The State of the Units or the Units of the States

Klaus Potsch, Team Leader, OMV

Opening the business section of my newspaper (a European one), my eyes spot an article on the oil price. It is over USD 60/bbl. In Europe, do we not measure amounts differently? Conveniently, the newspaper printed the conversion factor right along with the figure. So it is “just” a matter of a rough calculation to arrive at a number that I am used to.

That is just an everyday example. Sitting in an oil conference, reading a technical article on petroleum engineering, or digging through a technical operations manual you may find ample opportunity for all kinds of unit conversions. Is that necessary? Can we not agree in our industry once and for all to use only one system of units?

I still remember my years at the university when I had to fight with different systems of units, such as “cgs” or “mks” with “atm” or “at” as units for pressure. It took some time before the SI system (Le Système International d’Unités) began gaining recognition. Generally, this system is also called the metric system. The SI system was founded in 1960. Its roots date back to the French Revolution, when scientists like Lavoisier created a rational system of units. The numerical base was the number 10. Orders of magnitude are handled by prefixes, something that is missing in other systems. Seven basic units were singled out from which all other units of physical quantities can be derived. The units are not capitalized, except for some that are named after a person—K for Kelvin, Pa for Pascal. They are used only in the singular to avoid confusion with the unit “s” for seconds. In 1982, SPE issued a superb explanation of the SI system. Out of necessity, this SPE article is still my most frequently used article. Another detailed overview can be found at http://en.wikipedia.org/wiki/SI.

The oil industry sticks to its favorite oilfield (OF) units. Some call it “practical units.” But are they really practical? That depends probably on your point of view. The technology developed primarily in the US became so dominant in the industry that it also dictated how and in which system measurements have to be taken. The old countries, such as Azerbaijan, Poland, and Romania, where oil was produced even before the US, could not take over the technological lead. Now, people seem to use units as they please, even if it means mixing SI and OF units. Table 1 offers a glimpse of some units and terms found in a variety of journals and publications.

When I read some of those amusing creations, it is like being put back into former centuries when there were numerous different measuring systems. The measuring systems are consistent in themselves; the transition from one to another is the problem. Focus a moment on the basics of a system of units. The core is how to define mass, length, and time. To measure the space we are living in, we first need a length. The SI system defines it as the 40 millionth part of the circumference of the Earth at the equator. The next step is to define mass. Since water is a substance that is essential for life and is found almost everywhere on the globe, it is practical to define a certain volume of water as the unit of mass. In SI terms, that is 1 liter (=1 dm³) of water at 4°C has a mass of 1 kg. In the OF system, one pint of water has the mass of one pound at 62°F. But how does the volume of a pint relate to the unit of length? It has to be defined: 1 pint=27.7015 in.³ The next higher units in the OF system are formed by doubling, tripling, or multiplying by 12.

That might be handy for cooking, but not for engineering. In engineering we work extensively with numbers, and they are built on the basis of 10. When dividing amounts you often end up with decimals or even periodic numbers. That is the only drawback that I know in the SI system. Water, with its boiling and freezing points, is also the standard in measuring temperature. The difference between the two is 100°C. The Fahrenheit scale for temperature appears arbitrary. At least measuring time is universal.

Most countries have adopted the SI system. Among those in the former British Empire, only a few have switched to SI. The US once made an effort to promote the SI system. I remember gasoline stations in California in 1980 selling gasoline by the liter. But the idea did not catch on. Was it lack of marketing or did people just resist change? Nowadays, the same picture permeates not only the world of engineering but also daily life (Table 2). Are the differences preserved to protect the markets?

That the OF units are still frequently used, at least in our industry, can be explained by the sense dominance of the US economy and its technology. Change takes time. No one likes it, and most resist it. The system in which we measure is not an exception. I wonder why more pressure is not put on the industry to unify? That might happen only in a unified world government as a result of globalization. Which system would prevail?

How is the state of the units doing? As demonstrated, it is still a mess. How are the units of the States doing? Great! Meanwhile, I still have to sit in my cubicle and convert numbers, waiting for the next revolution to happen.

Klaus Potsch is Team Leader of the Reservoir Engineering Group in the E&P Laboratory of OMV where he is involved in R&D. He also serves as SPE Regional Director for South, Central, and Eastern Europe. His main interests in the field of reservoir engineering are pressure/volume/temperature studies, phase behavior, enhanced oil recovery, and multiphase flow. He earned an MSc degree in physics and a PhD degree in fluid mechanics from Technical University in Vienna.