Measurement principle of zirconia oxygen analyzer
1 Measurement Fundamentals
Zirconia oxygen analyzer (also known as zirconia oxygen analyzer, zirconia analyzer, zirconia oxygen meter, zirconia oxygen meter) is mainly used to measure the oxygen concentration of flue gas in the combustion process, and it is also applicable to Measurement of oxygen concentration in non-combustion gases. An electrochemical cell (oxygen concentration cell, also referred to as a zirconium head) with a constant temperature in the sensor generates a millivolt potential, which directly reflects the oxygen concentration in the flue gas.
The key component of the oxygen sensor is zirconia, and the oxygen concentration difference battery is made by coating the inner and outer sides of the zirconia element with porous platinum electrodes. It's on top of the sensor. To keep the battery at rated operating temperature, a heater is placed in the sensor. The temperature of the zirconia is kept constant with a temperature controller inside the oxygen analyzer.
At the rated temperature, the battery output potential is calculated using the following equation (Nernst equation):
RT P1
mV=------Ln--- +C
4F P2
In the formula, P1——the oxygen partial pressure of the reference gas (such as air) inside the battery;
P2——The oxygen partial pressure of the gas to be measured (such as flue gas) outside the battery;
R-----gas constant;
F-----Faraday constant;
T-----absolute temperature=( 273 + t ℃);
C-----battery constant.
The reference gas should be dry, clean, oil-free air (20.60% oxygen). When the reference gas side and the measured gas side have different oxygen concentrations, oxygen ions migrate from the high side to the low side. The output of the battery reflects the oxygen concentration in the measured gas in a logarithmic law.
2 System Composition
The composition of zirconia oxygen analyzer is composed of oxygen sensor (also known as oxygen probe, oxygen detector), oxygen analyzer (also known as transmitter, transmission unit, converter, analyzer) and the connecting cable between them etc. composition.
2.1 Oxygen sensor
The sensor device consists of a metal casing, a measuring battery, a heater, a thermocouple, a filter element and a cable terminal. The measuring cell body is divided into three layers: platinum (electrode)─zirconia (electrolyte)─platinum (electrode). Platinum electrodes are porous. The flue gas passes through the filter or the calibration gas through the conduction pipe and enters the measuring cell on one side of the measured gas, while the other side is the reference air (20.60% oxygen content).
When two gases with different oxygen concentrations act on the measuring cell, a logarithmic potential is generated (the greater the oxygen concentration difference between the two sides, the greater the potential signal). The millivolt signal is converted into 0-10mA or 4-20mA standard current by the oxygen analyzer. This current is output from the oxygen analyzer terminal block.
The working temperature of the measuring battery is set to a constant temperature higher than 650°C. In order to keep the working temperature constant, 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 in the oxygen analyzer.
When the measured flue gas temperature is higher than 700 ℃, the heater and temperature measuring thermocouple are omitted from the sensor composition.
2.2 Oxygen analyzer
In order to make the working temperature of the measuring battery reach 700 ℃, the oxygen analyzer receives the temperature mV signal output by the K-type thermocouple in the sensor, and compares it with the preset temperature (millivolt) of the microprocessor, thereby controlling the battery temperature. The oxygen analyzer uses the ambient temperature as the thermocouple cold junction comparison point.
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. According to the characteristic curve of the oxygen sensor pre-calibrated or preset by the oxygen analyzer, the microprocessor converts the digital signal into the corresponding oxygen concentration value and displays it on the display screen of the oxygen analyzer. At the same time, the digital signal is converted into a linear standard Analog current signal 0-10mA or 4-20 mA output.
The oxygen analyzer continuously performs a system self-check during operation, while the sensor is temperature controlled, overheated, and fault monitored through the cable. If a fault occurs, the fault is displayed on the analyzer display.
