OXYGEN SYSTEMS
demand with any certainty; only to provide an advisory output for scheduling O2
deliveries for VIE replenishment.
Installing their own measuring systems
Due to these concerns, and the limitations of the existing VIE (Vacuum Insulated Evaporator) systems to indicate real-time demand, NHS Trusts have seen the importance of installing their own dynamic oxygen flow demand measuring systems on their copper pipework. As a result of these issues, raised within the NHSE/I – 2020/001 alert relating to the Adequacy, Continuity and Quality of Oxygen as delivered through the Piped Medical Gas System, many Trusts have investigated how to achieve actual dynamic flow measurement at various delivery and take-off points, rather than a theoretical pipe flow/capacity methodology based purely on a pressure transmitter measuring the level within the VIE pressure vessel. Real-time data could be used to ensure efficient bed management and patient care by clinical staff.
An easily retrofitted solution Thankfully, the solution is easily retrofitted to existing oxygen delivery copper pipework without any pipe re-work, invasive system modifications, or critical system outages. Non-invasive ultrasonic oxygen flowmeters from Flexim are simply clamped onto existing copper pipework and introduce no new leak paths or pipe tapping points which could harbour health-threatening bugs. The FLUXUS G721CA Oxygen meter, invented, designed, and manufactured by Flexim in Berlin, works on the principle of Transit Time, otherwise known as ‘Time of Flight’ (see Fig 1). Transducers are robustly attached to the outer wall of the copper pipe within a protective mounting fixture. This pair of dual-matched and calibrated transducers simultaneously sends and
Figure 1: The Ultrasonic Transit Time measuring principle; ultrasound travelling in the direction of oxygen flow travels faster than ultrasound travelling against the flow. The time difference, ΔT, is directly proportional to the flow rate.
receives 1,000 bursts of ultrasound per second, with signals sent both with the flowing oxygen and against the gas flow. If we use the analogy of rowing a boat along a river, the boat travels faster in the direction of flow, and then slower when returning against the river current. So too, ultrasound sent with the oxygen flow in a copper pipe is sped up on its journey, and likewise slowed on its return journey, fighting against the O2
flow. This
difference in ‘time of flight’, the ΔT between the two journeys up and down stream, is directly proportional to the flow rate within the pipe. This velocity, when applied to the cross-sectional area of the pipe, can be expressed in instantaneous volumetric flow rate, e.g. L/min.
Flowmeters dispatched pre-calibrated
Clamp-on ultrasonic flowmeters from Flexim are dispatched from the factory pre-calibrated, with a calibration certificate proving laboratory accuracy of +/–0.3% of reading. Pre-calibration, drift-free electronics, and state-of-the-art signal processing evaluating signals every 10 ms result in transducers and transmitters with the highest possible zero-point stability. Installed accuracy, when commissioned by a factory-trained
engineer, will be in the range of +/–1%-3% of reading. Confidence in the local measurement can also be achieved via interrogation of a rich set of diagnostic values, including verification of the expected speed of sound of the gas being measured.
When applied to pipes within the hospital at a regulated 4 bar typical pressure, there is little need for any pressure or temperature compensation to the measurement to achieve acceptable levels of flow measurement that are a significant improvement over the basic data from the VIE pressure-based system of alert. In the rare cases where pressure fluctuates widely, most likely before the 4 bar regulators, i.e. on the ~14 bar pipework directly exiting from the VIE pressure vessel, it would be recommended to compensate for pressure and possibly temperature on such flow measurements to achieve greater accuracy during these fluctuating operating conditions.
Highly accurate and repeatable flow measurement data
Advanced digital signal processing and measurement algorithms ensure highly accurate and repeatable flow measurement data, instantly capturing changes in the flow regime. Data is either
An oxygen meter installed within a VIE compound – with the transmitter, top right, and the transducers, on pipe, bottom left.
56 Health Estate Journal January 2021
Flexim FLUXUS G721CA transmitters.
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