Therefore, the colored water is forced part of the way into the left arm of the tube. A scale between the two arms of the U indicates the difference in the height of the two columns of water. This difference tells the engineer the degree of vacuum-or how much below atmospheric pressure the pressure within the condenser is. A force is a push or a pull exerted on or by an object. You measure force in pounds.
Pressure is the force per unit area exerted on an object or exerted by an object.
You calculate pressure by the formula . Spring scales and lever balances are familiar instruments you use for measuring forces. Bourdon gauges, barometers, and manometers are instruments for the measurement of pressure. The normal pressure of the air is 14.7 psi at sea level. Pressure that is less than the normal air pressure is called a vacuum.