Constant Mass Flow – KISS mCCR

  1. constant mass flow Technical diving wellington-min

The KISS CCR is an mCCR or manual CCR. That means it does not rely on a solenoid to detect and inject oxygen when the PPO2 drops. The KISS PPO2 is controlled by two events. The first is an orifice through which oxygen flows constantly at a set rate. The second is the manual add valve (MAV) operated by the diver. The constant flow of O2 bleeding into our unit through the orifice replaces much of the 02 we metabolise. To help us keep a constant PPO2 we need a constant oxygen flow into the unit. This is called constant mass flow (CMF). If we need more O2, we can add that via the MAV.

To achieve this, our O2 1st stage is modified so that it operates at a constant intermediate pressure (IP) of 10 bar regardless of depth. This creates a CMF of about 1 L/min through the 00.0035 micron orifice.

When gas is pushed through the orifice, the maximum speed it can pass through at is called Sonic Speed.  It reaches its sonic speed when the IP is at least twice the outlet pressure (OP). With a fixed diameter orifice, the speed of the gas passing through can never be increased once sonic speed is achieved. That means the density or mass of the flow won’t change either so the PPO2 is very stable and requires minimal diver intervention.

constant mass flow Technical diving wellington-minWhen we reach 40m the absolute or ambient pressure in the water is 5 bars, but the O2 1st stage IP stays at 10 bars absolute. We still have “maximum CMF” as IP (10 bars) is still twice the OP (5 bars). When we descend past 40m the velocity of the gas will keep dropping until we reach 90m. At 90m, ambient pressure or OP equals the IP (both are 10 bars). As there is now no pressure difference between the IP and the OP, no gas will flow through the orifice, as the CMF stops.

As the O2 regulator 1st stage provides the O2 supply for both the orifice and MAV, neither will work at 90m/10 bar. An additional O2 supply method will need to be provided if the diver wishes to descend past 90m. The KISS Classic/Explorer can be modified simply with a manufactures approved fitting to allow oxygen to be manually injected. This allows the KISS Classic diver to descend deeper than 90m. The Spirit LTE does not have this ability.

Attributed to: http://www.advanceddivermagazine.com/articles/CMF/CMF.html

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