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How to Interpret Manometer Readings

Neha Gohad Feb 21, 2019
A manometer is used for measuring differential pressure. It's just a scale. To interpret a manometer's readings, you multiply the reading obtained on the scale by the calibration factor. This ScienceStruck post explains how to read a manometer along with some examples.

Testing Room Pressure With a Manometer

  • Close all the windows and doors to the room. Switch off the fans if any.
  • Insert the end of the hose of the manometer underneath the door. Connect the other end to the manometer.
  • Interpret the results according to the rise in water column. The room should maintain a slightly positive pressure of approximately 0.02 inch per water column.
In its simplest form, a manometer is a U-tube filled with an incompressible manometric liquid such as water or mercury. Piezometers, and inclined type manometers are used to measure small pressures, while differential U-tube manometers are used to measure large pressure.
Piezometers are also used to measure pressure difference in pipes where the liquid is in motion. Following are the steps to take readings in different manometers and calculate the pressure at a point in a fluid or the differential pressure between two points in a fluid.

How to Read a Manometer

● The reference level is indicated by a mark on the tube. Measure the distance between the rise in liquid on one side and the reference level. This value is to be multiplied by '2' as the distance of liquid lowering on one side is equal to the rise in liquid on the other side.
● Convert the result to inches. Water is considered to be standard manometric liquid for this conversion.

● The pressure to be measured in Pascal can be obtained as follows:

Pressure in N/m2 = ρ X g X h

where, ρ = density of liquid in kg/m3
g = acceleration due to gravity given by 9.8 m/s2
h = doubled height difference in meter.

How to Read a Mercury Manometer

● As mentioned earlier, measure the pressure in water inches.

● Specific gravity (SG) of a liquid is the ratio of the density of a substance to the density of water. The specific gravity of mercury is 13.6, which means it is 13.6 times denser than water.
● Divide the pressure measured in water inches by 13.6. This will give the pressure measured in inches of mercury.

● Gas pressure is given in millimeter ( an inch is 25.4 mm).

Numerical Example

A U-tube manometer filled with mercury is connected between two points in a pipeline. If the manometer reading is 26 mm of Hg, calculate the pressure difference between the points when water is flowing through the pipe. Density of mercury = 13.6 gm/cc, density of water = 1 gm/cc.
Data: Manometer reading (h) = 26 mm Hg = 0.026 m Hg
Density of mercury (ρm) = 13.6 gm/cc
Density of water = 1 gm/cc

For simple U - tube manometer,
p1 - p2 = Δ p = (ρm - ρ)gh

Water flowing through the pipe:
Δ p = (ρm-ρ)gh
= (13600 - 1000) X 9.812 X 0.026
= 3214.4 N/m2
Thus, manometer is one of the simplest devices to measure pressure difference. A few simple calculations can give you the result right away.