Pressure megaConverter #16

Introduction and Overview

Pressure describes the force per unit area of a system. The formula for pressure is force/area. Thus, pressure can be represented by any force unit divided by any area unit, but only the common ones have any practical application. The units in this megaConverter are based on the most common force and area units of today. For a description of the various units, see the megaConverters #8 Force and #15 Area.

Pressure can be used to describe both compression and tension. If one object or system is pressing on another along a surface or the object is being affected by forces in opposing directions, pressure units are used to describe the effects. Pressure is why sleeping on a bed of nails is less comfortable than sleeping on a water bed. Pressure is also why a wire or rope under tension breaks. Quite often we think of pressure in terms of a liquid or gaseous system. The atmosphere is a system of air molecules all pressing down on us. Imagine the air above your head as all contained in a single long tube into outer space. The weight of all that air is pressing down on you. The higher in the atmosphere you go the less air there is above you and the less pressure there is. Air is also compressible so that the higher you go the less dense the air is and so the air pressure changes in a non-linear fashion. Water, on the other hand, is relatively incompressible and so going down into the ocean the pressure change is more linear. Pressure in liquids and gases also are omni-directional, meaning that the pressure on the top of your head is essentially the same as the pressure on the side of your face. This is because the air pushes down on itself and because air and water can flow sideways the pressure the air exerts on your face is reflected by the pressure on the air or water molecules by the air or water above it.

Atmospheric pressure is measured by the forces acting on an object or system that we can measure. The origins of pressure measurement when someone realized that the weight of liquids in a closed system such as on upside down glass tube could mimic the effect of the atmosphere. If a tube of water with a certain cross-sectional area were turned upside down in a bucket of water, the water would only come down the tube to the point where the water remaining in the tube weighed the same as an equal sky-high column of air with the same cross-sectional area of the tube. The water in the tube and the air outside are pressing down on the surface of the water in the bucket with the same pressure. The same will be true of any liquid. Such a tube of water at normal atmospheric pressure is almost 34 feet high. Mercury is often used for such measurements because of its greater density. A water column 34 feet high is very unwieldy, but a column of mercury equivalent to 1 atmosphere is only about 76 centimeters high. Your TV weatherman typically reports the atmospheric pressure as so many inches or millimeters of mercury.

For a more comprehensive treatment of measurements, find "NTC’s Encyclopedia of International Weights & Measures" by William D. Johnstone at your local library. For a better discussion of pressure, see any college physics textbook.

* Much of our written history still refers to things in common units. The Bible does not refer to meters or kilograms, but to cubits and stadia, or shekels and drachma. Wouldn't it be nice to know what they were talking about way back then? Now you can use megaConverter! For a more complete listing of ancient, foreign, and obsolete measures, download our 'megaSpreadsheet' of conversions in MS Excel format.

Note: Because of round-off errors, converting from very large units to very small units or vice-versa may not be accurate (or practical). Conversion factors can be found by converting a quantity of 1 unit to another unit several steps above or below the first. You may need to string several conversion factors together to find the factor from a very large unit to a very small unit, and then you can use a calculator with sufficient digits to find your answer.