This will depend on the size of the installation. The LPC Rules for Automatic Sprinkler Installations, in Annex K, states: ‘At least one-metre length of range pipe should be inspected per 100 sprinklers.’ If you have 1,000 sprinklers in your building, it is therefore recommended to have at least 10 range pipes tested.
Each pipe sent should be as close to 1 metre in length as possible, regardless of diameter.