? The "people" over at the physics stack exchange said the question had nothing to do with pysics.
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davidcoton
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Re: calculating Volume using US units

Mon Feb 05, 2024 11:51 pm

mechmon wrote: ↑
Mon Feb 05, 2024 10:03 pm
L->ft3 is 0.7915429ft3.
Really?
A litre is a cube of 100mm edges, about 4".
12" to 1', so three litre cubes fit along 1'.
That makes about 27 litres in a cubic foot.
1/27 is not 0.79.
Either there is a problem there, or maths or physics has changed since I learnt it, or my brain has deteriorated more than I thought.
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mechmon
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Re: calculating Volume using US units

Mon Feb 05, 2024 11:57 pm

22.414L = 0.791543ft^3
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davidcoton
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Re: calculating Volume using US units

Tue Feb 06, 2024 12:01 am

mechmon wrote: ↑
Mon Feb 05, 2024 11:57 pm
22.414L = 0.791543ft^3
Maybe, but that is not what you wrote:
L->ft3 is 0.7915429ft3.
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mechmon
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Re: calculating Volume using US units

Tue Feb 06, 2024 12:04 am

22.414L = 0.791543ft^3
22.414L = 0.022414m^3

1 m^3= 35.314667ft^3
35.314667*0.022414=0.791543.

Okay.
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scruss
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Re: calculating Volume using US units

Tue Feb 06, 2024 12:05 am

I don't particularly condone using broken measure, but the units command will help you out here:

Code: Select all

units "R * tempF(59) / 14.696psi" "ft^3 / mol"
	* 0.83500638
	/ 1.1975956
or the same result, except in sensible units:

Code: Select all

units "R * tempF(59) / 14.696psi"
        Definition: 0.023644748 m^3 / mol
The units command has some weird syntax conventions around temperatures, and its manual is too long to be helpful. You can browse its conversion file using

Code: Select all

less `units -U`
but that may make you more confused than when you started.
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mechmon
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Re: calculating Volume using US units

Tue Feb 06, 2024 12:19 am

Yep I know all the ways back to front to convert from one unit to the other.

That is not my issue.

There is something simple I am not remembering because when I put the units into the equation above I do not get the correct value, or units.

I have a feeling it will be a "duh!" moment...
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scruss
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Re: calculating Volume using US units

Tue Feb 06, 2024 12:55 am

mechmon wrote: ↑
Tue Feb 06, 2024 12:19 am
Yep I know all the ways back to front to convert from one unit to the other.
I showed you a way that didn't need you to do any conversion.

You're doing something wrong in your calculation for sure, but I can't follow it. You've given us two measured values (59Β°F, 14.696psi): what you haven't told us is what you're trying to calculate from those values.

Are you sure you mean to be using R, and not Rs?

Edit: Molar mass of air: 0.06385469 lb, Molar volume of air at (59Β°F, 14.696psi): 0.83500638 ft^3 β€” so there would be 13.0766648463879 ft^3 to the pound.
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davidcoton
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Re: calculating Volume using US units

Tue Feb 06, 2024 8:54 am

If you want your calculation checked, then you need to show it, like in a maths test -- line by line, labelling what you think you are calculating and the intermediate result. Like the earlier example where you labelled a figure as "L->ft3" when it was actually 22.414L->ft3.

At the moment, you have not shown enough of your working for anyone to follow, apart from confirming that your initial answer is wrong.
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