Principles And Construction 2a:
Tech: Units of Measure

Measuring Things

In order to describe many things, we measure them. The most common system of measurement used worldwide is the standardised metric system known as "SI metric". I'll be using this system, since I am writing for an international audience from a country (Canada) which is mostly metric. However, as time goes on, I'll be adding 'translations' into US Customary and British Imperial units.

Every SI unit can be used with the usual SI prefixes such as mega (M) for a million, kilo (k) for a thousand, milli (m) for one one-thousandth, and so on. These are set out in many locations on the web, such as in the article "SI Prefixes" at Wikipedia.

Length, Area, and Volume

Length is measured in metres (with the symbol m). Area is in square metres (m²). Volume is in cubic metres (m³).

Mass, Weight, and Pressure

In SI metric, "mass", the amount of substance in something, is measured in kilograms (with the symbol kg).

"Weight", how heavy something is, is measured in newtons (with the symbol N).

(In the US and British systems of units, the "pound" was originally a unit of weight. When the concept of mass arose, people came up with different ways to represent it--the pound-mass (lbm), the "slug", and others.

"Pressure" is weight spread over a certain area, and is measured in newtons per square metre (N/m²). This unit is also called the pascal (with the symbol Pa). In the US and British Imperial systems, one way to represent pressure is "pounds per square inch" (psi).

Air pressure is the weight of the atmosphere pressing on the surface of the land, so it's measured in pascals as well. This is the number that shows up on the weather reports: "the barometric pressure in Toronto is 101.3 kPa and falling." One kPa (kilopascal) is 1000 pascals.

Energy

Energy is measured in units called joules (with the symbol J).

The well-known watt (W), which shows up in headlines whenever electricity usage breaks records, is actually a rate of energy flow. It is defined as 1 joule per second (J/s).

People are much more familiar with watts than joules, so electricity suppliers prefer to mention watts instead of joules. But they are dealing with quantities of energy supplied rather than rates of energy usage, so they multiply the rate of energy usage by the time the usage lasts, and end up with quantities of energy expressed in watt-hours (W·h) or kilowatt-hours (kW·h). But if you work it out (1 W·h = 1 J/s x 1h = 1 J/s x 3600 s = 3600 J), one watt-hour is 3600 joules. They are both quantities of energy.

Temperature

Temperature is a little more unusual. When we speak of temperature, we say things like, "It's 23 degrees outside" (if it's a nice summer day). The usual scale for temperature (outside the US, anyway) is the Celsius scale; 23 degrees Celsius is the temperature of a nice summer day.

But for reasons originally relating to the physics of gases (which I won't go into), people doing technical work often measure temperature in kelvins (with the symbol K). Zero degrees Celsius (the freezing point of water) is defined as 273.15 K. The kelvin is the SI unit of temperature.

One degree Celsius is the same size as one kelvin. So a change in temperature, whether in degrees Celsius or in kelvins, will be expressed with the same number. It's only the starting point that is different. In subsequent tech pages, calculations may accept differences in temperature expressed in kelvins, but the differences in degrees Celsius can be used.

The temperature 0 K is called "absolute zero", and things actually can't get colder than that. So one advantage of using kelvins is that you never have to deal with temperatures below zero. It makes the math less confusing. (On the other hand, a nice room temperature of 21 degrees Celsius becomes 294.15 K.)

So now we have units for expressing mass, temperature, and energy. These will come in handy as we advance to the next section, and talk about heat capacity...

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