• This is the U type manometer.pressure image by Dusan Radivojevic from Fotolia.com
  
  According to the HVAC Protech website, manometers are "unique, in being both basic pressure measurement instruments and standards for calibration of other instruments." They measure pressure by using "the hydrostatic (standing liquid) balance principle", which uses the height of mercury or other liquids in a tube to establish the amount of pressure. HVAC Protech describes five types of manometers: U Type, Well Type, Inclined Tube, Dual Tube and Sealed Tube (Absolute).
  
  

U Type Manometer

• This looks like a thermometer bent into a U shape. On either side, the tube is open, so the pressure on both sides is 0, thus any liquid in the tube will be at the same level. Apply pressure to either side, however, and the liquid will rise in the other side. Which liquid is used to measure and how much pressure is applied to one side of the tube will determine how many inches the liquid rises in the other side.
  
  

Well Type Manometer

• One leg of the manometer is much longer than the other; it resembles an upside-down candy cane. All the pressure is measured through the small curved leg on the bottom, and the measurement of pressure, in inches, happens in the long leg. Again, the liquid and amount of pressure determine how many inches the liquid rises in the long leg.
  
  

Inclined Tube Manometer

• This features a short, straight length of tube that then curves downward at a 135-degree angle for about 12 inches. After 12 inches, it curves upright again, and this leg is the same length as the first. The purpose of this manometer is to use 12 inches of manometer to measure only one inch of pressure, for low-pressure measurements.
  
  

Dual Tube Manometer

• If the inclined tube manometer is for low-pressure measurements, the dual tube manometer is the opposite--but without an extremely long tube. There are two tubes, side by side. One manometer resembles a U type manometer, while the other looks like a well type manometer. The measurement is the reaction to pressure between the two and where they come together.
  
  

Sealed Tube (Absolute) Manometer

• Though this can resemble a U type or well type manometer, the HVAC Protech site explains that the difference is that it is sealed on both ends, for "the complete absence of any gas." This is a "mercury-filled tube over 30 inches high" that is submerged in a mercury-filled container and "exposed to the atmosphere." This is used for any process that requires measurements "near or below atmospheric pressure."