Mercury Barometer

Figure 2.9 : Mercury Barometer

 

Mercury Barometer (Fig.2.9) is the simplest device to measure atmospheric pressure at a location. It consists of a glass tube closed at one end immersed in a container filled with mercury. Because of the atmospheric pressure mercury rises in the tube as shown. If $h$ is the height of mercury above the fluid level in the container, we have

$$P_{atm} - p_{A} ~=~\gamma~h$$ (2.27)

where $p_A$ is the pressure at A and will be equal to the vapour pressure of mercury, $p_{vap}$, which is around 0.16pa at a temperature of $20^0C$. It is usual to neglect $p_A$ when the atmospheric pressure is given as

$$p_{atm}~=~\gamma~h$$

Sometimes atmospheric pressure is expressed as "mms of mercury" being equal to $h$. At sealevel conditions where the pressure value is 101,327 Pascals and the specific weight of mercury is 133,100 N/m³, the barometric height is 761 mm Hg. Water could be used as the barometer fluid, but in that case the height of water will be around 10.36m!

(c) Aerospace, Mechanical & Mechatronic Engg. 2005
University of Sydney