The Gaslow Butane 21.8LH Adapter Gauge screws onto a Butane 21.8LH cylinder connection.It connects directly onto a 4.5kg Butane cylinderCan also be used with the relevant adapter to fit 7kg and 15kg Butane cylinders (01-1670 21mm Clip-on) and Camping Gas cylinders (01-1665)Compatible with all Gaslow 21.8LH Stainless Steel and Rubber hoses and 21.8LH Butane 28mbar regulators (01-1728).It provides a visual warning of low gas levels with the Green/Yellow/Red displaySecondly, it operates as a quick and easy leak detector!