Velocimetry refers to the study of the flow of fluids.  It is very important in the field of fluid dynamics and in the design of equipments, such as, pumps and turbines. 

The flow of liquids can be studied by adding small materials such as bits of paper or styrofoam in water.  This helps the observer to track the movement of the flow. 

The particles for observations used should have the same density as the fluids. 

Velocimetry can also be done using non contact methods such as with the use of lasers.  Laser Doppler velocimetry is a method, which uses two beams of lasers to analyze the flow of a fluid.

The pound, which is a unit of weight, is indicated by the letters lbs.  lbs comes from the latin Libra which is a short form of Libra Pondo (trans. a pound by weight).  This was a unit of weight used in the Roman empire.  

Over time, the libra got shortened to lbs.

The bar is a very common unit of pressure.  It is used in the industry for normal operational measurements. It is popular as one bar is approximately equal to the normal atmospheric pressure (slightly lower, 0.987 atmosphere).  This helps people to visualize the pressure easily.

A millibar is 1/1000th of a bar.  A bar is equal to 1.01972 kg/cm2

Pounds per square inch is a very common unit of pressure.  It belongs to the Imperial System of measurement.  

It is widely used in industries, tyres and other applications.  The unit indicates that the pressure is equivalent to a certain number of pounds applied to a square inch of area.  

There are two variations of the unit, the absolute and the gauge versions. 

psia (pounds per square inch absolute) indicates that the pressure value is made vacuum as the reference while psig (pounds per square inch gauge) indicates that the atmospheric pressure is the reference.