The Mares XR Over Pressure Relief Valve (OPV) releases any build up of pressure inside your first stage regulator. Fitted to one of your LP ports this vavle will open when your interstage pressure goes above 13 bar, nominally, with +/-0.5 bar tolerance.
The Mares XR OPV is for any 1st stage that doesn't have a 2nd stage to relieve excessive pressure if the 1st stage fails. Primarily used for argon and air drysuit inflation cylinders that don't have a 2nd stage, you need an OPV to prevent excess pressure building up in the 1st stage and hose leading to your suit that could cause damage or uncontrolled inflation.
Technical Divers need redundant safety back-ups on most all their equipment to assure the safest dive techniques allowed. A first stage regulator should work well when it is maintained properly, yet if it does malfunction added safety features can make the difference between a controlled emergency and a diving accident. Incorporating a safety relief valve on your 1st stage regulator makes an over pressurisation of a 2nd stage regulator hose nearly impossible.
The 3/8"-24 UNF Male end of this Mares OPV screws into a standard LP port on a first stage regulator.
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