The provisioning of medical gases to hospitals, mobile health providers, and in-patient care environments requires absolute conformance to stringent industry standards. As a result, manufacturers, distributors, and facility safety managers must have complete confidence in supply quality and continuity.

Fig. 1 – Medical gas banks.
It is important to note that, while accredited governing bodies do set critical industry health and safety guidelines, they neither evaluate nor inspect medical gas equipment, facilities, pipelines, nor other infrastructure. Compliance is, therefore, the direct responsibility of the medical gas OEM, along with the individual facility into which such gases are delivered. To achieve required high-accuracy monitoring and control, design engineers typically specify pressure sensors, switches, and isolation valves within finished system designs. For larger or more complex projects, they also out- source the design of bespoke manifolds. These integrate sensors, switches, valves, and other components into a single control module.

Applications that OEMs typically confront address the often-complex instrumentation requirements of medical gas delivery systems. These include the monitoring of oxygen, helium, xenon, carbon dioxide, and nitrous oxide; medical and surgical air; and compressed air vacuum systems. Typical requirements can range anywhere from a few dozen pressure switches and valves, to the custom OEM volume manufacturing of fully degreased, oxygen compatible sensors, to the full instrumentation of medical gas distribution pipelines. This article examines, by application, several key considerations for medical gas delivery systems.

Portable Medical Gas Cylinder Changeover Devices

Without exception, medical gas OEMs must specify instrumentation that is in strict conformance with the maximum allowable and stability concentrations of delivered patient oxygen. Smaller and more mobile healthcare environments, such as ambulances and temporary triage centers, rely upon portable, replaceable supply cylinders for patient-inhaled oxygen delivery. (See Figure 1)

Due to the wide external temperature ranges over which portable changeover systems must operate, cylinder setups are also instrumented with safety alarms. High-pressure alarms signal higher temperatures and increased risk of gas store depletion caused by evaporation. Low-pressure alarms can signal pending cylinder changeover requirements.

Fig. 2 – Shown is a typical medical vacuum system.
Pressure monitoring ranges vary, depending upon medical gas type. Thus, careful attention to proper sensor selection criteria is essential. Specified instrumentation can accurately monitor the cylinder pressure levels of oxygen, nitrous oxide, and other medical gases. Typically installed between the cylinders and gas delivery system, they also help to determine proper cylinder supply changeover criteria and timing. One transducer is used to measure incoming delivered gas supply pressure. Another measures supply feed pressure to the distribution system. This process ensures that consistent pressure levels are maintained within a specified range.

Medical grade pressure switches are also installed on cylinder alarms to provide effective early warning of possible supply system risks, as well as leaks or blockages. They are also used to facilitate the switch of cylinders from standby to on-duty modes. Complete pressure manifolds, consisting of a pressure switch and various isolation valves, are also specified for these applications. Here, several medical gas cylinder banks are monitored simultaneously. Pressure switches used within manifold applications typically incorporate special Kapton® Polyimide diaphragms, which allow them to maintain their physical properties and performance stability over a wide operating temperature range. This also allows them to be directly compatible with a variety of medical gases.

In addition, pressure sensing instrumentation is used to monitor cylinders, alarms, and inhaled nitrous oxide supplies. Inhaled nitrous oxide has proven especially effective within neonatal intensive care units for the treatment of newborn hypoxemic respiratory failure.

Medical Air Generation Compressors and Vacuum Systems

Surgical and medical air are essential for safe operating theatre, patient hospital ward, and intensive care unit functionality. Compressed air helps medical professionals to deliver proper surgical anesthesia and inhaled patient oxygen concentrations. It also powers diagnostic and surgical equipment, dental drills, and other non-critical medical devices.

Medical and surgical air is typically produced via specialty water- or air-cooled compressor systems. In order to remain effective, a compressor must be clean and dry, as well as remain dust-, mold-, and oil-free throughout its operation. It also must reliably perform to varying set pressure levels. To ensure this, compressors are instrumented with both an automatic manifold panel and a manifold reserve, each programmed to a predefined set buffer pressure range. In the event that an automatic manifold is exhausted, a secondary emergency reserve manifold engages, ensuring uninterrupted airflow. Both manifolds require accurate, continuous pressure level monitoring.

Rugged stainless steel pressure sensors are often specified within these generator systems. The sensors form part of a control circuit that activates and deactivates the compressor, allowing it to automatically monitor supply pressure. In doing so, they helps to ensure that buffer pressure remains within its predefined set range. These sensors can reliably operate over a variety of pressure settings, with low thermal errors and a wide temperature compensation range. Sensors may also be customized with application-specific pressure ranges, ports, connectors, cables, and electrical outputs. In addition, the sensors can act as an alarm, providing early warning should levels fall below acceptable values. Data from these sensors also provide important overall compressor health assessments.

Medical vacuum systems, like compressed air, operate from a centrally controlled source. The systems are essential for surgical suction, as well as for generating negative pressure conditions within environmental chambers. Here, pressure transducers are used for critical level monitoring, alarm activation, and support of the central vacuum generator control circuit. (See Figure 2)

Distributed Medical Gas Pipeline Systems

Medical gas pipeline systems (MGPS) are used to transport a variety of often-combustible medical gases, including oxygen, medical air, and anesthesia from central and secondary stores into designated areas. In North America, medical gas pipelines are also designed for conformance to National Fire Protection Association guidelines. Typically constructed of copper, MGPS support high- and low-pressure medical gas delivery within hospitals, laboratories, and other clinical environments.

Basic medical gas pipeline infrastructure consists of a main line connecting the gas media supply to the risers; risers, which connect the main line to lateral pipelines; and lateral pipelines, which feed into a branch room, or set of rooms, within the medical care environment. These pipes are installed within the care setting in plain view, versus behind walls.

All MGPS must be extensively tested prior to use. This is to ensure adequate gas delivery pressure, supply regulation, and control. Particularly, gas supplies to anesthesia ventilators are most critical, as those rely upon sufficiently high pressure levels for proper functionality. To ensure such functionality, MGPS must also be continuously monitored for pressure and temperature variations and extremes. Sensors implemented within these environments are used to provide critical early warning of impending pressure drops or overpressure conditions.

Fig. 3 – A variety of transducers for low pressure measurement are shown.
The highly corrosive nature of certain gas media makes MGPS prone to corrosion, moisture, mold, pressure stresses, and other mechanical damages. Over time, these factors can contribute to pipe leakages. Within an oxygen delivery environment, such leaks can pose an especially high risk of fire or combustion, as well as patient health damage caused by depleted oxygen stores. Any cross-leakage of medical gases can further increase patient hypoxia risks.

Main MGPS supplies are controlled via manifolds. The manifolds allow for the manual or automatic switching of primary and secondary supplies between alternating stores. Gas is then transported through the main line. These systems also include a manual shutoff function, in the event of an emergency.

Instrumentation requirements call for the monitoring of individual pipelines, with their specifications based upon the type of measured gas media and untapped emergency stores. Vacuum air pressures are also monitored. Pressure switches and transducers are the key safety components to alarm for any leaks or overpressures, as well as to monitor general pressure levels. The main trunk system from the source often operates at 400 bar or 200 bar, with typical step-down distribution channels to 25 bar and 16 bar at a room level (sometimes as low as 10 bar). Pressure transducers and switches are placed strategically within the system to monitor pressure levels and alarm for any unusual conditions. Selected instrumentation must be able to withstand higher-than-typical pressure levels. Pressure sensors typically would operate at 12V or 24V, with 4 to 20 mA or 0.5 to 4.5V output. An IP67 rating ensures continued sensor reliability within possible splash environments. (See Figure 3)

Sensors specified within these applications feature high-accuracy and reliability with low thermal errors and a wide temperature compensation. Specification of sensors with compact size facilitates their ease of installation within the space constraints of a typical MGPS environment. Sensors degreased prior to shipment ensure their compatibility with specialty monitored gases.

Ensuring Absolute Compliance

The specialty nature of medical gas delivery systems, along with their critical end use, creates often-complex requirements. As the burdens of stringent regulatory compliance are left to the OEMs and health care facilities themselves, effective system monitoring is essential. Specified sensors and switches must be fully gas media compatible, as well as highly robust, accurate, and compact. Instrumentation must be free of contamination risks. It must also have the option for rapid customization.

This article was written by Vincent Ellis, Key Account and Marketing Manager, Gems Sensors & Controls, Basingstoke, Hampshire, England, and Molly Bakewell Chamberlin, President, Embassy Global, LLC, Hamburg, NY. For more information, Click Here " target="_blank" rel="noopener noreferrer">http://info.hotims.com/49751-162.

Medical Design Briefs Magazine

This article first appeared in the November, 2014 issue of Medical Design Briefs Magazine.

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