Pressure Basics – Air and Hydrostatic


We encounter various forms of (science-related) pressure in our daily lives, but many of us probably fall short of an above-basic understanding of what it’s all about. Both air and water fall under “fluids” in terms of classification, and relating to pressure, they are the two more common substances we come across. I will lightly cover two systems that many of you might find familiar.

Air Compressor

Air compressors are relatively common machines that have practically become a household item owing both to its increasingly affordable price as well as the various functions it’s capable of performing from pumping bicycle tires to air-cleaning to the running of air tools. Loosely speaking, air compressors consist of a compressor unit which compresses the air and an air tank which stores the compressed air.

Gauge showing psi and kpa

Various air compressors have different capabilities and run at different capacities, with the high-end ones naturally having superior specifications. Air pressure is built-up, with the upper limit of the given machine more often than not reflecting its price. Depending on the region, it is denoted by either pounds per square inch (psi), kilogram force per square centimeter (kgf/cm2), or megapascal (Mpa).

Standard air pressures generally range from around 0.7 Mpa to 3 Mpa. Many smaller DIY-type pneumatic tools, such as nail and staple guns, will run on 0.4 to 0.7 Mpa, whereas higher end professional-grade tools will require much more pressure. The standard SI measurement for pressure is the pascal, but some countries like the USA use psi. 1 Mpa is equal to roughly 145 psi, or 10.2 kgf/cm2. This will help you convert pressure units into whatever you’re most familiar with. (See Hydrostatic level sensors

Assuming that the fluid is incompressible – water compresses 46.4 ppm per atmosphere of pressure – maintaining a constant density throughout the fluid, and that the column height is such that any variation of the gravitational acceleration is negligible, the formula can be expressed as pressure = pgh, with p being fluid density, g being the gravity of Earth, and h being the height of the water column.

One thing to keep in mind – and something that is often confused – is that the overall volume of the water has no bearing on the hydrostatic pressure at a given point – say a spigot outlet for example. Pressure is the force divided by the area, P = F/A, and is denoted by a unit of force per unit of area. This means that it is defined by a force acting on a given area, which won’t be changed by the volume of its container.

Pounds per square inch is an easy way to remember hydrostatic pressure, as a square inch is fairly close to an average hose, tap, or outlet. For every foot the upper surface of the water rises from the output, you multiply by 0.433. If you were to break this down to simple terms, this is the converted weight-sum of vertical cubic inches to pounds, from the top surface of the water to the water output.

A cubic inch of water weighs roughly 0.036 pounds. So if you were to multiply 0.036 by the total amount of vertical inches as explained above, you will end up with a number that will describe your pounds per square inch at your output. As reference, normal city water pipes have a water pressure of around 50 psi – a rather daunting amount of pressure to come up with for a DIYer.

Water Towers

Although not as important in recent times, water towers were extremely vital components of city water distribution and provided a natural pressurization that would deliver water at useful pressures to households. They are also used as secondary or backup water delivery systems during peak consumption hours when electric pumps aren’t able to keep up.

A typical city water tower that feeds water mains has to have a pressure of about 50 psi – same as the mechanized water pressure. In order to produce this level of pressure however, the water tower has to be about 35 meters high. An alternative to building a miniature skyscraper is to install the tank on a hill that sits at an equivalent height in relation to the consumer-end. This is only possible for hilly regions but essentially produces the same pressure.

Many taller buildings are required to have their own “water tower” or tank, usually on the roof, pumping up water from the city mains and distributing it throughout the building. Without this requirement, city water mains would have to have considerably higher pressure, putting piping in danger of premature fatigue and even bursting – not to mention the additional cost in terms of more powerful pumps and taller water towers.

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