Gas Metering System

An MOV (Manual Override Valve)

The ABEL Mk5 is a simple mechanical SCR system and uses the principle of HPD (high pressure drop) gas flow, to establish a set gas flow across an orifice. Simply stated when the inlet pressure is more than twice the outlet pressure across an orifice the speed of the gas flowing through the orifice is equal to the speed of sound. This is the maximum flow that can be obtained and any further increase in inlet pressure or decrease in outlet pressure will not alter the flow rate.

The maximum flow rates are also sometimes called sonic flow, critical flow, choked flow or constant mass flow. Most standard regulators correctly maintained and serviced will deliver approximately 10bars pressure to the LP side of the regulator. This then is quite adequate to operate the principle of HPD gas flow. This facility is then used in the ABEL Mk5 SCR to drive the valve train which is connected to one of the LP ports on the first stage of the regulator. All that is required is to have the regulator and cylinder cleaned and serviced for Nitrox use.

The Valve Train for a single gas mix is of a flow regulator / metering valve and MOV (Manual Override Valve). The MOV is sometimes called the additional or add valve. The minimum flow rate should be set at a rate that will support the diver at a maximum physical exertion rate of 75lts per min tidal volume with an expected metabolic consumption rate of 3lts per minute. This is about as hard as it gets. Recommended flow rates are calculated to take into account worst-case scenarios.

A Flow Regulator / Metering Valve

The flow rates that we suggest are in fact the ones we use. 5lts/min for 80%, 7.5lts/min for 60%, 10lts/min for 50%, 15lts/min for 40% and 20lts/min for 32.5% for Nitrox. These are the minimum flow rates set at the metering valve, should the diver for whatever reason decide that the flow rate is not high enough then he has the option of cracking the MOV slightly to increase the flow rate. The MOV is also used to compensate for a reduction in gas volume in the counter lungs due to increasing depth and for flush through during the dive and preceding assent.

The valve train for a twin set, dual gas mix is a more complicated version of the above. It has two flow regulators, two one way valves and a common MOV. Gas mix selection during the dive is made by opening and closing the relevant pillar valves, when this is done the system is flushed through to ensure the correct mix is then at the MOV and in the breathing loop. The one way valves ensure that the gas mix selected is only fed to it's corresponding flow regulator and hence into the breathing loop.