A bed of granular medium (usually sand, but other materials can also be used, such as Ballotini or anthracite) is placed in a readily demountable tube through which water can be made to flow in either a downward or upward direction.
Flow from a laboratory tap passes through a constant head tank, which also allows air bubbles to be released, and is controlled by hand with a needle valve. The rate of flow is indicated by a variable area meter.
Pressure drop across the bed can be measured either by a 50cm-long water differential manometer or a 50cm-long mercury manometer (see recommended accessories), depending on the magnitude. Valves are fitted for isolation of various parts of the circuit, along with air release valves. The test section tube and all tubing connections are transparent so that operation can be observed and the presence of air bubbles easily detected. All metal fittings are corrosion resistant. No power connections are required and the whole apparatus can be lifted and transported by one person. Test material (Ballotini) for the packed bed is supplied but mercury for the manometer is not included.