Application of the SI Metric System: Part 1 The Basic System
Authors: Campbell John M. , Campbell Robert A. 
Published: December 2007

The SPE Symbols and Metrication Committee has developed several voluntary standards that are used worldwide. Like all detailed standards, these can be tedious to use unless one is very familiar with them. This is the first in a series of articles designed to enhance the convenience of using-and converting to or fromSI metric units (for Le Systeme International d'Unites). Each subsequent article in the series will address SI applications in a specific area: drilling, logging, production facilities, etc. We hope these articles will production facilities, etc. We hope these articles will prove helpful to SPE members. prove helpful to SPE members.

Introduction

There is little question that the SI metric system of units is technically superior to any other unit system. It is coherent and nonambiguous. A series of base units is defined carefully and then related to a group of derived units in an orderly manner, as shown in Fig. 1. Prefixes are used to relate the numerical value of these units as "powers of ten." The key advantage of this system, however, is the opportunity to have a single, worldwide set of units that enhances technical communication. But the system is new. To some extent an old familiar set of problems has been exchanged for some new, unfamiliar ones. The old, comfortable units possess a meaning through long usage that must be acquired for new units. To a U.S. engineer, a pressure of 1,000 psi denotes an order of magnitude, a feeling not yet possible when using 6900 kPa or 6.9 MPa. Reactions to the SI metric system vary. At best, the system sometimes can be a bit of a nuisance; at worst, it is very exasperating. Interestingly enough, those who heretofore have used one of the traditional metric systems may offer more resistance to change than those who have used some form of customary units. So, stow away your biases, forget about the pros and cons, and let us move forward with our continuing examination of how we in SPE can cope with the SI metric system more efficiently.

Relationship Between Units

Fig. 1 shows the basic relationship between units. The base units are shown in the left column. The next column shows four forms of length and time terms used to define the derived units shown in the fight column. Each circle representing a derived unit has one or more lines entering or leaving it. Each line is connected to a base unit or to another derived unit. A solid line indicates multiplication; a dashed (broken) line indicates division. Notice the circle for FORCE in the upper right comer. It has two solid lines entering it from mass (kg) and acceleration (m/s). This means that force in newton (N) is the product of these two quantities. Notice also that there are two solid lines leaving the FORCE circle. One goes to the PRESSURE circle and one to the ENERGY, WORK circle. Consider pressure, which by definition is force per unit area. The PRESSURE circle has two lines entering it, a solid one from FORCE and a dashed one from AREA. This means that pressure is force divided by area: pascal (P) equals N/m. The same system is used throughout Fig. 1. Energy in joule (J) is Nm, power is energy per unit time, so watt (W) equals J/s, etc. Table 1 contains the definitions of the base and derived units shown in Fig. 1. Notice that Fig. 1 involves no numerical values such as 12, 32.2, or the like. Only the number one (1.0) is implied. * 1.0 newton (N) is the force that, when applied to a mass of 1.0 kilogram (kg), causes an acceleration of 1.0 m/s. * 1.0 joule (J) is the energy involved when a force of 1.0 N is moved a distance of 1.0 meter (m) in the direction of the force. * 1.0 watt (W) is the power resulting from energy production at the rate of 1.0 J/s. * A resistance of 1.0 ohm ( ) produces a current flow of 1.0 ampere (A) with an electrical potential of 1.0 volt (V).

This is a simple, coherent system that requires no memorization of numerical values when relating units. Although difficult to adopt later in life, it is easy to learn as a youngster. Although the primary purpose of Fig. 1 is to interrelate SI metric units, it also serves to relate the basic dimensions. An equivalent figure could be used for customary units since the basic dimensions do not change with the system of units employed. Table 2 shows the four basic dimension symbols used by SPE in its symbol standard. Note that dimension symbols are not italicized. These are examples of the principle involved. One can make a similar list of the terms used in a given area of expertise. The SPE letter symbol standard shows the basic dimensions for all terms included therein. Of course, some terms are dimensionless.

JPT

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