Gas Analyzers Principle and Different Types

Principle of Gas Analyzer

Gas analyzers are process analytical instruments that measure gas composition. In many production processes, especially in production processes 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 analyzed gases and the variety of analysis principles, there are many types of gas analyzers. Commonly used are thermal conductivity gas analyzers, electrochemical gas analyzers and infrared absorption analyzers.

Gas sensors are mainly used to detect the types of gases existing in the environment. Gas sensors are sensors used to detect the composition and content of gases. It is generally believed that the definition of gas sensors is based on the classification of detection targets, that is to say, any sensor used to detect gas composition and concentration is called a gas sensor, regardless of whether it uses physical methods or chemical methods. 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. According to the principle that different gases have different thermal conductivity, it calculates the content of some components by measuring the thermal conductivity of the mixed gas. This kind of analytical instrument is simple and reliable, applicable to many types of gases, and is a basic analytical instrument. However, it is difficult to directly measure the thermal conductivity of gas, so in fact, the change of thermal conductivity of gas is often converted into the change of resistance, and then measured with a bridge. The heat sensitive 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. The bead-shaped metal oxide is sintered on the platinum coil as a sensitive element, and then the same platinum coil with equal internal resistance and calorific value is wound with a material that does not react to gas as a compensation element. These two components form a bridge circuit as two arms, which is the measurement circuit. When the semiconductor metal oxide sensitive element absorbs the measured gas, the electrical conductivity and thermal conductivity will change, and the heat dissipation state of the element will also change accordingly. The temperature change of the element changes the resistance of the platinum coil, and the bridge then has 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-concentration flammable gases, they can also be used as detectors in chromatographic analyzers to analyze other components.

2. Thermal magnetic type

The principle is to use the physical property of the particularly high magnetic susceptibility of oxygen in the flue gas components to measure the oxygen content in the flue gas. Oxygen is a paramagnetic gas (a gas that can be attracted by a magnetic field is called a paramagnetic gas), and it is attracted in an inhomogeneous magnetic field and flows to a place with a stronger magnetic field. There is a heating wire here, so that the temperature of oxygen here rises and the magnetic susceptibility decreases, so the magnetic attraction force decreases, and the unheated oxygen molecules with higher magnetic susceptibility behind push and discharge the magnetic field, resulting in The phenomenon of "thermomagnetic convection" or "magnetic wind". 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 heat-sensitive element (platinum wire) is used not only as the resistance of the two arms of the unbalanced bridge, but also as the heating resistance wire, a temperature gradient appears under the action of the magnetic wind, that is, the temperature of the bridge arm on the inlet side is lower than that on the outlet side bridge. arm temperature. The unbalanced bridge will output a corresponding voltage value according to the oxygen content in the gas sample.

The thermomagnetic oxygen analyzer has the advantages of simple structure, easy manufacture and adjustment.

3. Electrochemical formula

A chemical gas analysis instrument. It measures gas composition based on changes in the amount of ions caused by chemical reactions or changes in electrical current. In order to improve the selectivity, prevent the surface of the measuring electrode from being stained and maintain the performance of the electrolyte, a diaphragm structure is generally used. Commonly used electrochemical analyzers include constant potential electrolysis and galvanic battery. The working principle of the constant potential electrolytic analyzer is to apply a specific potential on the electrode, and the measured gas will produce electrolysis on the surface of the electrode. As long as the potential applied to the electrode is measured, the specific electrolytic potential of the measured gas can be determined, so that The instrument has the ability to select and identify the gas to be measured. The Galvanic battery analyzer electrolyzes the measured gas diffused into the electrolyte through the diaphragm, and measures the formed electrolysis current to determine the concentration of the measured gas. The selectivity to gases with different electrolysis potentials is achieved by changing the internal voltage on the electrode surface by selecting different electrode materials and electrolytes.

4. Infrared absorption type

An analytical instrument that works according to the selective absorption of infrared rays of different wavelengths by gases of different components. Measuring this absorption spectrum can identify the type of gas; measuring the absorption intensity can determine the concentration of the gas being measured. The infrared analyzer has a wide range of applications. It can not only analyze gas components, but also analyze solution components. It has high sensitivity, rapid response, continuous online indication, and can also form an adjustment system. The detection part of the infrared gas analyzer commonly used in industry is composed 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 light paths simultaneously or alternately at a certain period through the light cutting plate. After the gas to be measured is introduced into the measurement chamber, the light with the specific wavelength of the gas to be measured is absorbed, thereby reducing the luminous flux that passes through the optical path of the measurement chamber and enters the infrared receiving gas chamber. The higher the gas concentration, the less the luminous flux entering the infrared receiving gas chamber; while the luminous flux passing through the reference chamber is constant, the luminous flux entering the infrared receiving gas chamber is also constant. Therefore, the higher the measured gas concentration, the greater the difference in luminous flux that passes through the measurement chamber and the reference chamber. This luminous flux difference is projected to the infrared receiving air chamber with a certain periodic vibration amplitude. The receiving gas chamber is divided into two halves by a metal film with a thickness of several microns. The gas of the measured component with a relatively high concentration is sealed in the chamber, which can absorb all the incoming infrared rays in the absorption wavelength range, so that the pulsating luminous flux becomes The periodic change of temperature can be converted into a change of pressure according to the gaseous state equation, and then detected by a capacitive sensor, which indicates the concentration of the gas to be measured after amplification. In addition to capacitive sensors, quantum infrared sensors that directly detect infrared rays can also be used, and infrared interference filters are used for wavelength selection and adjustable lasers are used as light sources to form a new all-solid-state infrared gas analyzer. This analyzer can complete the measurement of gas concentration with only one light source, one measurement chamber and one infrared sensor. In addition, if a filter disc equipped with multiple different wavelengths is used, the concentration of each gas in the multi-component gas can be measured separately at the same time.

Similar to the principle of infrared analyzers, there are ultraviolet analyzers, photoelectric colorimetric analyzers, etc., which are also widely used in industry.