Pressure measuring devices

Lina and Maria were biking, when Maria noticed there wasn’t enough pressure in her tyre. Luckily, they found a pump at a nearby petrol station. - Do you think this is enough pressure? - I don’t know… But look, this pump has some sort of dial here, with a scale and a pointer… The dial Lina noticed is a pressure measuring device — a pressure gauge. This is exactly what the girls need in order to know if they’ve pumped the tyre enough, or not. Let's see how it works. Pressure gauges measure a force exerted by gases or liquids on a surface.

For instance — how much force the air inside the tyre exerts on the inner surface of the tyre. The gauge in this pump is called a Bourdon pressure gauge. Inside, it has a flattened thin-walled tube, which is bent into a C shape. This tube is fixed at one end to a pressure inlet. The other end is closed, and attached to a system of gears and levers.

This system, in turn, is connected to a needle on the dial. When a pressurised fluid, such as air in a bike tyre, is connected to the gauge, the pressure inside the tube becomes higher than the pressure outside the tube. This causes the tube to straighten a little. This motion is picked up by the system of levers, which move the pointer on the dial. The dial has a scale, which shows the measurement in units such as pounds per square inch — psi — or bar.

Besides the Bourdon gauge, there are other types of pressure gauges. The diaphragm gauge uses a thin, flexible, metal membrane and measures even very low pressure precisely. Digital gauges rely on digital sensors and show the measurement on an electronic display. Most of these gauges measure pressure of something relative to the pressure around us — that is, the pressure that air in the atmosphere exerts. We call this pressure atmospheric pressure.

Atmospheric pressure is measured relative to the point at which there is no pressure at all — as in a vacuum. How can we measure atmospheric pressure? With a device called a barometer! There are two common types of barometers. One of them consists of a glass tube sealed at the top.

The bottom of the tube is placed in a small reservoir filled with liquid. When the atmospheric pressure exerts a force on the liquid in the reservoir, the liquid is pushed up the tube. High atmospheric pressure forces the liquid higher up in the tube, and low pressure causes the level of the liquid in the tube to drop. The liquid used in the past was mercury, so this type of barometer is known as a mercury barometer. The second type of barometer doesn’t use liquid.

Instead, it contains a small, flexible metal box that has had the air pumped out of it to create a vacuum. This box is the barometer’s aneroid cell. Changes in air pressure outside the cell cause the cell to expand or contract. A system of pulleys and levers connects the aneroid cell to a rotating pointer, which indicates the pressure on a dial. This device is an aneroid barometer.

Which device you use depends on the type of pressure you need to measure: for instance atmospheric pressure, or gauge pressure in a tyre, how accurate your measurement needs to be, or how large the pressure you’re measuring is. The pressure gauge on the pump says we're good to go!