Gas analysis instrument principle and types

What is a gas analyzer?

Gas analyzers are process analyzers that measure gas components. In many production processes, especially those with chemical reactions, automatic control based on physical parameters such as temperature, pressure, and flow is often not enough. Due to the wide variety of gases being analyzed and the variety of analysis principles, there are many types of gas analysis instruments. Commonly used are thermal conductivity gas analyzers, electrochemical gas analyzers and infrared absorption analyzers.

The principle of biogas analyzer

The gas sensor is mainly used to detect the type of gas existing in the environment. The gas sensor is a sensor used to detect the composition and content of the gas. It is generally believed that the definition of a gas sensor is based on the classification of the detection target, that is to say, any sensor used to detect the gas composition and concentration is called a gas sensor, no matter whether it uses a physical method or a chemical method. For example, sensors that detect gas flow are not considered gas sensors, but thermal conductivity gas analyzers are important gas sensors, although they sometimes use roughly the same detection principle.

Types of gas analyzers

1. Thermal conductivity

A physical gas analysis instrument, which calculates the content of certain components by measuring the thermal conductivity of mixed gases according to the principle of different thermal conductivity of different gases; this kind of analytical instrument is simple and reliable, and is suitable for a variety of gases . It is a basic analytical instrument. However, it is difficult to directly measure the thermal conductivity of the gas, so in fact, the change in the thermal conductivity of the gas is often converted into a change in resistance, and then measured by a bridge.

The thermal elements of the thermal conductivity gas analyzer mainly include semiconductor sensitive elements and metal resistance wires. The semiconductor sensitive element has small volume, small thermal inertia, and large temperature coefficient of resistance, so it has high sensitivity and small time lag. When the semiconductor metal oxide sensitive element adsorbs the measured gas, the electrical conductivity and thermal conductivity will change, and the heat dissipation state of the element will also change. The temperature change of the element changes the resistance of the platinum coil, and the bridge generates an unbalanced voltage output, which can detect the concentration of the gas.

Thermal conductivity gas analyzers have a wide range of applications. In addition to the usual analysis of hydrogen, ammonia, carbon dioxide, sulfur dioxide and low concentrations of combustible gases, it can also be used as a detector in chromatographic analyzers to analyze other components.

2. Thermomagnetic

The principle is to use the physical property of the extremely high magnetic susceptibility of oxygen in the flue gas components to determine the oxygen content in the flue gas; oxygen is a paramagnetic gas (the gas that can be attracted by a magnetic field is called a paramagnetic gas). It is attracted in the uneven magnetic field and flows to the place where there is a strong magnetic field. There is an electric heating wire here, which increases the temperature of oxygen and reduces the magnetic susceptibility, thereby reducing the attractive force of the magnetic field, pushing the unheated oxygen molecules with higher magnetic susceptibility to release the magnetic field, producing "thermomagnetic convection" or "magnetic wind" Phenomenon.

Under a certain gas sample pressure, temperature and flow rate, the oxygen content in the gas sample can be measured by measuring the size of the magnetic wind. Since the thermal element (platinum wire) acts as both the two bridge arm resistors of the unbalanced bridge and the heating resistance wire, a temperature gradient is generated under the action of the magnetic wind, that is, the temperature of the intake side bridge arm is lower than that of the exhaust side bridge. . arm temperature. The unbalanced bridge will output the corresponding voltage value according to the oxygen content in the gas sample. The thermal magnetic oxygen analyzer has a simple structure and is easy to manufacture and adjust.

3. Electrochemical

A chemical type. It measures the gas composition according to the change of the ion amount or the current change caused by the chemical reaction. In order to improve the selectivity, prevent the surface of the measuring electrode from contamination and maintain the performance of the electrolyte, the diaphragm structure is generally used. There are two types of electrochemical analyzers commonly used: constant potential electrolysis and galvanic cell. The working principle of the constant potential electrolytic analyzer is to apply a specific potential on the electrode, and the gas under test will produce electrolysis on the surface of the electrode. The meter has the ability to selectively identify the gas being measured.

The Galvani cell analyzer electrolyzes the gas to be measured that diffuses into the electrolyte through the diaphragm, and measures the resulting electrolytic current to determine the concentration of the gas to be measured. The selectivity to gases with different electrolysis potentials is achieved by changing the internal voltage of the electrode surface by selecting different electrode materials and electrolytes.

4. Infrared absorption type

It is an analytical instrument that works according to the selective absorption of different wavelengths of infrared rays by different components of the gas. Measuring the absorption spectrum can identify the type of gas; measuring the absorption intensity can determine the concentration of the measured gas. The infrared analyzer has a wide range of uses. It can not only analyze the gas composition, but also the solution composition. It has high sensitivity, fast response, continuous online indication, and can also form a regulating system.

The detection part commonly used in industry consists of two parallel optical systems with the same structure. One is the measurement room and the other is the reference room. The two chambers open and close the optical path simultaneously or alternately within a certain period through the optical cutting plate. After the gas to be measured is introduced into the measurement chamber, the light of the unique wavelength of the gas to be measured is absorbed, thereby reducing the luminous flux entering the infrared receiving chamber through the optical path of the measuring chamber. The higher the gas concentration, the less the luminous flux entering the infrared receiving chamber; The luminous flux of the chamber is constant, and the luminous flux entering the infrared receiving chamber is also constant.

Therefore, the higher the concentration of the gas being measured, the greater the difference in the luminous flux passing through the measuring chamber and the reference chamber. This difference in luminous flux is projected onto the infrared receiving airbag with the amplitude of a certain periodic vibration. The receiving gas chamber is divided into two halves by a metal film with a thickness of several microns, and the gas of the measured component is sealed in the chamber, which can absorb all incident infrared rays in the absorption wavelength range, so that the pulsating light flux becomes a periodic change of temperature, which can be adjusted according to the gas. The equation of state is converted into a pressure change, which is then detected by a capacitive sensor, which is amplified to indicate the concentration of the gas being measured.

In addition to capacitive sensors, quantum infrared sensors that directly detect infrared rays can also be used, infrared interference filters are used for wavelength selection, and tunable lasers are used as light sources to form a new all-solid-state infrared gas analyzer. The analyzer only needs one The light source, a measuring chamber and an infrared sensor can complete the measurement of gas concentration. In addition, if multiple filters of different wavelengths are used, the concentration of each gas in a multi-component gas can be measured simultaneously. Similar to the principle of infrared analyzers, there are also ultraviolet analyzers, photoelectric colorimetric analyzers, etc., which are also widely used in industry.

5. Non-dispersive infrared analysis

The non-dispersive infrared gas analyzer adopts the combination of two non-dispersive spectral analysis technologies, narrow-band filter and gas filter correlation method, which is suitable for different measurement ranges of gases. The filter correlation method can measure low-range gases and effectively avoid cross-interference. This unique technology can eliminate the cross-interference of weakly absorbing gases such as CO and high-absorbing gas CO2. The infrared light emitted by the heat source is filtered by a rotating filter, causing a series of pulsed signals to pass directly through the cell containing the sample gas. As the filter wheel rotates, the solid-state detector reflects the signal changes and amplifies the signal for output and display.