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How much gas is one cubic foot or meter?

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klugesmith

Thu Dec 31 2015, 05:39AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

This is about "Not So Standard Temperature and Pressure". Can anyone show us a reference for _which_ STP values apply when industrial gases are sold by the cubic foot, cubic meter, or liter?

In high school I was taught that "STP" = 0 Â°C and 1 atmosphere. But apparently in 1982, IUPAC redefined _their_ standard pressure to be exactly 100 kPa (1 bar; 1/1.01325 atm). That changed the "ideal gas molar volume at STP" from 22.4 to 22.7 liters.

In this snip from Matheson catalog page for helium,

, the ratios between ft3 and m3 values range from 38.0 to 38.1. Huh? Geometrically, a m^3 is only 35.3 ft^3.

Matheson tables for nitrogen have the same funny ratio between the ft3 and m3 numbers.

[update] That funny ratio is consistent with cubic feet measured at 70 Â°F and cubic meters measured at 0 Â°C. See below. [/update]

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. One reference says that a natural gas standard CF is at 60 Â°F and 14.73 psia ( > 1 atm today).

Those Matheson gas catalog pages incidentally give densities and specific volumes at 70 Â°F and 1 atm.

I would search Compressed Gas Association standards if they could be viewed for free.

[edit] Found, in US NIST handbook 130-2012, "Regulation for the Method of Sale of Commodities as adopted by The National Conference on Weights and Measures.

2.16. Compressed or Liquefied Gases in Refillable Cylinders ...
2.16.2. Net Contents. â€“ 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 ÂºC (70 ÂºF) and a pressure of 101.35 kilopascals (14.696 psia), except for liquefied petroleum gas as stated in Section 2.21.

We saw at least one industrial gas supplier uses 70 Â°F when selling by the cubic foot, and 0 Â°C when selling by the cubic meter. How about quantities given in liters, as with rare and specialty gases? I'm waiting for an answer from tech support at Sigma Aldrich.

Dr. Slack

Thu Dec 31 2015, 09:32AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

It's great to see what happens when people tie themselves in knots as a result of not doing the unambiguous thing, which is to sell it by mass, instead of the easy thing which is to sell it by the handful, and then worry about the size of the hands.

In the UK, for domestic users at least, there's a further complication. I don't buy my NG by the cubic metre, kg, or even mol. I buy it by the kWh, which means there are a further two fiddle factors. One that appears on my bill is the heat equivalent, which varies slightly as the composition ratio of the gas varies from month to month as the supplier buys it from different sources. It's still metered by volume of course, at 25mBar above my local atmospheric pressure, I don't know whether the average over the last billing period enters the calculation anywhere.

Back in the 60s, when I was a littl'un, my mother used to buy fresh prawns at the local market by the pint! I recall even then being concerned at the scope the vendor had for varying the measure, by how loose or tight the stuff was packed down into his calibrated tin mug. My mother was quite glamorous then (if I look back at old photos and employ Donald Trump's family eye filter) and she seemed to get very good measure when in such measurement situations.

Bored Chemist

Sun Jan 03 2016, 04:51PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

As long as they state the temperature, pressure and volume,you can always do the arithmetic if you need to.

klugesmith

Thu Jan 14 2016, 02:41AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

The trouble is, as a rule compressed-gas vendors tell us the volume for sale WITHOUT giving the associated temperature and pressure conditions. They use implicit STP(s) whose values can be hard to find.

I did get an answer, without a citable reference, from Sigma Aldrich for gases sold by the liter. 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, 0Â°C and 1 atm. I have yet to see that STP called out explicitly in a gas vendor catalog or trade standard. Any pointers would be welcome.

So a lecture bottle loaded with 25 liters of gas, including the half liter you 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, 0Â°C and 1 bar since 1982, seems to be generally ignored. Why should perfectly good, world-wide conventions be disrupted just because some ivory tower guys wanted them to be "more metric"?

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