How much gas is a liter, or cubic foot?

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Rich Feldman
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How much gas is a liter, or cubic foot?

Post by Rich Feldman »

Now updated & posted here by invitation from RH. Half of the subject question is still open!

This is all about "Not So Standard Temperature and Pressure". The other day I learned that in USA, for most gases in refillable cylinders, "cubic feet" are at 70 °F and 1 atm. Am still seeking a proper reference giving the (different) STP values that apply when industrial and specialty gases are sold by the liter or cubic meter.

In high school I was taught that "STP" = 0 °C and 1 atmosphere. But back in 1982, IUPAC redefined the P in their STP http://goldbook.iupac.org/S05910.html to be exactly 100 kPa (1 bar; 1/1.01325 atm). That changed the value of "ideal gas molar volume at STP" from 22.4 to 22.7 liters. Did thousands of other references immediately become wrong?

This snip from Matheson catalog page for helium, http://www.mathesongas.com/industrialga ... strial.pdf,
math_he1.PNG
gives ft3 capacities numerically 38.0 to 38.1 times greater than m3 capacities. Same funny ratio appears in the tables for nitrogen, with all gas volumes slightly greater than for He (different story). Huh? Geometrically, a m^3 is only 35.3 ft^3. My unreviewed calculations say the 38:1 ratio is consistent with cubic feet measured at 70 °F and cubic meters measured at 0 °C. , but that doesn't make it official. Nor does seeing the same catalog pages give gas density & specific volume at 70 °F, 14.696 psia. Newbies must learn not to make unjustified inferences. That would be like reading different blocks in a neutron detector datasheet, and assuming that the sensitivity range in block A applies throughout the detectable energy range given in block B. :-) (yes, I design calibration systems for expensive instruments for a living).

There's a similar discrepancy between ft^3 and m^3 units in bulk natural gas, because the "US" and metric NG trade units have different standard temperatures and pressures. Apparently the natural gas standard CF was defined, about a century ago, at 60 °F and 14.73 psia. I think the metric STP here is at 15 °C.

Unfortunately, Compressed Gas Association standards can't easily & honestly be viewed for free. After posting the original question, I found US NIST handbook 130-2012, "Regulation for the Method of Sale of Commodities" as adopted by The National Conference on Weights and Measures. http://www.nist.gov/pml/wmd/pubs/upload ... -final.pdf
2.16. Compressed or Liquefied Gases in Refillable Cylinders ...
2.16.2. Net Contents. – The net contents shall be expressed in terms of cubic meters or cubic feet, kilograms, or pounds and ounces. See Section 2.21. Liquefied Petroleum Gas for permitted expressions of net contents for liquefied petroleum gas. A standard cubic foot of gas is defined as a cubic foot at a temperature of 21 ºC (70 ºF) and a pressure of 101.35 kilopascals (14.696 psia), except for liquefied petroleum gas as stated in Section 2.21.
Note that LPG, colloquially but inaccurately called propane, is not natural gas.

I am waiting for an answer from tech service at Sigma Aldrich about STP conditions for specialty gases by the liter.
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Rich Feldman
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Re: How much gas is a liter, or cubic foot?

Post by Rich Feldman »

I got an answer, but not a legal or trade code reference, from Sigma Aldrich. After a couple of emails each way:
There are several different standards out there for STP and NTP liters, but we use the traditional one.
That is to say, 0°C and 1 atm. Carl Willis gave the same un-referenced answer here, a few years ago. I have yet to see STP declared explicitly in any gas vendor catalog or trade standard.

So a lecture bottle loaded with 25 liters of D2 gas (including the half liter we have to suck out) contains 1.115 moles.

Geometrically, 25 liters is 0.88 cubic feet.
The gas would be listed as 0.95 cubic feet under US weights and measures law, as explained in OP.

The IUPAC definition of STP, 0°C and 1 bar, seems to be generally ignored.
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Richard Hull
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Re: How much gas is a liter, or cubic foot?

Post by Richard Hull »

Sometimes science and scientists set an ad hoc "working standard" that is accepted by default due to convenience. Millions in the field have done things for so long one way with the ad hoc standard that a newer silly standard set by vaunted societies in full agreement are just not recognized or take a long time to make their way into common usage in a field. Witness the Torr and micron in vacuum speak by those actually in the field.

Personally, I still use and prefer the CGS system due to the laboratory sized units contained. The only SI unit I like is the Bequerel, a great, long needed unit of activity, even though I still talk Curies. The Tesla is ridiculously over sized being 10,000 of my normal talk units (gauss).

I think SI was created by a consortium of cosmologists and big science nuclear physicists and not guys in the lab.

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Rich Feldman
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Re: How much gas is a liter, or cubic foot?

Post by Rich Feldman »

...accepted by default due to convenience ... done things for so long one way ... newer silly standard set by vaunted societies ... not recognized or take a long time to make their way into common usage ... Witness the Torr and micron ...
Another example: after a couple of generations we state frequencies in hertz. Previous generations did fine using cycles per second, kilomegacycles per second, etc., which are sufficient and unambiguous.

I wonder whether today's ambiguous use of "megabit" and "megabyte" and multiplier symbol M will ever fade out?
For powers of 1024 the mebi, gibi style prefixes (multiplier symbols Mi and Gi) might catch on, long after being adopted by IEC, SI, etc.

The change would eliminate a real and increasingly significant ambiguity. Please don't say everybody knows mega means 1,048,576 in "tech". It has always meant 1,000,000 for data transmission rates, CPU clock frequencies, and most data storage not connected to a physical address bus.

Historical note: I think that schism began in the early 1980's. When DRAMs advanced from 16384 bits to 65536 bits per IC, the introductory press releases and datasheets (e.g. from Intel) called them 65K chips. The user community, mostly short-sighted nerds like me, said are you kidding? Marketing listened and soon the datasheets said 64K.
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Re: How much gas is a liter, or cubic foot?

Post by Dan Tibbets »

Note that the above numbers is for dry air. The water content is often ignored, though it is significant. The percentage of air that is made up of water vapor may exceed ~ several percent by weight, or is that volume?

My recollection is that the partial pressure of water in expired air from humans is at ~ 40 mm Hg. This is near saturation at that temperature. I'll leave conversions to others as I am too lazy.

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Rich Feldman
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Re: How much gas is a liter, or cubic foot?

Post by Rich Feldman »

Yup, though we are losing sight of the original question, in which the word air does not appear. OP is about the various so-called standard temperatures and pressures in use today, by law and by convention, for sale of gases by volume.

I took Dan's bait. Inquiring minds want to know. Spreadsheet calculators and Internet are useful tools. :-)
Yes, 40 torr partial pressure of water vapor is 100% relative humidity at 34 °C, according to Arden Buck.
The mixture is 5.3% water by volume and 3.3% by weight, with average MW 2.0% less than that of dry air.
At 0 °C, the vapor pressure of water is only 4.6 torr http://www.respirometry.org/calculator/ ... alculators,
making saturated air 0.23% less dense than dry air at the traditional STP in physics.

The humidity thing leads us to yet another wrong-answer website, superficially more respectable than smarter.com and ask.com. http://chemistry.about.com/od/gases/f/W ... At-Stp.htm says
... However, air typically contains a lot of water vapor, which would make it more dense than the cited value.
Wrong! Then it says
Answer: The density of dry air is 1.29 grams per liter (0.07967 pounds per cubic foot) at ... 0 °C ... 760 millimeters.
That gram value is correct to 3 digits. Pound value has too many significant digits and the first one is wrong, even if converted from 1.29 g instead of 1.293.
Pound value is about right for IUPAC's STP (0 °C and 100 kPa ~= 750 torr ~= standard atm 92 meters above sea level). Maybe IUPAC wanted a standard that would still be relevant after plenty of global warming.
All models are wrong; some models are useful. -- George Box
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