2. MANOMETERS
A manometer is an instrument that uses a column of liquid to
measure pressure.
The manometer utilizes the hydrostatic (standing liquid)
balance principle wherein a pressure is measured by the height of the
liquid it will support.
There are many types of manometers, some of which are as follows:
Simple Manometer
Well Type Manometer
Inclined Manometer
Two liquid manometer
3. SIMPLE MANOMETER
It consists of a transparent U-tube containing the fluid A of density
p whose pressure is to be measured and an immiscible fluid B of
higher density pm.
The limbs are connected to the two points between which the
pressure difference (P2- P1) is required.
If P2 is greater than P1, the interface between the two liquids in limb
2 will be depressed a distance hm (say) below that in limb 1.
The pressure at the level a — a must be the same in each of the
limbs and, therefore;
𝑷 𝟐 + 𝒛 𝒎 𝝆𝒈 = 𝑷 𝟏 + 𝒛 𝒎 − 𝒉 𝒎 𝝆𝒈 + 𝒉 𝒎 𝝆 𝒎 𝒈
∆𝑷 = 𝑷 𝟐 − 𝑷 𝟏 = 𝒉 𝒎 𝝆 𝒎 − 𝝆 𝒈
If fluid A is a gas, ∆𝑷 = 𝒉 𝒎 𝝆 𝒎 𝒈
5. WELL TYPE MANOMETER
In order to avoid the inconvenience of having to read two limbs,
the well-type manometer can be used.
If Aw and Ac are the cross-sectional areas of the well and the
column and hm is the increase in the level of the column and hw
the decrease in the level of the well, then;
∆𝑷 = 𝑷 𝟐 − 𝑷 𝟏 = 𝒉 𝒎 𝝆𝒈(𝟏 +
𝑨𝒄
𝑨𝒘
)
7. INCLINED MANOMETER
The inclined manometer enables the sensitivity of the
manometers described previously to be increased by
measuring the length of the column of liquid.
If 𝜽 is the angle of inclination of the manometer and L is
the movement of the column of liquid along the limb,
then;
𝑳 =
𝒉𝒎
𝒔𝒊𝒏𝜽
8. TWO LIQUID MANOMETER
Small differences in pressure in gases are often measured with two liquid
manometer.
It consists of reservoir at the top of each limb of a sufficiently large
cross-section for the liquid level to remain approximately the same on
each side of manometer.
∆𝑷 = 𝑷 𝟐 − 𝑷 𝟏 = 𝒉 𝒎 𝝆 𝒎𝟏 − 𝝆 𝒎𝟐 𝒈
The sensitivity of the instrument is very high if the densities of the two
liquids are nearly the same.
Liquids used: 1. Paraffin Oil and Industrial Alcohol
2. Benzyl Alcohol and Calcium Chloride
