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Measuring fluid pressure. (a) Using a manometer
(i) Measurement of Liquid pressure
One arm of the manometer is connected to a thistle funnel whose base is covered with a thin membrane and the other end remains open to the atmosphere.
The difference in the liquid surface levels, h gives the pressure at point A and it is called the gauge pressure or absolute pressure. 𝐀𝐛𝐬𝐨𝐥𝐮𝐭𝐞 𝐩𝐫𝐞𝐬𝐬𝐮𝐫𝐞 = 𝐇 + 𝐡𝛒𝐠
(ii) Measurement of Gas pressure
-Turn on the gas.
-The gas exerts a pressure at a point A. This causes the liquid to rise in the opposite arm until the pressure in both arms is the same.
-The gas pressure in one arm (limb) is equal to the pressure in the opposite limb. Pressure at A=Pressure at B
(b) Using a bourdon gauge
This gauge measures the very high pressures of liquids or gases, e.g. the pressure of steam in boilers.
It is a hollow curved tube of springy metal closed at one end. The tube straightens slightly when pressure acts on the inside.
The closed end of the tube is joined to a series of levers and gear wheels which magnify the slight movement.
A pointer moving over a scale (usually graduated in 105 pa, which is about 1 atmosphere pressure) records
Then, the recorded pressure is the excess pressure of liquid or gas over atmospheric pressure, but some gauges can record the actual pressure.
Bourdon gauges are commonly used at filling stations.
Example:1
Mercury was poured in a U- tube such that it finds its own level. When a column of 20cm of alcohol was poured on one side of the tube, it was necessary to pour 16cm of water on the other side to maintain equal mercury levels on both sides as shown below. Find the density of alcohol.
Solution
Expressing cmHg or mmHg pressure in Nm-2 or Pa
This is done by applying of formula pressure = hρ𝒈 where h is the liquid column which should be in meters, ρ is the density of the liquid and it should be in kgm-3 and 𝒈 is the acceleration due to gravity (𝒈=𝟏𝟎𝒎𝒔−𝟐).
Example 1
Express a pressure of 75cmHg given that the density of mercury (Hg) is 13600kgm-3.
Example 2
The manometer contains mercury so the atmospheric pressure is 76cm Hg. Calculate the gas pressure in cm Hg and Nm-2.
