Analysis methods and principles for high-purity nitrogen

It uses zirconia; the principle behind it isn’t quite clear – one would need to ask the people in charge of the instruments

Use chromatography to analyze the trace components, and then perform subtraction.

If it is a nitrogen production unit, it should be equipped with a dedicated detector

Measurement principle of zirconia oxygen analyzer 1. Measurement basis: The zirconia oxygen analyzer (also known as zirconia oxygen meter, zirconia analyzer, zirconia oxygen gauge) is primarily used to measure the oxygen concentration in flue gases during combustion processes; it can also be used to measure the oxygen concentration in non-combustion gases. An electrochemical cell with a constant temperature inside the sensor (an oxygen concentration difference cell, also referred to as a zirconium electrode) generates a millivolt potential, which directly reflects the oxygen concentration in the flue gas. The key component of an oxygen sensor is zirconia; porous platinum electrodes are coated on the inner and outer sides of the zirconia element to form an oxygen concentration difference cell. It is located at the top of the sensor. To maintain the battery at its rated operating temperature, a heater is installed in the sensor. The temperature of zirconia is kept constant using the temperature controller in the oxygen analyzer. At the rated temperature, the battery’s output potential is calculated using the following formula (Nernst equation):　　　　　　　 mV = RT / (4F) * ln(P2/P1) + C. Here, P1 represents the oxygen partial pressure of the reference gas (such as air) on the inside of the battery ； P2──Oxygen partial pressure of the gas under test (such as flue gas) on the outside of the battery ； R-----gas constant ； F-----Faraday constant ； T-----Absolute temperature = (273 + t °C) ； C-----Battery constant. The reference gas should be dry, clean, oil-free air (20.60% oxygen content). When the oxygen concentrations on the reference gas side and the gas under test side are different, oxygen ions migrate from the side with a higher concentration to the side with a lower concentration. The battery output responds to the oxygen concentration in the gas being measured in a logarithmic manner. 2 System Composition The zirconia oxygen analyzer consists of an oxygen sensor (also known as an oxygen probe or oxygen detector), an oxygen analyzer (also known as a transmitter, transduction unit, converter, or analyzer), as well as the connection cables between them. 2.1 Oxygen sensor: The sensor unit consists of a metal housing, a measurement battery, a heater, a thermocouple, filter elements, and cable terminals. The battery cell consists of 3 layers: platinum (electrode) – zirconia (electrolyte) – platinum (electrode). Platinum electrodes are porous. Flue gas passes through a filter, or calibration gas passes through a conductive tube to enter the side of the measurement cell where the gas to be measured is located; the other side contains reference air with 20.60% oxygen. When two gases with different oxygen concentrations are applied to the measuring cell, a potential that follows a logarithmic pattern is generated (the greater the difference in oxygen concentration on the two sides, the larger the potential signal). The millivolt signal is converted by the oxygen analyzer into a standard current of 0–10 mA or 4–20 mA. This current is output from the oxygen analyzer terminals. The operating temperature of the battery is set at a constant level above 650°C. To maintain this constant operating temperature, a K-type thermocouple is used to measure it, and the heating voltage of the heater is adjusted by the temperature controller built into the oxygen analyzer. When measuring flue gas temperatures above 700°C, the heater and the temperature-sensing thermocouple are omitted from the sensor configuration. 2.2 Oxygen Analyzer: To maintain the operating temperature of the cell at 700°C, the oxygen analyzer receives the temperature mV signal generated by the K-type thermocouple in the sensor, compares it with the preset temperature (in millivolts) set by the microprocessor, and uses this comparison to control the cell temperature. The oxygen analyzer uses the ambient temperature as the reference point for the thermocouple cold junction. The oxygen analyzer amplifies the oxygen mV signal input from the oxygen sensor, and then converts the amplified voltage signal into a digital signal using an A/D converter. Based on the characteristic curve of the battery measured by the oxygen sensor, which has been pre-calibrated or preset by the oxygen analyzer, the microprocessor converts the digital signal into the corresponding oxygen concentration value, which is displayed on the oxygen analyzer’s screen. At the same time, it converts the digital signal into a linear standard analog current signal of 0–10 mA or 4–20 mA for output. The oxygen analyzer performs continuous system self-checks during operation, while temperature control, overheat protection, and fault monitoring are provided for the sensor via cables. In the event of a fault, it is displayed on the analyzer’s screen