The two halves of the vessel and the diaphragm are retained by a crimped collar to give a pressure tight seal on sizes 8 to 25 litre and on larger sizes by a welded joint.Non-replaceable diaphragm.Epoxy coated in grey.Suitable for flow temperatures up to 120°C, resistant to ethylene orpropylene glycol mixtures and has low gas permeability.Altecnic expansion vessels are all tested according to the Pressure Systems Directive.