Pressure measuring devices

Jul 10, 2008 13:13 GMT  ·  By

We might not feel it, but Earth's atmosphere is putting a tremendous amount of pressure on our bodies every day. A column of gas some 100 kilometers tall exerts a staggering 101,353 Newtons per square meter on any surface at sea level. Our body cannot generally notice it because this pressure is exerted evenly from all sides, giving us the impression that everything is completely normal. However, when pressure is increased inside a flexible container, its existence and effects are immediately observed.

Take for example a tire or a balloon filled up with air. Outside the pressure is 'normal', albeit inside the increased air density determines the atoms to draw closer together, making them collide with each other more frequently and at much higher speeds, exerting more pressure on the inner walls of the container. As a result of the imbalance created between the forces exerted on the outer walls and those on the inner walls the container, a tire or a balloon for example, inflates.

The same principle applies to most pressure gauges, including the tire pressure gauge, which, as its name says, is a simple device used to measure the pressure inside tires. Basically it consists of a piston whose movement can be tracked on a scale with the help of a calibrated rod connected to it.

As the pressure gauge is coupled to the valve stem of the tire, pressurized air enters the piston and starts to expand. When this happens, the air pressure applied on the piston of the gauge determines it to move in such a way that the volume of the cylinder increases. In order to keep the piston from suddenly moving to the far end of the cylinder, a spring must be used in order to balance the force of the air pressure inside the cylinder by pushing against the piston with equal force from the other side.

When a balance between the two forces acting on the piston is achieved, the value of the pressure in the tire can be read on the calibrated rod which moves simultaneously with the piston.

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Image of a typical pressure gauge
Artistic impression showing the construction of a simple pressure gauge
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