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Electrochemical sensor

Instrument for measuring various chemical components

The earliest electrochemical sensors can be traced back to the 1950s, when they were used for oxygen monitoring. By the mid-1980s, small electrochemical sensors began to be used to detect a variety of differences within the PEL range Toxic gas , and showed good susceptibility And selectivity. To protect Personal safety For the sake of, various electrochemical sensors are widely used in many static and Mobile applications Occasion.

Chinese name: Electrochemical sensor
Foreign name: Electrochemical sensors

Traceability: 1950s
operation mode: Working by electrical signal
Composition: Sensor electrode, counter electrode, thin electrolytic layer

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seven Summary

working principle

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The electrochemical sensor reacts with the measured gas and generates electrical signal To work. A typical electrochemical sensor consists of a sensing electrode (or Working electrode ）And a reverse electrode, and is separated by a thin electrolytic layer.

The gas first passes through the tiny capillary type Opening It reacts with the sensor, then the hydrophobic barrier layer, and finally reaches the electrode surface. This method allows a proper amount of gas to react with the sensing electrode to form a sufficient electrical signal And prevent the electrolyte from leaking out of the sensor.

The gas diffused through the barrier reacts with the sensing electrode, and the sensing electrode can adopt oxidation mechanism or reduction mechanism. These reactions are catalyzed by electrode materials designed for the gas being measured.

Connected by electrodes resistor The current proportional to the concentration of the measured gas will flow between the positive pole and the negative pole. Measure the current to determine gas concentration 。 Because of this process China Council To generate current, electrochemical sensors are often called current gas sensors or Micro fuel cell 。

In practice, due to the continuous electrochemical reaction on the electrode surface Electrode potential It cannot be kept constant. After a long period of time, it will lead to sensor performance degradation. In order to improve the performance of sensors, reference electrodes are introduced.

The reference electrode is installed in the electrolyte and adjacent to the sensing electrode. The fixed stable constant potential acts on the sensing electrode. The reference electrode can maintain this fixed voltage value on the sensing electrode. There is no current flow between reference electrodes. The gas molecule reacts with the sensing electrode, and the counter electrode is measured at the same time, measurement result It is usually directly related to the gas concentration. The voltage value applied to the sensing electrode can make the sensor aim at the target gas.

form

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The electrochemical sensor consists of the following main elements:

A. Breathable membrane (also called hydrophobic membrane): the breathable membrane is used to cover the sensing (catalytic) electrode, and in some cases to control the molecular weight of the gas reaching the electrode surface. Such barriers are usually low porosity Teflon Film. Such sensors are called coated sensors. Alternatively, it can be covered with high porosity Teflon film, and Capillaries Control the molecular weight of gas reaching the electrode surface. These sensors are called capillary type sensors. In addition to providing mechanical protection for the sensor, the film also has the function of filtering unwanted particles. In order to deliver the correct gas molecular weight, it is necessary to select the correct pore size of the film and capillary. The pore size shall allow a sufficient number of gas molecules to reach the sensing electrode. The pore size should also prevent liquid electrolyte Leakage or rapid drying.

B. Electrode: It is important to select electrode material. The electrode material shall be a catalytic material capable of performing semi electrolytic reaction for a long time. Generally, electrodes are made of precious metals, such as platinum or gold, which react effectively with gas molecules after catalysis. Depending on the design of the sensor, the three electrodes can be made of different materials to complete the electrolytic reaction.

C. electrolyte : The electrolyte must be able to conduct electrolytic reaction and effectively transfer ionic charge to the electrode. It must also Reference electrode It forms a stable reference potential and is compatible with the materials used in the sensor. If the electrolyte evaporates too quickly, the sensor signal will be weakened.

D. Filter: sometimes a scrubbing filter is installed in front of the sensor to filter out unwanted gases. The selection range of filters is limited, and each filter has different efficiency degrees. Most commonly used filter materials are Activated carbon 。 Activated carbon can filter out most chemicals material , but cannot be filtered carbon monoxide 。 By choosing the right filter material, the electrochemical sensor can have higher selectivity for its target gas.

There are many manufacturing methods for electrochemical sensors, which ultimately depend on the gas to be detected and the manufacturer. However, the main characteristics of sensors are very similar in nature. The following describes some common characteristics of electrochemical sensors:

1. On the three electrode sensor, there is usually a jumper To connect Working electrode And reference electrode. If removed during storage, the sensor will take a long time to remain stable and ready for use. Some sensors require the presence of bias In this case, the sensor is shipped with a 9-volt battery powered electronic circuit 。 It takes 30 minutes to 24 hours for the sensor to stabilize, and it takes three weeks to continue to stabilize.

2. Majority Toxic gas The sensor requires a small amount of oxygen to remain functional. There is one on the back of the sensor vent To achieve this goal. It is recommended to recheck with the manufacturer when using non oxygen background gas.

3. The electrolyte of the battery in the sensor is a kind of water solvent, which is isolated by a hydrophobic barrier, which can prevent the leakage of water solvent. However, like other gas molecules, Water vapor It can pass through the hydrophobic barrier. Under high humidity conditions, prolonged exposure may lead to excessive moisture accumulation and leakage. In low humidity conditions, the sensor may dry out. Sensors designed to monitor high concentrations of gases have low Porosity Barrier to limit the molecular weight of gas passing through, so it is not affected by humidity. Like the sensor used to monitor low concentration gas, this sensor has a high porosity barrier and allows gas molecules free flow 。

application

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1、 humidity sensor Humidity is an important indicator of the air environment Heat of evaporation There is a close relationship between them. When the temperature is high and the humidity is high, the human body feels stuffy due to the difficulty of water evaporation. When the temperature is low and the humidity is high, the heat dissipation process of the human body is intense, which is easy to cause cold and frostbite 。 The most suitable temperature for human body is 18~22 ℃, and the relative humidity is 35%~65 %RH 。 In the environment and monitoring of hygiene Medium, often used for wet bulb Temperature and humidity meter , manual wet thermometer, ventilation wet thermometer and other instruments air humidity 。 In recent years, a large number of literatures reported that air humidity was measured by sensors. Coating for determining relative humidity Piezoelectric quartz crystal With sensor, through Photolithography And chemistry Etching technology Made small quartz Capture Transistor , 10MHZ quartz cut at AT crystal There are 4 substances coated on it, which have high quality for humidity susceptibility . The crystal is Oscillating circuit In Resonator The frequency varies with the mass. Select an appropriate coating. The sensor can be used to measure the relative humidity of different gases. The sensitivity, response linearity, response time, selectivity Hysteresis phenomenon And service life depend on the nature of coating chemicals.

2. Nitrogen oxide sensor Nitrogen oxide It is composed of various oxides of nitrogen Gas mixture The general name of, usually expressed as NOX. In nitrogen oxide, different forms of nitrogen oxide chemical stability Different, what is often windy in the air is chemical property Relatively stable nitric oxide and Nitrogen dioxide Their hygienic significance is more important than other forms of nitrogen oxides. stay environmental analysis Nitrogen oxide generally refers to nitric oxide and nitrogen dioxide. Monitoring of nitrogen oxides in China standard method yes Naphthalenediamine hydrochloride colorimetry , the sensitivity of the method is 0.25ug/5ml, and the method conversion coefficient is determined by the composition of the absorption solution, the concentration of nitrogen dioxide Gas production rate , absorption tube structure, coexisting ions, temperature and other factors are not completely unified. Sensor measurement is a new method developed in recent years. It is reported in the literature that microelectronics Integrated circuit and Chemical activity e-beam evaporation Combined with phthalein bronze film, a new type of gas sensitivity is obtained Microsensor , selective detection of mg/m3 level nitrogen dioxide and thionyl chloride Methylphosphonic acid Salt (DIMP).

3. Hydrogen sulfide gas sensor hydrogen sulfide It is colorless and has a special rotten egg odor combustible gas It is irritant and suffocating, which is harmful to human body. Mostly used colorimetry and Gas chromatography Determination of hydrogen sulfide in air. For those whose content is often as low as mg/m3 Air pollutants Measurement is a major application of gas sensors, but in a short period of time Semiconductor gas sensor It can not meet the sensitivity and selectivity requirements for monitoring some polluted gases. The silver doped thin film sensor array is composed of four sensors. The signals of the universal analytical device based on library titration and the semiconductor gas sensor array are recorded simultaneously sulfur dioxide And the concentration of hydrogen sulfide. The practice shows that the silver doped film sensor with constant temperature at 150 ℃ is effective for monitoring the content of hydrogen sulfide in urban air.

4、 Sulfur dioxide sensor Sulfur dioxide is one of the main substances that pollute the air. It is a regular work to detect sulfur dioxide in the air. Use sensors to monitor sulfur dioxide. From reducing detection time to reducing detection limit , all show great advantages. Using solid polymer as Ion exchange membrane , one side of the membrane contains Counter electrode and Reference electrode Inside of electrolyte , insert on the other side Platinum electrode , forming a sulfur dioxide sensor. The sensor is installed in the flow cell and oxidizes sulfur dioxide at 0.65V. Indicate the amount of sulfur dioxide. This Sensing device High current sensitivity. Short response time, good stability, Background noise Low, Linear range Up to 0.2mmol/L, the detection limit is 8 * 10-6mmol/L, Signal-to-noise ratio Is 3. The sensor can not only measure sulfur dioxide in the air, but also measure low conductivity Sulfur dioxide in liquid. Organic modification silicate The gas sensing coating of thin-film sulfur dioxide gas sensor is made by sol process and spin technology, which has good reproducibility and reversibility The response time is less than 20s, the interaction with other gases is small, and the influence of temperature and humidity is small.

Pressure and temperature

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The electrochemical sensor is little affected by the pressure change. However, due to the differential pressure The sensor may be damaged, so the whole sensor must maintain the same pressure. The electrochemical sensor is also very sensitive to temperature, so it usually adopts internal temperature compensation 。 But it's best to keep Standard temperature 。

Generally speaking, when the temperature is higher than 25 ° C, the sensor reading is higher; Below 25 ° C, the reading is low. The temperature effect is usually centigrade 0.5% to 1.0% depending on manufacturer and sensor type.

selectivity

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Electrochemical sensors usually have high selectivity for their target gases. The degree of selectivity depends on the type of sensor, the target gas, and the gas concentration 。 The best electrochemical sensor is the one that detects oxygen. It has good selectivity, reliability and long life expectancy. Other electrochemical sensors are vulnerable to interference from other gases. Interference data is calculated using relatively low gas concentrations. In practical applications, the interference concentration may be high, which may lead to wrong readings or False alarm 。

life expectancy

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The expected life of the electrochemical sensor depends on several factors, including the gas to be detected and the use of the sensor environment condition 。 Generally speaking, the specified life expectancy is one to three years. In practice, the life expectancy mainly depends on the total amount of gas exposed in the use of the sensor and other environmental conditions, such as temperature, pressure and humidity.

Summary

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Electrochemical sensors have low requirements for working power supply. In fact, in Gas monitoring Of all the sensor types available, they have the lowest power consumption. Therefore, this sensor is widely used in mobile instruments containing multiple sensors. They are Finite space Sensors used most in applications.

The expected life of the sensor is predicted by its manufacturer based on conditions they consider normal. However, the expected life of a sensor depends largely on environmental pollution, temperature and the humidity to which it is exposed.

Typical electrochemical sensor specifications

Sensor type: 2 or 3 electrodes, usually 3 electrodes

Scope: OK Permissible exposure limit 2 of -10 times

Life expectancy: normally 12 to 24 months, depending on the manufacturer and sensor

Temperature range: – 40 ° C to+45 ° C

relative humidity : 15-95%, no condensation

response time : less than 50 seconds

Long term offset: move down 2% every month

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