Measurement of Pressure; Gauges & the Barometer
open-tube manometer – U-shaped tube partially filled with mercury or water; P being measured is
related to the difference in height, h, of the 2 levels
P = Po + ρgh
ρgh is the GAUGE pressure–the amount by which P exceeds the atmospheric pressure
mm Hg or in Hg in this country–often ρgh is NOT calculated and only h is
reported [think weather reports]
1mm Hg = 133 N/m2 = 1 torr
torr–1 mm Hg and is in honor of Evangelista Torricelli (1608-1647),
the inventor of the barometer
barometer –one end of a tube of mercury is closed, the open end is
suspended in a pool of mercury. The atmospheric pressure supports a
column of mercury 76 cm (760 mm) tall @ sea level on a clear day.
ΔP = ρgΔh = (13,600 kg/m3)(9.8 m/s2)(0.760 m) = 1.013 × 105 N/m2 = 1.0 atm
When other quantities are in SI units, use the Pa as your pressure unit
aneroid GAUGE–pointer linked to the flexible ends of an evacuated thin metal chamber.
If electronic, P is applied to a thin metal diaphragm whose resulting distortion is detected electrically.
No matter how good a vacuum pump is, it cannot lift water more than 10 meters.
Torricelli was a student of Galileo and explained that a vacuum does NOT suck water up a tube, but rather reduces the P at the top of the tube.
Atmospheric P pushes the water up the tube if its top end is at low P
More videos on solving different problems involving pressure measurements are available on Ms. Twu's Physics Site:
https://sites.google.com/site/twuphysicslessons/home/fluids/fluids-page-2