Top Hat Commissioning Filters are typically deployed during the early stages of a gas installation and are designed to prevent system debris from entering the turbine or rotors of a gas meter during start up. The temporary use of a top hat (sometimes referred to as 'witches hat') commissioning filter is a vital way of protecting the internals of the meter. UK Metering recommend the use of top hat filters for turbine gas meter installations and insist their use with rotary gas meters.