EN IEC 61207-2 pdf free download
EN IEC 61207-2 pdf free download.Expression of performance of gas analyzers – Part 2: Measuring oxygen in gas utilizing high-temperature electrochemical sensors.
3.2 Concepts
3.2.1 High-temperature electrochemical sensor
3.2.1.1 General
The sensor is usually controlled at a stable, high temperature, typically in excess of 500 DC. This high temperature is normally maintained by an electric heater, however, in some high- temperature in-situ applications, the sensor may require cooling to be applied. The sensor may also be run in passive mode with temperature sensing, where the heating is provided by the sample environment and the measured temperature is used in the calculation of the oxygen concentration. The high-temperature electrochemical sensor can be constructed in two basic forms:
a) galvanic concentration cell;
b) ion pump cell.
3.2.1.2 Galvanic concentration cell (gauge cell)
3.2.1.2.1 General
Most commercially available analyzers employ the galvanic concentration cell consisting of two gas volumes or chambers, separated by an oxygen ion conducting solid electrolyte, and provided with a porous electrode on each side. The two sides are filled with sample gas on the one side and a fixed oxygen partial pressure reference gas on the other side. The reference gas shall contain some oxygen. The reference gas is usually air, but could be another constant oxygen partial pressure mixture or even a sealed reference where the oxygen partial pressure is maintained by a metal/metal oxide mixture.
The electrodes are catalytic and the electrode/solid electrolyte interface at elevated temperature allows the formation of oxygen ions (Q2j which can then diffuse across the solid electrolyte interface. This interface remains an impenetrable barrier for the other gases present and thus provides a selective means of determining the partial pressure of oxygen present in the sample gas. The solid electrolyte is typically yttrium oxide (yttria)-stabilized zirconium oxide (zirconia), and the porous electrode is platinum based, although other materials may be used. The signal magnitude is temperature dependent and thus requires a low uncertainty of temperature measurement of the solid electrolyte interface by employing temperature sensors as given in IEC 60584 and IEC 60751, and stability of heating provided to achieve the high temperatures required for efficient and sensitive operation.
Thus, provided the oxygen partial pressure is known at one electrode (P1), then the potential difference between the two electrodes will enable the unknown oxygen pressure to be determined at the other electrode (P2).
Note that in the above formulae, it is the partial pressure of oxygen on the two sides which is important, not the fractional component of the oxygen. Therefore, if equal component mixtures containing oxygen (e.g. air), but at different absolute pressures, are applied to either side of the solid electrolyte barrier, the signal will not be 0 mV, but proportional to the logarithm of the ratio of the absolute pressures of the gases on each side.
The Nernstian response of the high-temperature electrochemical ceramic sensor holds over a very wide range of oxygen partial pressures differences, and the sensor output increases logarithmically with linear reduction of the oxygen’s partial pressure at a given temperature. The sensor output is directly proportional to temperature, and hence for quantitative analysis, the temperature of the cell should be closely controlled and/or measured, and the necessary corrections applied in Formula (1).
Theoretically, the output e.m.f. of the sensor, when the partial pressures of the sample gas and reference gas are equal, is 0 V. However, in some sensors, a zero offset is measured and is considered as being largely due to thermo-electric effects and thermal gradients across the electrodes. This offset can be considered theoretically as an extra constant (asymmetry potential).EN IEC 61207-2 pdf download.
