Sensor is a kind of detection device, which can feel the measured information and transform the sensed information into electrical signals or other required forms of information output according to a certain law, so as to meet the requirements of information transmission, processing, storage, display, recording and control(Definition of sensor,Wikipedia). Sensors have many applications in daily life. For example, the brightness of the lamp can be controlled by touching the edge of the lamp, which applies tactile sensors. With the continuous improvement of science and technology, sensors are applied to many industries, such as machinery, aerospace, medicine, robot and so on. Today, I will introduce the application of sensors in carbon monoxide detector.
Development of sensors for carbon monoxide alarm
The early carbon monoxide sensor was a piece of white cushion. When the concentration of carbon monoxide gas in the air reached a certain value, the cushion would fade to brown or black. This carbon monoxide sensor is very cheap, but the disadvantage is that it only provides visual alarm. With the increasing number of people poisoned by carbon monoxide in the 1900s, the carbon monoxide sensor gradually changed from visual detector to auditory alarm, which is called CO alarm. Nowadays, the carbon monoxide alarm in society can not only monitor the value of carbon monoxide concentration in the air in real time, but also remind people to evacuate in time through the alarm sound before the danger is coming.
Types of carbon monoxide sensor
There are four kinds of carbon monoxide sensors. Their cost, measurement accuracy and response speed are different. Therefore, it is very important to select the right carbon monoxide sensor to design a suitable CO alarm.
Opto-Chemical CO Sensors:
Opto-Chemical sensor can also be called spot sensor. Its principle is that carbon monoxide is easy to combine with oxygen to oxidize itself and generate carbon dioxide. Carbon dioxide reacts with the chemical salt on the spot sensor to make the spot turn black.
The cost is very low, about $4;
First of all, it only provides a visual alarm, not an alarm sound. You need to constantly observe the change of color to judge whether there is high concentration of carbon monoxide around; Secondly, the sensor needs to be replaced every 3 to 6 months, which means that the cost in a few years is almost the same as that of buying a normal carbon monoxide detector.
Biomimetic CO Sensors:
The working principle of biomimetic sensor is that it simulates the change of carbon monoxide concentration with time after entering the human body. If the concentration exceeds 100ppm, the alarm will be triggered. If the concentration is low, the alarm will not be triggered. This is to prevent some false alarms of low concentration. For example, carbon monoxide produced by smoking will not cause the alarm. The biomimetic sensor uses cyclodextrin, chromophore and many metal salts. When there is a certain concentration of carbon monoxide around, the color of the sensor will change, which can be observed directly or connected to an infrared photon source (such as IR LED), and then monitored by photodiode.
Biomimetic sensing technology is a technology without false alarm after testing. Its high-end technology and high accuracy are favored by large facilities such as hospitals, hotels and apartments;
The R & D cost is too high and it is difficult to popularize in families; The service life of biomimetic sensor is 6 years, and battery life of conventional alkaline is 2 to 3 years, therefore, the battery needs to be replaced 2 to 3 times; biomimetic facilities need to use air freshener, alcohol and other disinfection, and the artificial disinfection cost is high, which is to avoid greater false alarm cost due to false alarm.
Electrochemical CO Sensors:
This sensor is a kind of fuel cell. Its working principle is that when it comes into contact with carbon monoxide, it will generate corresponding current to reflect the concentration of carbon monoxide. The biggest feature of electrochemical gas sensor is that the current is completely proportional to the concentration of carbon monoxide, and the output signal has a good linear relationship with the gas concentration, so the signal processing and display are very convenient. Another feature is that due to the normal temperature reaction, there is no need for a heater, so the voltage between the electrodes can use a dry battery, there is no need for mains power, and it is easy to carry.
The electrochemical sensor has a highly accurate and linear output of carbon monoxide concentration, and the power required for operation at room temperature is also very low, so it has a long service life, which makes most carbon monoxide alarms use electrochemical sensors;
There are great differences in price and performance among brands.
Semiconductor CO Sensors:
This kind of sensor is also called resistance sensor. Its working principle is to detect the composition or concentration of gas by using the change of resistance value when semiconductor contacts gas. The sensitivity of semiconductor sensor to gas depends on the temperature at which the sensitive element is heated. For carbon monoxide, the temperature at which the sensitive element is heated is less than 100 degrees. This temperature is much lower than the detection temperature of other gases(such as methane and hydrogen). However, the test response speed of carbon monoxide at low temperature is slow, and its sensitivity is easily affected by water vapor in the atmosphere. Therefore, the sensitive element is heated alternately from high temperature to low temperature.During high temperature, water vapor and other mixed gases are removed from the surface of sensitive components; During low temperature, the sensitive element can detect carbon monoxide well, and has excellent sensitivity and reproducibility.
It can detect many gases, such as methane, ethane, propane, alcohol, etc; Long service life and low cost. Suitable for civil gas detection, high-quality semiconductor sensors can also meet the needs of industrial detection;
Poor stability due to preheating, high power consumption, not suitable for battery power supply, vulnerable to temperature, humidity, air flow, etc. Poor anti cross interference ability and high false alarm rate.
Which type of carbon monoxide sensor can be better combined with EN50291
In the civil field, carbon monoxide sensor is widely used in carbon monoxide alarm and gas water heater to detect the concentration of carbon monoxide in real time and prevent carbon monoxide poisoning.
If you want to sell carbon monoxide detectors in the European market, you must pass the CE standard. The latest CE standard is EN50291:2018, which lists the requirements for the appearance, battery and various coefficients of carbon monoxide detector in detail. For example, the requirements for alarm value: when the concentration of CO in the air reaches 50ppm or above, the alarm that can be muted is triggered; When the concentration reaches 300ppm, a continuous alarm is triggered and cannot be muted, which makes the accuracy of the carbon monoxide sensor very high. At present, the product trend of carbon monoxide detector is mainly durability and anti false alarm, throughout the above four types of sensors, electrochemical sensors are better. Firstly, it does not need power supply, which makes the appearance of carbon monoxide detector very small and easy to carry; Secondly, its output signal has a good linear relationship with gas concentration, so the signal processing and display are very convenient and the measurement accuracy is high; Finally, the battery has a long service life. Therefore, the electrochemical sensor is very consistent with EN50291 standard.