Pressure is the amount of force a liquid or gas exerts on the surfaces of its container. We can express the amount of pressure in a number of ways including different units (psi, bar, etc.) or different reference points (gauge, absolute, differential). Pressure can be measured with many instruments including gauges which show the pressure at the point of measurement or with transmitters, transducers and sensors which transmit pressure readings to panel meters or indicators located away from the point of measurement. For the best accuracy, it is important that pressure instruments are regularly calibrated.