• Pressure gaugemanometer image by Inger Anne Hulb?|kdal from Fotolia.com
  
  Differential manometers measure atmospheric pressure. Most manometers are u-shaped glass tubes with open ends on each side. Each leg is filled about halfway up with liquid, usually water or mercury.
  
  

Atmospheric Pressure

• The most common use of differential manometers is to compare pressure levels between two atmospheres. When both ends are left open to the same environment, the water level in each tube is the same. If one tube is attached to an atmosphere of high pressure, the liquid in that side of the manometer gets pushed down. The liquid on the other side rises. By measuring liquid displacement, a relationship between pressure levels in both atmospheres can be understood.
  
  

Comparing Altitudes

• As distance from sea level increases, atmospheric pressure decreases. This explains why it can be harder to breathe at high elevations and why it takes water longer to boil. Differential manometers can illustrate this difference in air pressure. Since manometers only measure a relationship between pressure levels and not absolute pressure, the numbers used to calculate "zero" pressure at varying elevations necessarily differ. By taking manometer readings at varying altitudes, students should be able to see how height affects pressure, as even simple manometers take minute and accurate readings.
  
  

Demonstrating Vacuums

• By definition, a vacuum is a pressure level less than atmospheric. To someone standing at sea level, pressure levels equivalent to those at the top of a mountain would qualify as vacuums, as they are considerably less. This phenomenon increases as you move further into the atmosphere, which explains why space is called a vacuum. It also explains why astronauts' lungs would collapse if exposed to the atmosphere in space without intervention of a suit and breathing apparatus, as our lungs are not equipped to deal with pressure levels that low. Manometers can be used to explore this concept in microcosm.