# Measuring the gas supply pressure

##### Demonstration

Using U-tube manometers to measure the gas supply pressure.

#### Apparatus and materials

U-tube manometer, 0.5 m tall

Slotted base

Methyl orange

Length of Bunsen tubing

0.5 metre rule

Trap (to prevent the water getting into the gas supply)

#### Health & Safety

Ensure the lab is well ventilated.

A thin film of oil on top of the mercury will prevent the vapour escaping.

The 0.5 m manometer is filled with water coloured with a few drops of methyl orange to make it clearly visible. It is set up vertically using the slotted base to hold it.

You should place a trap between the gas tap and manometer to prevent water from getting into the gas supply.

#### Procedure

a Connect the manometer to the gas supply by Bunsen tubing, via the trap.

b Turn the gas tap on, slowly at first, and keep it on. Measure the difference in levels in the manometer arms.

#### Teaching notes

1 Before the gas tap is connected, the levels of the water in both arms of the manometer should be the same, showing that atmospheric pressure pushes down equally on both columns of water. When the gas tap is connected to one arm and the gas is turned on, one arm has a downward pressure due to the atmosphere and the gas and the other arm has a downward pressure due to the atmosphere only. The difference in water levels is caused by the difference in pressures on the water in each arm.

2 If both arms are connected to the gas supply, then the level of water in each tube is once again the same and so the pressure difference is zero.

3 The manometer tube can be filled with oil or mercury to show that the gas still produces a difference in the level of the liquid but the value is greater in the case of oil and less in the case of mercury. Has the gas pressure changed? No! The gas company is unaware of the instrument which is connected. However, pressure is measured as force/area and the weight of extra liquid supported by the gas pressure is equal to the volume of extra liquid (related to height of liquid) x density of the liquid.

The experiment was safety-checked in July 2007