You know, in over 25 years of servicing scuba kit, I have never damaged, or seen a seat damaged, by not pressing the purge button......I must admit I usually do press the button or depress the lever, but by no means do I always do it.
Maybe it's one of those mask on the forehead deals that just get perpetuated by the community.
I've heard this caution for 25 years as well - and again I've never actually seen it happen. Like you I normally follow this practice as a good practice, but not universally.
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Actually, on the MK25 the IP will increase with decreasing tank pressure.
"Intermediate Pressure Limits: IP limits must be between 130-142 PSIG at both 500and 3000 PSI supply. Verify and expect an IP inversion/increase of approximately 5 PSI from 3000 PSI to 500 PSI supply. IP inversion: As tank supply decrease, IP will increase. For best results, set IP @ 142 PSI, with a supply pressure of 500."
They did not mention that in March of 2011.
This would be unfortunate as it would suggest Scubapro has in fact "overbalanced" the design.
The only valid argument for doing this is to improve flow rates at low supply pressures, but the Mk 25 already delivers bucket loads of gas at low tank pressures.
As noted then, if the IP increases at low tank pressures, the second stage must be detuned to prevent any free flow at low tank pressures - and that in essence detunes the reg across the entree range of tank performance.
In that regard it's not an improvement or a design feature, but rather an engineering mistake. What is this fascination with screwing with IP when IP should be a very stable thing to maximize second stage performance? It makes me suspect the marketing folks are back in charge of the engineering department.
It's also a powerful argument for getting a Mk 17.
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However, I would suggest actually checking the IP on a sample of Mk 25s and see what really happens.
And, they are apparently suggesting 142 psi at 500 psi to maximize the IP and allow greater air delivery. However, many techs with cold wearer customers reduce the IP to the 120 psi range to reduce gas flow and reduce the freeze flow potential, and often detune the second stage to prevent any inadvertent free flow that might cool the reg and lead to a freeze flow.
