place to stand

By William E. Steinman

Weights and Measures:

November 27, 2006:

Before I go on with my normal study notes, it is important to lay the groundwork for my studies. One of the steps in that is to settle on some basic tools and methods. Underlying everything we do in science is the discipline of accurate measurements. I must have some standards of weights and measures so I can be consistent in my work. Fortunately, our United States government has done a great deal of that work for me.

Within the United States Department of Commerce is a Technology Administration and within that is the National Institute of Standards and Technology (NIST). Each year this NIST produces a document called handbook 44. Its official name is a bit more ostentatious. It is Specifications, Tolerances, And Other Technical Requirements For Weighing And Measuring Devices.

Here is the forward from that handbook:

Handbook 44 was first published in 1949, having been preceded by similar and books of various designations and in several forms, beginning in 1918. Handbook 44 is published in its entirety each year following the Annual meeting of the National Conference on Weights and Measures (NCWM). The Committee on Specifications and Tolerances of the NCWM developed the 2006 Edition with the assistance of the Weights and Measures Division (WMD) of the National Institute of Standards and Technology (NIST). This handbook includes amendments endorsed by the 90th National Conference on Weights and Measures during its Annual Meeting in 2005.

NIST has a statutory responsibility for “cooperation with the States in securing uniformity of weights and measures laws and methods of inspection.” In partial fulfillment of this responsibility, NIST is pleased to publish these recommendations of the NCWM. This handbook conforms to the concept of primary use of SI (metric) measurements recommended in the Omnibus Trade

and Competitiveness Act of 1988 by citing SI units before inch-pound units where both units appear together and placing separate sections containing requirements in SI units before corresponding sections containing requirements in inchpound units. In some cases, however, trade practice is currently restricted to the use of inch-pound units; therefore, some requirements in this handbook will continue to specify only inch-pound units until the NCWM achieves a broad consensus on the permitted SI units.

In accord with NIST policy, the meter/liter spellings are used in this document. However, the metre/litre spellings are acceptable, and are preferred by the NCWM. It should be noted that a space has been inserted instead of commas in all numerical values greater than 9999 in this document, following a growing practice, originating in tabular work, to use spaces to separate large numbers into groups of three digits. This avoids conflict with the practice in many countries to use the comma as a decimal marker.

Now, the big thing from all of this is that I will use the Metric (SI) measuring system everywhere that it is appropriate. I have found that system to be in widespread use in the scientific community of America and of the rest of the world. So, what is the SI or metric system of measurement?

The metric system is a system of weights and measures, which was first planned and adopted in France in 1799. As I said, it is now used by most of the countries of the world. This SI system is based on a unit of length called the meter (m) and a unit of mass called the kilogram (kg). The meter is now defined as the distance light travels through a vacuum in 1/299,792,458 of a second. The kilogram is defined as the mass of the International Prototype Kilogram, a platinum-iridium cylinder kept at Sèvres, France, near Paris. From those basic units, other metric units can be defined. The nice thing about this SI system is fractions and multiples of the metric units are related to each other by powers of 10. This makes it easy to convert from one unit to a multiple of it just by shifting a decimal point.

If you have ever had to work in the English system of measurement you will realize what a boon this simple fact is. It means no more cumbersome arithmetical operations. Of course there are standard prefixes for specifying these units, multiples, and fractions. All of this is spelled out in appendix C of the NIST handbook, which will be posted as my next study notes. It is a big file consisting of 21 pages of charts and tables. For those who want the entire handbook, here is a link to their home page. http://ts.nist.gov/WeightsAndMeasures/owmhome.cfm

From there you can download the entire handbook for this year or previous years in PDF format.

Although this handbook covers a great deal of information about measuring, it is not all inclusive. For example, it does not directly deal with measurement of energy at all. There are several things I will need to deal with in this area of energy. Understanding electrical wire and wire sizes is just one. The thing that makes this a bit hairy for many people is the standards of wire size used. In place of diameter we use wire gauge which is inversely proportional to wire diameter. Small gauges are large diameters and large gauges are small diameters.

In addition, we used different gauge standards depending on what we are going to deal with. The standard scale for copper and aluminum wire is the Brown and Sharpe gauge, which ranges from gauge number 0000, which corresponds to a diameter of 0.460 in to gauge number 40, which corresponds to a diameter of 0.00314 in. That scale is the one I will be most interested in. There are other standards for steel and iron wire, which I will ignore if I can.

I will be mostly interested in wire as electrical wire for carrying current to and from electrical devices. In current capacities, there are several different tables depending on how and where the wire will be used. The most common of these is the table for house wiring and most commercial applications. Remembering that gauge is inversely proportional to diameter here is the current capacity table for the most common wire sizes. (A means amperes.)

Wire current capacities.

size 14 wire 15A

size 12 wire 20A

size 10 wire 30A

size 8 wire 40A

size 6 wire 55A

size 4 wire 70A

size 2 wire 95A

size 1 wire 110A

These current capacities help to explain why they have 300 volt batteries in those electric cars. I will get into the laws governing electrical systems later. For now, ohms law for DC circuits gives us the equation E = IR (Voltage is equal to the current required times the resistance of the circuit). The higher the voltage applied, the less current will be needed to perform a particular task. You don’t need to be an electrical engineer to understand that higher voltage means smaller wire diameters. Size 1 wire will deliver 110 amps but it is about ½ inch in diameter. Take it from me, that size wire is very hard to handle.

To complete this picture, I can tell you about comparing horsepower to Kilowatts (KW). The horsepower, as used in describing automobile engines, is equal to 745.7 watts. Think about that for a minute. This means the output horsepower of a 200 horsepower engine is equivalent to about 150 KW. Now go look at your electric bill for last month. 150 KW is enough electricity to run 10 or 15 normal households. That’s the kind of energy an auto uses just to get you to the grocery store. To produce that kind of energy with electricity presents a very unique problem to the engineering community.