# Measuring gas pressure using manometers

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**Demonstration**

Does the area of a manometer tube matter when measuring pressure?

#### Apparatus and materials

Mounted U-tube manometer, 1.5 m tall

Manometer with unequal limbs, 0.75 m tall

Aspirator (10 litres)

Food colouring

#### Healthy & Safety and Technical notes

**1** Set up the 1.5 m manometer vertically using the slotted base to hold it. One arm should be removable, so that it can be replaced with a tube of the same length but greater diameter. If this is not possible, then join together two unequal arms.

Fill it with water, coloured with a few drops of food colouring to make it clearly visible.

**2** The gas pressure is generally about 25 cm of water. When filling the tubes there must be sufficient water to allow for this change in level.

#### Procedure

**a** Connect the local gas supply to the manometer.

**b** Replace one arm of the manometer with a tube of greater diameter and repeat.

**c** Connect the gas supply to the aspirator. It should be connected to each end of the aspirator in turn, as indicated in the diagrams below, to show that the difference in level is still the same.

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Teaching notes

**1** These three experiments should be set up side by side.

Ask, *would it make any difference to the manometer reading for a given pressure if the cross-section area of the liquid in the tube were changed on one side?* Surprisingly for young students, the level difference is always the same.

The manometers with unequal arms should be connected up to the gas supply first from one side and then from the other to see if it matters which side is the input.

**2** For those who can manage a mathematical argument about the pressure being equal to the extra weight of water supported/area then they will see that the height of the extra column of water is independent of the cross sectional area. For some the pressure equation *p = hρg* can be explored.

For some particular pressure, explore the equation *p = hρg*.

*h* = difference in height of columns

*ρ* = density of liquid in the columns

*g* = gravitational field strength = 10 N/kg

*The experiment was safety-checked in July 2007*