Tech talk #3: “Hose illusions” Whether caused by wear and tear or damage, each diver amongst us most likely can relate to an occasion where it has been necessary to replace a regulator hose. Changing a hose is no big deal we all can agree upon. However shall we replace the hose with an original manufacturer’s one or just use a generic one at the fraction of the price is the question? As you might already guess, the answer isn’t that simple... A key-point to keep in the back of your mind is that manufacturers design their regulators to supply a certain volume of air at a given depth and maximum demand conditions. For regulators to pass EN250, simplified, this respiratory minute volume is 62.5 l/min at the surface, a test depth of 50m and a supply pressure of 50 bar. In order to solve this riddle, we’ll have to look into air flow. Primarily the amount of air flow through a hose is directly related to the diameter of the hose (and its restrictions, e.g. the fittings) and the air pressure supplied through the hose. A smaller diameter or lower pressure will result in a lower air flow through the hose. Secondary factors that will limit that flow include but are not limited to the length of the hose, tight bends, hose imperfections, material selection and so on. All those factors create turbulence in the air flow, increase the drag and thus increasing overall frictional losses in the hose. Finally as a diver we should understand that as the depth increases, not only our relative air demand increases, but so does the air density. Hence, at depth, those frictional losses will increase even more. All those losses under dynamic or flow conditions (i.e. a breathing diver) can result in a reduced pressure at the second stage end compared to the first stage. As mentioned before, a reduced pressure will reduce the airflow even more. A lot of talk to absorb, so let’s try to visualize this: I took some regulators and hooked them up to the flow bench in order to simulate air flow scenarios at the surface (atmospheric pressure) through original and generic hoses at different supply pressures. The goal is to establish the maximum achievable air flow through different hoses. Where applicable I did not connect the second stage hose up to the preferred port as not to bring another factor into play. Finally our test were restricted to a supply pressure of 1000 psi / 69 bar as most of the dynamic variations will occur in this lower supply pressure range. Increasing the supply pressure above 1000 psi did not result in any higher flow rates in any of the test. Note again that the test was performed at atmospheric pressure and therefore the results should not be extrapolated to predict the performance at depth; I’m convinced that there is a correlation between atmospheric and depth test results however no study is available to establish this relationship. - Scenario 1: Scubapro MK2 first stage with manufacturers hose versus generic hose. Both hoses have a length of 76 cm. The smallest diameter of the original hose is 5.1mm and the smallest diameter of the generic hose is 4.5mm. The maximum flow achieved through the original hose is 38 SCFM / 1076 lpm versus 34 SCFM / 963 lpm for the generic hose. On the graph the original hose performance is plotted in green, the generic hose in red. - Scenario 2: Mares MR12ST first stage with manufacturers hose versus generic hose. The original hose has a length of 80 cm and the generic hose has a length of 76 cm. The smallest diameter of the original hose is 5.1mm and the smallest diameter of the generic hose is 4.5mm. The maximum flow achieved through the original hose is 50 SCFM / 1416 lpm versus 42 SCFM / 1189 lpm for the generic hose. On the graph the original hose performance is plotted in green, the generic hose in red. - Scenario 3: Diverite RG3 first stage with manufacturers long hose versus 2 generic long hoses. All hoses have a length of 210 cm. The smallest diameter of the original hose is 4.5mm, the smallest diameter of the generic hose 1 is 4.5mm and the smallest diameter of the generic hose 2 is 4.9mm. The maximum flow achieved through the original hose is 39 SCFM / 1104 lpm versus 39 SCFM / 1104 lpm for the generic hose 1 and 45 SCFM / 1274 lpm for generic hose 2. On the graph the original hose performance is plotted in green, generic hose 1 in red and generic hose 2 in yellow. Conclusion: this experiment confirms that firstly smaller diameter hoses will restrict the maximum achievable flow through the hose and hence changes the performance characteristics of your regulator. As well, let’s not lose out of sight that that the maximum achievable flow will depend on the regulator/brand. In doubt or you’d like to know the flow through different hoses before making your purchase decision, consult your professional repair centre; they should be able to flow test the hoses for you. This article does not favour any brands nor claims performance comparisons between brands or condemns brands, it’s just an experiment. For those who want the full test results (including: methodology, dynamic intermediate pressures and mathematical back-up), please send us a PM.
Posted on: Tue, 27 Jan 2015 01:38:30 +0000
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