Getting some sun as I return from Washington to New York has been enjoyable. This time my car is facing forward so I can see what is coming (instead of what is going). From prevous issues (see website), gasoline purchased at a service station was measured by volume in the USA, and by mass in Canada. Perhaps you could not see that coming. Gas flow measurement units often exhibit simillar idiosyncracies.
Let’s have a quiz.
|cubic feet per minute|
|cubic centimeters per minute|
|cubic meters per hour|
If you answered volumetric to the first three questions, mass to the next three questions, and yes to the last three questions, you are on track.
Some explanation of mass units is in order (time is ignored to simplify the discussion). A cubic foot of fluid is an expression of volume. However, when a (seemingly) volumetric unit infers mass, it should be considered a mass unit. A standard cubic foot is is an expression of a fluid volume at a standard pressure and standard temperature. Knowledge of the standard pressure, standard temperature, and composition of the gas is sufficient to determine its density at these conditions, and hence infer its mass. “Normal” units exhibit similar properties.
And what about cubic feet of natural gas? Cubic feet is a volume and only the fluid (natural gas) is defined, so it would seem to be a volumetric unit. Not so fast, because a standard temperature and standard pressure may be stipulated in the tariffs governing the distribution of natural gas. Even though standard conditions are not clearly stated, the application (natural gas) may dictate the use of a tariff containing certain standard conditions — inferring a mass measurement.
Adding complexity is that different standard conditions may be applicable, depending on the country, state, industry, and customer of the measurement. For example, when not cited in the contract, the standard pressure could include 14.65 psia (AR, AL, TX), 14.73 psia (CA, CO), and 15.325 psia (OH). In addition, a company headquartered in one state may use the home state’s standard pressure in their contracts for all of their plants in the country.
Mention was made of standard units, but what are they? Next time. The train has started to pull out of Newark, NJ northwards to New York City.
Next Issue: What’s in a Unit? (Flow Measurement – Part 5)
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