Sensor, oxygen, accuracy

Relevant issues still to be addressed in constructing amperometric enzyme sensors either using the electrical wiring of enzymes with redox polymers or with flexible polymeric electron mediators are sensor efficiency, accuracy, reproducibility, selectivity, insensitivity to partial pressure of oxygen, detectivity (signal-to-noise ratio) as well as sensor hfetime and biocompatibility [47]. Then we can address manufacturability and the cost of use of either in vitro or in vivo sensors. 

Figure 9.14. Precision and accuracy of the instrument at various concentrations of oxygen as compared to a standard oxygen analyzer (Servomex). The fiber optic sensor monitored the concentration of oxygen in saline solution (continuous stair) in equilibrium with various nitrogen-oxygen gas mixtures monitored by the gas analyzer (dotted staircase). The absolute concentration of oxygen in the gas phase is about 60 times larger than the corresponding equilibrium concentration in the liquid phase. The bath temperature was 37 C For the purpose of comparison both measurements have been scaled to percent oxygen. (From Ref. 21 with permission.)...

In order to use semiconductive metal oxides as an oxygen sensor, both thermal stability at elevated temperatures and atmospheric stability under reductive environments are required for reproducibility and accuracy of the sensor. 

An example of this problem is shown in Fig. 5.27, where the transitionmode closed-loop control of central oxygen partial pressure is performed by adjusting the discharge power. The graph shows that the central oxygen partial pressure is kept constant with high accuracy. Additional A-sensors have been used to monitor the top and the bottom O2 partial pressure. These pressures vary for the two consecutive runs by several millipascal. As a consequence, the films are neither homogeneous nor reproducible. 

The uncertainty of commercial calibration gas mixtures is typically 1% of the amount fraction, and the contribution of this to the overall measurement uncertainty is much greater than the intrinsic error of the oxygen sensor for amount fractions of oxygen that are 0.1 ppm and above [2]. Consequently, test procedures that require calibrated gas mixtures will be limited by the uncertainty of those gas mixtures, and therefore carmot be used to test the accuracy of oxygen and other zirconia-based gas sensors, assuming the uncertainty of the sensor calibration is no worse than that of the test mixtures. At present, the uncertainty of commercial calibration gas mixtures is a limitation to verifying the accuracy of these gas sensors. 

This paper will concentrate on the unique requirements of aeronomic spectroscopy and on the application of image devices to these measurements. Spectrometer 1, Table I, was developed for rocket experiments intended to measure the NIR absorption spectra of 1 0 and 02 molecules in the middle atmosphere. A photodiode array was used as the spectrometric sensor. With this spectrometer we were able to measure the NIR solar radiation spectrum with an altitude resolution better than 2 km. Spectrometer 2, Table I, was basically of the same design as spectrometer 1, except that an image intensifier was optically coupled to the diode array to permit low light-level measurements. The resolution of this spectrometer was adequate for measurements of rotational profiles of the A-band absorption spectra of 02 molecules. We were able to measure the rotational temperature of oxygen molecules, in the stratosphere and the lower mesosphere with an accuracy of + 1.5°, and a spatial resolution better than 2 km. These experiments provided the basis for study of the dynamic processes of atmospheric molecules. Spectrometer 3,... 

In both cases the accuracy requirement is shifted from the sensors to the oxygen dosage or constant-flow generator. Processing the respiratory quotient, RQ, requires inaccuracies of oxygen uptake and CO2 production of less than 3% to result in RQ values with deviations of less than 6-8170, which can be used to control the nutrition of patients in the post-operation phase. To complete the parameters needed, the nitrogen excretion has to be determined by clinical laboratory methods. 

Recently, combustion control by wide range air/fuel ratio sensor with an oxygen pumping function as measurement in the atmosphere of high oxygen volume has been developed, but without much accuracy. 

For oxygen determinations within the framework of WOCE (World Ocean Circulation Experiment) an algorithm to convert the CTD oxygen sensor measurements into oxygen profiles based on the documented sensor physics and in situ oxygen data of discrete samples is described in the WOCE Opierations Manual (WHP, 1994). The achievable accuracy and precision is < 1 % and 0.1 %, respjectively. 

The association of an electrochemical oxygen pump and an oxygen sensor allows the monitoring of oxygen partial pressure in a flowing gas in the range 1 bar - 10-27 with an accuracy of 2 %. 

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