TESTCENTRE
Second Stage
The second stage works in much the same way as the first . On inhaling , pressure is reduced , the diaphragm is pushed inward by water pressure and gas is supplied to the diver at ambient pressure . When the diver stops breathing , pressure in the second stage increases , relaxes the diaphragm and closes the valve .
Second stages are also available in balanced and unbalanced models . They offer similar pros and cons to those of their corresponding first stages . The balanced first stage will supply gas at a constant pressure whatever the depth or pressure gauge tells you . They do tend to be more expensive than their unbalanced counterparts , but many divers regard this as a price worth paying .
An unbalanced first stage offers marginally more resistance as depth increases and tank pressure decreases . Although slightly more physical effort may be required , it ’ s easier to tell when you ’ re getting low on gas , because you really feel the difference . Being cheaper to manufacture , they usually present an affordable option for those on a budget .
A third variant is the overbalanced regulator , which provides improved gas flow with increasing depth . A downside to this system is an increased sensitivity which may lead to free-flow , particularly in shallower depths . This can be counteracted using a second stage inhalation adjustment control .
Upstream v downstream
The majority of today ’ s regulators are of downstream design . In the event of a first stage failure , the downstream second stage valve is forced into the open position , continuing to supply the diver with air , albeit in the form of a free-flow .
In a similar scenario , the upstream design forces the valve to close . In years gone by , this meant air supply would be cut off completely . Modern upstream regulators however , feature an over-pressure relief valve that allows the diver to continue breathing .
An environmentally sealed first stage with heat exchange fins on both first and primary / alternative second stages .
Venturi effect and inhalation adjustment
There ’ s often confusion surrounding the functions of the Venturi lever and the inhalation / opening effort adjustment knob . The Venturi Effect , without going into too much detail , harnesses fast-flowing air to create a vacuum behind the second stage ’ s diaphragm as means of enhancing a diver ’ s ease of breathing effort .
The downside is a tendency to free-flow more easily , particularly at the surface and even more so when not securely clamped in the diver ’ s mouth . To counteract this , a small lever typically labelled pre-dive ( or
- ) and dive ( or +) allows the Venturi effect to be disabled prior to immersion and then activated on descent .
Opening or ‘ cracking ’ effort is essentially the amount of work it takes to begin the flow of air from your second stage . The purpose of the rotating knob , usually labelled with plus and minus symbols , is to allow fine-tuning of your breathing effort .
A typical advantage of being able to tune down would be while swimming into strong current or using a scooter . In such cases , increased water pressure against the purge button may result in a free-flow . If you ’ re diving deep or have increased your workload , then simply open it up for an easier breathe .
Some first stages offer both options . Inhalation adjustment knob ( left ) and Venturi lever ( right ).
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