What does bar pressure stand for and psi?

Answer 1

In the SI system the pressure unit is the Pascal (after the French maths genius) which is defined as 1 Newton per square meter. This is a very small unit. Atmospheric pressure is about 105 Pascals, and this is called the Bar. You have probably seen in weather forecasts the pressure given in millibars, the normal atmospheric pressure is 1000 mb, and the equal pressure contours on the weather maps give the millibar figure. These contours are called Isobars, as they are lines of constant pressure.

Psi stands for pounds per square inch, which is I imagine easier to grasp, and is probably more widely used in the US. In Europe and the UK, in power plants and other industries, the pascal - kilopascal - bar units are used. I was brought up on psi and had to convert mid life, I have got used to bars but find kilopascals don't come easily.

It is useful to remember that 1 bar = 14.5 psi. You will also come across psig, which just means pressure aboveatmospheric, it stands for psi gauge, as a gauge normally reads zero at atmospheric pressure. If you see psia, this means psi absolute, the pressure above vacuum, where atmospheric pressure = 14.5 psia.