Pressure – Fundamentals Module #11
The definition, how we measure it. This is next in our series of engineering fundamentals. Pressure is awesome, it is everywhere. It exists between your house and the outside, it changes as you drive and stop your car.
Pressure, like temperature, is one of the basic engineering measurements. As with temperature readings, pressure readings provide you with an indication of how something is operating. It might be the operation of an entire system or the running condition of single piece of equipment. Pressure changes even indicate weather changes right?
Pressure – the simple definition
Pressure is defined as the force per unit area. I used to hate that song Under Pressure by Queen but
now I love it, it’s crazy. Wait.
The simplest pressure units are the ones that indicate how much force is applied to an area of a certain size. Familar units include pounds per square inch and pounds per square foot. Maybe you’ve heard of ounces per square inch. And if you use newtons per square millimeter or dynes per square centimeter, you are most likely already a NASA physicist and why are you here?
Now, this part might seem to come out of left field, talking about crazy pressures. Depending on what you do, maybe engineering, you need to measure the pressure of air between spaces. ANY difference in pressure causes some amount of flow.
Pressures. if they can, always want to equalize. It is crucial sometimes that you know pressures. Will you be able to pump that fluid uphill? Is your pump capable enough to use electrical energy, convert it to mechanical energy, move the weight of the fluid against the internal friction of the pipe, uphill against the force of gravity? Do you see how many energy exchanges there are?
Here is the left field part:
You use another kind of pressure unit that involves length. These units include inches of water (” H2O), inches of mercury (” Hg), and inches of some other liquid of a known density. Actually, these units do not involve length as a fundamental dimension. Rather, length is taken as a measure of force or weight.
For example, a reading of 1 ” H2O means that the exerted pressure is able to support a column of water 1″ high, or that a column of water in a U-tube would be displaced 1″ by the pressure being measured. Similarly, a reading of 12 ” Hg means that the measured pressure is sufficient to support a column of mercury 12″ high.
And here is where it makes sense:
What is really being expressed (even though it is not mentioned in the pressure unit) is that a certain quantity of material (water, mercury, and so on) of known density exerts a certain definite force upon a specified area. Pressure is still force per unit area, even if the pressure unit refers to inches of some liquid.
Different Types of Pressure Readings
When we try to read our pressure measurements, sometimes confusion arises because the zero point on
most pressure gauges represents atmospheric pressure rather than zero absolute pressure. It’s often
necessary to specify the kind of pressure being measured under any given conditions.
To clarify the numerous meanings of the word pressure, the relationships among gauge, atmospheric, vacuum, and absolute pressures are shown to the right.
- is the pressure actually shown on the dial of a gauge that registers pressure relative to atmospheric pressure.
An ordinary pressure gauge reading of zero does not mean there is no pressure in the absolute sense; rather, it means there is no pressure in excess of atmospheric pressure.
- is the pressure exerted by the weight of the atmosphere.
At sea level, the average pressure of the atmosphere is sufficient to hold a column of mercury at the height of 76 centimeters or 29.92 inches. Since a column of mercury 1 inch high exerts a pressure of 0.49 pound per square inch (psi) at its base, a column of mercury 29.92 inches high exerts a pressure that is equal to 29.92 x 0.49 or about 14.7 psi.
Since we are dealing now in absolute pressure, we say that the average atmospheric pressure at sea level is 14.7 pounds per square inch absolute (psia). It would show zero on the ordinary pressure gauge. Notice, however, that the figure of 14.7 psia represents the average atmospheric pressure at sea level; it does not always represent the actual pressure being exerted by the atmosphere at the exact moment a gauge is being read.
Since fluctuations from this standard are shown on a barometer (device used to measure atmospheric pressure), the term barometric pressure is used to describe the atmospheric pressure that exists at any given moment.
- is the term used to describe the actual atmospheric pressure that exists at any given moment.
A space in which the pressure is less than atmospheric pressure is said to be under partial vacuum. The vacuum is expressed in terms of the difference between the absolute pressure in the space and the pressure of the atmosphere. Most commonly, vacuum is expressed in inches of mercury, with the vacuum gauge scale marked from 0 to 30 ” Hg.
When a vacuum gauge reads zero, the pressure in the space is the same as atmospheric pressure — or, in other words, there is no vacuum. A vacuum gauge reading of 29.92 ” Hg would indicate a perfect (or nearly perfect) vacuum. In actual practice a perfect vacuum is impossible to obtain even under laboratory conditions. A reading between 0 and 29.92 ” Hg is a partial vacuum.
- is atmospheric pressure plus gauge pressure, or absolute pressure minus vacuum.
For example, a gauge pressure of 300 pounds per square inch gauge (psig) equals an absolute pressure of
314.7 psia (300 + 14.7).
Consider a space in which the measured vacuum is 10 ” Hg; the absolute pressure in this space is figured by subtracting the measured vacuum (10 ” Hg) from the nearly perfect vacuum (29.92 ” Hg). The absolute pressure then will be 19.92 or about 20 ” Hg absolute. Note that the amount of pressure in a space under vacuum can only be expressed in terms of absolute pressure.
By common convention, gauge pressure is always assumed when pressure is given in pounds per square inch, pounds per square foot, or similar units. The g (for gauge) is added only when there is some possibility of confusion. Absolute pressure, on the other hand, is always expressed as pounds per square inch absolute (psia.) It is always necessary to establish clearly just what kind of pressure we are talking about.
And maybe to put you in a good mood or to get your toes tapping, here is the Queen song. That bass guitar baby! No wonder Vanilla Ice ripped it off.