The manometer

The manometer is a pressure gauge.  It is different than some other pressure gauges in that it measures very low pressures.  One PSIG equals 28 inches water column.  So, what is water column?  Look at the image below.  The tubing has water inside.  It is called the Utube manometer.  If I fill the tubing with water to the point that the water on either side rises to the zero mark, the pressure is zero inches water column.  If I put pressure on one tube and the other is open to the atmosphere, the water will move down on the pressurized tube and up on the atmospheric tube.  As the water moves down on the pressurized tube due to the pressure applied.  the open side also moves up.  If the column moves down one inch on the pressurized tube, the column on the open tube will will also move one inch up.  That equals 2 inches water column pressure.  Looking at the illustration below, the pressure tap from the tool is connected to the pressure source.  The other end is left open to the atmosphere. If you are testing the pressure drop across 2 different pressure areas, you would connect one on one side and one on the other pressure source.  This tool is suitable to test water column pressures used in natural gas.  

The slope gauge below is similar to the Utube manometer except that the tube is sloped to read much smaller pressures.  This tool also uses a special oil inside the tubing.  These would test pressures such as duct pressures.  

These tools above have been the standard for many years.  They are reasonably accurate and never need to be calibrated.  However, they must be filled with water, which may freeze or leak out.  You can use a special oil in the tube that does not freeze but it too leaks.  There are many other types of gauges designed to measure these pressures.  Some are shown below.  

Above is the Magnehelic brand manometer.   The one above is calibrated for 2 inches wc.  They come in many different pressure ranges.  This gauge has the advantage of not needing water or oil filling.  Overall, they are a step up from the Utube manometer.  However, you would need several of these if you were measuring water column from fractions of an inch to several inches.  There are somewhat delicate and did not tolerate dropping well.  The most common use for these are for pressure differential across duct filters to determine if the filter needs to be replaced.  

Above is a Yellow Jacket brand manometer.  It is somewhat similar to the Magnehelic.  These gauges were used primarily for testing gas line pressure or manifold pressure.  

Above is an electronic manometer.  It has the advantage of being able to measure wide variations of pressure.  It is used much like the Magnehelic with 2 sampling ports.  
​Below is a description of one of these manometers.

Above is a Fieldpiece stick meter with an electronic manometer head.  

Below I have described several different types of manometers and show how they work.