Question

Alcohol is used in manometers because?

Answer 1

Manometers are mainly used to measure the difference in the pressures. This device works on the principle of hydrostatic equilibrium. The hydrostatic equilibrium means that the pressure at any point of a fluid at rest will be equal to the weight of the overlying liquid.

Complete answer:
The fluid manometers are the devices which readout the pressure differential as the length of the column. This is most commonly used in the U-Tube manometers. This is simply a U-shaped tube filled with manometer fluid. The pressure differential is applied at both the ends of the tube, to move the fluid downward in one limb and upward in another, until both the forces balance out each other.
The manometer can be shown as:

In manometers where the density of the manometer fluid is much higher than the density of the air, the difference in the pressure is given by the equation: $\Delta p = \rho gh$
Where $\rho$ is the density of the fluid, g is the acceleration due to gravity and h is the height of the fluid in the manometer. The sensitivity of the manometer can be adjusted by changing the angle. The most common manometer fluids used are water, alcohol and mercury. The density of alcohol and water are quite close to each other, whereas mercury is much higher.
Alcohol is used in the manometer because alcohol is a low-density liquid and low-density liquids can be used to adjust and amplify the deflections of the manometer. Water and alcohol manometers are used to measure lower pressure. Alcohol has a suitable and fairly visible meniscus in the inclined tube. Also, it has a low density thus can provide a longer length of pressure difference. The accuracy is obtained because of this.

Note:
There are many factors that we consider while deciding the manometer fluid. These include: Density, Specific gravity, surface tension, corrosive properties, coefficient of thermal expansion , viscosity changes with temperature, and the influence of absorption of water vapour from the atmosphere into the fluid.