Measurement principle of oxygen concentration analyzer

Measurement principle

Oxygen concentration analyzer (also known as zirconia oxygen analyzer, zirconia analyzer, zirconia oxygen meter, zirconia oxygen meter), Mainly used to measure the oxygen concentration of flue gas during combustion, it is also suitable for measuring the oxygen concentration of non combustion gases. The electrochemical cell (oxygen concentration cell, also known as zirconium head) with a constant temperature inside the sensor generates a millivolt potential, which directly reflects the oxygen concentration value in the flue gas.

The key component of the oxygen sensor is zirconia, which is coated with porous platinum electrodes on both sides of the zirconia element to create an oxygen concentration difference battery. It is located at the top of the sensor. In order to maintain the rated operating temperature of the battery, a heater is installed in the sensor. Use the temperature controller inside the oxygen controller to control the constant temperature of zirconia.

At rated temperature, the output potential of the battery is calculated using the following formula (Nernst equation):

In the formula,

 P1- the oxygen partial pressure of the reference gas (such as air) inside the battery;

P2- oxygen partial pressure of the gas (such as smoke) being measured outside the battery;

R - Gas constant;

F - Faraday constant;

T - Absolute temperature=(273+t ℃);

C - Battery constant.

The reference gas should be dry, clean, and oil-free air (containing 20.60% oxygen). When the oxygen concentration of the reference gas and the measured gas is different, oxygen ions migrate from the high side to the low side. The battery output reflects the oxygen concentration value in the measured gas in a logarithmic pattern.

system composition

The composition of an oxygen concentration analyzer is composed of oxygen sensors (also known as oxygen probes, oxygen detectors), oxygen controllers (also known as transmitters, transmission units, converters, analyzers), and connecting cables between them.

oxygen sensor

The sensor device consists of a metal casing, measuring battery, heater, thermocouple, filtering element, and cable terminal blocks. The measuring battery body is divided into three layers: platinum (electrode) - zirconia (electrolyte) - platinum (electrode). Platinum electrodes are porous. The flue gas enters one side of the measured gas in the measurement battery through a filter or calibration gas through a transfer tube, while the other side is the reference air (containing 20.60% oxygen).

Two gases with different oxygen concentrations act on the measuring battery, generating a logarithmic potential (the larger the difference in oxygen concentration on both sides, the larger the potential signal). The millivolt signal is converted into 0-10mA or 4-20mA standard current through an oxygen controller. This current is output by the oxygen controller wiring terminal.

The working temperature of the measuring battery is set to a constant temperature higher than 650 ℃. In order to maintain a constant working temperature, a K-type thermocouple is used to measure the working temperature of the battery, and the heating voltage of the heater is adjusted by the temperature controller inside the oxygen controller.

When the measured flue gas temperature is above 700 ℃, the heater and temperature thermocouple are omitted from the sensor composition.

oxygen analyzer

In order to achieve a working temperature of 700 ℃ for the measuring battery, the oxygen analyzer receives the temperature mV signal output by the K-type thermocouple in the sensor and compares it with the preset temperature (millivolts) of the microprocessor to control the battery temperature. The oxygen analyzer uses ambient temperature as the cold junction comparison point for thermocouples.

The oxygen analyzer amplifies the oxygen mV signal input by the oxygen sensor, and then converts the amplified voltage signal into a digital signal through an A/D converter. Measure the characteristic curve of the battery based on pre calibrated or preset oxygen sensors of the oxygen analyzer, The microprocessor converts digital signals into corresponding oxygen concentration values and displays them on the oxygen analyzer display screen, At the same time, convert the digital signal into a linear standard analog current signal of 0-10mA or 4-20 mA output.

The oxygen analyzer continuously performs system self check during operation, while temperature control, overheating protection, and fault supervision are carried out on the sensor through cables. If a fault occurs, it will be displayed on the analyzer display screen.