Oxygen measurement in diving technology

Transcription

1 Oxygen measurement in diving technology Oxygen and nitrogen partial pressure are important parameters for divers, when it comes to calculating maximum depth ranges, decompressions and intervals between dives. With traditional open circuit scuba, the gas composition is known to the diver and the computer. In rebreathers it is possible to use different gas mixes (Nitrox, Heliox or Trimix). Moreover the oxygen level in the system is dependent on the divers consumption. Therefore the measurement of the oxygen level would be a practical feature. Optical sensors exploit the different spectra of absorption of the gases: Here the absorption is measured at a frequency which is typical for the gas to be measured, and the absorption is associated to a concentration. Oxygen molecules trespass a membrane and are reduced at a gold- cathode. The necessary electrons are set free through the oxidation of lead. The higher the O2 partial pressure the more molecules will trespass the membrane and will be reduced at the goldplate: The electronical power supplied by the sensor increases. The strength of the current gives the O2 partial pressure in the gas. A disadvantage of this principle is the temperature dependence: With increasing temperature the sensor signal rises. This performance is compensated with resistors that have inverse temperature performance. Here it is important to make sure the resistor and the membrane are subject to exactly the same temperature. With some of the available sensors, the resistor is located inside the sensor- body. The temperature compensation of these sensors is very good, as long as the temperature of the gas equals the temperature of the sensor. In

2 rebreathers these temperatures can differ from each other. The sensor- body will assume the water temperature like most of the materials used in the system. The temperature inside the breathing bags will be different from this, since the temperature generated in the scrubber- canister will alter the temperature of the gas that flows through it. The result is an error of measurment of the sensor, that can be as much as 10%. Since the sensor is a consumable part (just like a battery), it is necessary to make predive tests before every dive and calibrate the sensor. For this calibration it is necessary to expose the sensor to a known gasmix. Normally it is possible to use surrounding air, however it is better to use pure oxygen, to minimise a linearity error. The development of microbubbles in the sensor is the same as in the human body: During the dive, the inert gas (N2, He) trespasses the membrane due to the difference in partial pressure, and dissolves in the contained liquid. Slowly the inert gas fraction increases. At the surface, less inert gas leaves the sensor, since the partial pressure does not differ as much as it did before. Due to this, the inertgas accumulates inside the sensor when it comes to frequent dives.at a certain critical oversaturation microbubbles develop during fast pressure decrease and can affect the performance of the sensor. This phenomenon may occur only after many dives, and must be tested in long-term field-tests. Drägerwerk AG uses oxygen sensors in many different areas of medical and safety technology very successfully. Experiences in this new area must be gained. For this it is necessary to simulate dives in compression chambers the same as field-tests under controlled circumstances during the entire lifetime of the sensor. The collected results give feedback on the quality of the sensor and are used to further

3 develop the sensor. This procedure is very time intensive and requires a good co-operation between the partners. This way of proceeding is absolutely necessary in order to produce a high quality sensor. Electrochemical gas sensors have a lifetime of 1-2 years under normal conditions, with prices around 450 DM per sensor. Certainly the useful life of sensors for diving use is lower, due to the more extreme conditions of utilisation. Reliable statements will not be possible until long-term testing has been made. Realistically the useful life can be estimated to be 6-12 months. A well known method to measure oxygen is to exploit its paramagnetic behaviour. Oxygen is being pulled into a magnetic field and moves a crystal ball that is attached. The extent to which the crystal ball is moved is used as basis to calculate the partial pressure of the oxygen.a further possibility is based on the different thermal performances of different gases. An iron wire is constantly heated with electric power and the gas to be measured is flushed around the wire. According to the gas composition the temperature changes. However this method can only be used with dual gases having big differences in thermal performances. These three different sensors require relatively much electric energy and are furthermore fairly expensive compared to electrochemical O2- sensors. This sensor is an electronic current source that is dependent on the O2- partial pressure. It has been used in medical technology for over 20 years for monitoring the oxygen level in breathing gases. It does not require a power supply and is very robust. This static compensation- fault will even be increased by a dynamic compensation fault: The sensor and the resistor will react with different speeds to sudden temperature changes, and will lead to unacceptable errors until a

4 stable static situation is reached. The level of this error is dictated by the time difference in reaction of the sensor and the resistor and the speed of the temperature change. The error can reach as much as ( 20%. Sudden temperature changes can occur, when the diver enters cold water after preparing for the dive in a warm air environment. The error can be reduced by using an additional temperature sensor and an acceptable mathematical basis. In this case however, it is necessary to fine-tune the sensor to the special necessities of the concerned diving apparatus. A very good co-operation between the partners is very important. In order to reach further exactness for the oxygen partial pressure measurement it is useful to measure the ambient pressure. Even under hard conditions, the sensor must maintain its reliability over the entire lifetime. Therefore it is essential, that neither frequent pressure changes with a connected microbubbledevelopment in the sensor nor condensing water or corrosion of the electric contacts may influence the performance of the sensor. The available double- cathode sensors do not fulfil the requirement of redundant measurement. In these sensors, oxygen is reduced at two different cathodplates, and the measurement signal is transferred separately. However, other important parts are single. The computer in a redundant system compares the signals of two or more sensors and evaluates whether or not the measurements match with each other. In cases of error, the diver must be warned and be brought to the surface safely. The quantity of redundant measurement-systems depends on the importance of the message. Messages that are supposed to really gain safetyrelevant importance will certainly require three independent

5 measurement- units as in space applications. In this case however, the message may be compared to other values (such as depth) for plausibility reasons. For monitoring the scrubber in the unit, it is in principle possible to introduce a CO2- sensor. Fundamentally this sensor is subject to the same factors as the above mentioned oxygen sensor. The sensor must be fine-tuned exactly to the requirements of diving technology and the needs of the specific unit. For a safety relevant message a redundant system is important. The calibrating procedure for a high quality electrochemical CO2- sensor is more complicated than for an O2 sensor: Due to higher variances, it is necessary to make a two point calibration. In a first step the sensor is calibrated in CO2 free gas at the offsetpoint. In a second step, the sensor is calibrated in a gas with a known CO2 concentration (for example 5%). Both values must be memorised by the computer, which then evaluates the sensor impulses into reasonable values. Certainly the application of gas sensors in diving apparatusses is interesting. The monitoring of partial pressure and maybe even depth dependent gas mixing considering the workload of the diver seems to be at least possible in the future. At the moment no conventional O2 sensor specifically for diving application is known to us out of serial production. We have the know- how, but we know that the development of such a sensor will require some time. Dipl. Ing. Ulrich Schmidt Sensor development Drägerwerk AG, Lübeck