Also called a manometer, this device is used to measure slight changes in a low-pressure system, such as an HVAC system, or for remotely measuring a level in a tank of liquid. One end of the U-shaped tube, which contains fluid and is marked with a graduated scale for measuring, is open and the opposite end is connected to the low-pressure source. The fluid in the tube is moved or displaced by slight pressure changes and the change can be read on the scale, which is calibrated to read directly in the desired units of measure.
