Sensors & transducers


CO2 sensors for ventilator and capnography monitoring


Gas Sensing Solutions offers a number of technologies that are well matched to the emerging requirements of


next generation CO2 monitoring applications, including in demanding ventilator and capnography equipment


C


O2 sensors are used in a variety of medical applications, ranging from


simple pH sensitive paper for


detecting dissolved levels of CO2 through to ultra high-speed sensors for capnography for


helping diagnose heart and lung function. Gas Sensing Solutions (GSS) is a leader in


designing CO2 sensors ideal for a range of these medical applications. A ventilator helps the patient to breathe. An


airway is connected to the patient either with a mask or through tubes in the mouth or nose. Ventilators blow air into the lungs replicating the normal breathing. Releasing pressure causes the lungs to relax and exhale naturally. It is possible to synchronise the output from the machine with the patient’s own breathing patterns. When people breathe normally, a tightening of


the diaphragm and other muscles inhales air into the lungs. Oxygen then diffuses into the


bloodstream through the lung walls. CO2 is diffused into the lungs from the blood and exhales when the muscles relax. The level of


CO2 exhaled by the patient is a key indicator their health condition. There is an on-going


desire to improve the fidelity of CO2 measurements to help diagnose patient condition in real time and with greater certainty.


INTRODUCTION TO NDIR


CO2 SENSORS GSS sensors use a technique called non- dispersive infra-red (NDIR). The sensors work by measuring the amount of infra-red light


absorbed by the CO2 gas. The concentration is proportional to the amount of light absorbed as it passes through the gas. GSS has a long track-record of developing and


manufacturing commercially advanced mid- infrared LEDs and photodetectors using its state-of-the-art molecular beam epitaxy (MBE) facility in the UK. The mid infra-red LEDs are tuned to emit a centre wavelength of 4.25um,


which is strongly absorbed by the CO2 gas. Light emitting diodes (LEDs) and the companion


Photodiodes (PDs) detectors are semiconductor Figure 2: Ventilator Operation


devices that are formed by the sequential epitaxy of semiconductor layers onto the surface of a crystal substrate using the MBE machine. LED radiation is generated in the active layer and the emission wavelength of the LED and the spectral response of the PD are determined by the energy gap of the material in the active layers. GSS uses a variety of techniques to optimise


the semiconductor designs to meet the sometimes market specific requirements of its


CO2 sensors. These semiconductor devices are incorporated into standard sensors or specially configured for demanding customer applications.


USING CO2 SENSORS IN MECHANICAL VENTILATORS


The COVID-19 pandemic highlighted the lack of mechanical ventilators for critically ill patients in many countries including advanced economies. At the beginning of the pandemic, many countries ramped up mass-production of ventilators in the belief the health care system would not have enough capacity to cope. However, a poorly considered part of this strategy was the unexpected impact on the ability of hospital infrastructures to be able to fully utilise an increase in the number of ventilators. A ventilator is a machine that is designed to


Figure 1: Cross Section of LED Structure 44


deliver air or an air oxygen mixture to a patient that is physically unable to breathe for themselves or requires some help. The gas


mixture is pumped into the patient, either directly into the lungs, or via some form of face mask or mouthpiece. The gas delivered from a ventilator is a


mixture of approximately 21 per cent oxygen, 78 per cent nitrogen, trace amounts of carbon dioxide and the remainder from other inert gases. The air that is exhaled however is different with a composition that depends on the health of the patient. Typically, it is 16 per cent oxygen, 78 per cent nitrogen, five per cent carbon dioxide and about one per cent of other gases. About five per cent of the oxygen is


exchanged by the patient into carbon dioxide, but the remaining oxygen is exhausted. Most ventilators will exhaust into the atmosphere, with the exhaled oxygen wasted. Whilst this may be deemed acceptable in existing applications, the dramatic increase in the number of ventilators precipitated by the COVID-19 pandemic has highlighted concerns about the ability of the patient care infrastructure hospitals to deliver enough oxygen to the ventilators. Oxygen and other gases are typically piped under pressure around a hospital and delivered to where they are needed. Installation of large numbers of ventilators may put a severe strain on the ability of the care infrastructure to deliver enough oxygen to the point of need. One way to overcome this potential limitation is to re-cycle the exhausted oxygen instead of


February 2021 Instrumentation Monthly


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74