What’s In A Unit? Flow Measurement – Part 3.2


A brief review — Part 3 addressed mass flow measurement, volumetric flow measurement, and inferred mass flow measurement. The measurement of gasoline was given as an example of the inferred mass flow measurement (using volumetric units). Comments resulted in Part 3.1 that addressed some issues associated with retail gasoline measurements. This sparked a flurry of comments regarding how gasoline is measured at the pump that in the previous issues.

Gasoline pumps in the USA measure volume and are calibrated using volumetric means. In other words, they are true volumetric devices — they measure volume and indicate gallons. Even the New York Times offered advice to the consumer on this one with “… buy gasoline during the coolest time of the day — early morning or late evening — while the gasoline is most dense…” (New York Times, September 24, 2001, Empowered II – Smart Energy Management, A clean car is an efficient car, page 7).

Gasoline pumps in Canada measure volume. This volume is then compensated for the actual temperature to indicate the volume of the gasoline as if it were a certain temperature. The compensated volume is an implied mass measurement. I suspect (but do not know) that these pumps are calibrated using volumetric means that are temperature compensated. In other words, they are inferred mass measurement devices and are calibrated as such — they measure volume and indicate in (temperature-compensated) liters.

In Canada, the inferred mass of the gasoline received should be the same (within the limitations of the equipment) regardless of gasoline temperature. Note however that composition differences (and additives) may cause the density at a given temperature to be different than its nominal value. As an example, a 1% increase in gasoline density from its nominal value does not affect the actual volume measured, but will cause the inferred mass measurement to be 1% lower than the actual mass flow.

My comments on some readers’ responses follow:

One reader questioned whether the “wee hours of the am” would be the time when the gasoline would be at its lowest temperature in an underground tank. Thermal lag for underground gasoline storage tanks is an issue, but may not be significant. For science class, my daughter measured the temperatures 1 meter above and 1 meter below grade in the fall/winter (in the New York area). I seem to remember the ground temperature changing by only 1-2 degC over a period of months. The above ground temperature changed by 20 degC (or more?) during the same period. This issue is likely to be significant for above ground storage tanks (as suggested by other readers). Note however that filling the tank may cause larger (transient) effects caused by such issues as the quantity and temperature of the gasoline prior to fill, and the quantity and temperature of gasoline added.

Not being able to sell compressed natural gas measured with a Coriolis mass flowmeter in kg or lbm (pounds mass) because it was not considered ‘marketable’ to the the public illustrates resistance to change. By the way, when will gasoline be sold by the kg or lbm — or better yet, by the BTU or Joule (as suggested by another reader)? I suspect that it will not be soon.

The comments and observations about beating the measurement were amusing. Society allows people to (reasonably) operate in their own self-interest. Parting with less money for a product is clearly in the purchaser’s self-interest. (Engineers sometimes call this an “optimization problem”, but that is an issue for another day.) Comments on how to beat the system were inevitable.

The safety point regarding gasoline expansion causing explosions and fires (after topping off a gas tank in a cold climate and then parking in a warm garage) is important. Virtually everything is potentially dangerous — even a small puddle of water that turns to ice…

ISSN 1538-5280

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