Product Description
The SY21 combustible gas sensor adopts a multi-layer thick film manufacturing process. A heater and a metal oxide semiconductor gas-sensitive layer are made on both sides of the Al203 ceramic substrate and encapsulated in a metal shell. The conductivity of the sensor changes when the detected gas exists in the ambient air. The higher the concentration of the gas, the higher the conductivity of the sensor. This change in conductivity can be converted into an output signal corresponding to the gas concentration using a simple circuit.
Sensor Features
This product has good sensitivity to alcohol in a wide concentration range, and has the advantages of low power consumption, fast response recovery, excellent selectivity, simple detection circuit, good stability and long life.
Main Applications
Used for monitoring drunk motor vehicle drivers and other risk workers, and for detecting ethanol vapor in other places .
Technical Specifications Table 1
Product Model
SY21
Product Type
Planar semiconductor gas sensor
Standard package
Metal packaging
Detection gas
Alcohol
Detection concentration
0-500ppm (alcohol)
Loop voltage
Vc
≤ 24V DC
Standard circuit condition
Heating voltage
Vh
2.5V±0.1V AC or DC
Load resistance
R1
4.7K (adjustable)
Heating resistor
Rh
11Ω±2Ω (room temperature)
Gas sensor charact-
eristics under standard test conditions
Heating power consu-
mption
Ph
≤350mW
Sensitive body resistance
RS
5KΩ~50KΩ（in 50ppm alcohol）
Sensitivity
S
RSA/RSG (50PPM)＞8
Concent-
ration slope
a
≤0.6 (R300PPM/R50ppm alcohol)
Temperature, humidity
20℃±2℃; 65%±5%RH
Standard test conditions
Standard test circuit
VC: 2.5V±0.1V;
VH: 2.5V±0.1V
Service life
>5 years
Figure 1: Sensor structure diagram
Basic Circuit
SY21 test circuit
Description: The figure above is the basic test circuit of SY21 sensor. The sensor needs to apply 2 voltages: heater voltage (V₄) and test voltage (Ve). V₄ is used to provide a specific operating temperature for the sensor, which can be a DC power supply or an AC power supply. Va is the voltage on the load resistor (R) connected in series with the sensor. Vc is the test voltage for the load resistor R. A DC power supply is required.
Sensor Characterization
Figure 3: Response recovery curve
In the figure, Rs represents the resistance value of the sensor in different concentrations of gas: R represents the resistance value of the sensor in 0.4mg/L alcohol atmosphere. All tests in the figure are completed under standard test conditions. Note: The figure above is the basic test circuit of the SY21 sensor. The sensor needs to apply 2 voltages: heater voltage (V+) and test voltage (Ve). V+ is used to provide a specific operating temperature for the sensor, and can be powered by DC or AC. Va is the voltage on the load resistor (R) connected in series with the sensor. Vc is the voltage provided for the load resistor R, and a DC power supply must be used.
Figure 4: Sensor linearity curve
In the figure, Rs represents the resistance value at various temperatures/humidities containing 400ppm alcohol; Rso represents the resistance value at 20°C/65%RH containing 400ppmi alcohol.
Long-term stability
Figure 5: Repeatability curve
Note: All tests in the figure were completed under standard test conditions. The horizontal axis is the number of tests, the interval between each two tests is 5 minutes, and the vertical axis is the Va value.
Precautions
1. Situations that must be avoided
1.1 Exposure to volatile silicon compound vapor The sensor should avoid being exposed to silicone adhesives, hair spray, silicone rubber, putty or other places where volatile silicon compounds exist. If the surface of the sensor is adsorbed with silicon compound vapor, the sensitive material of the sensor will be wrapped by silicon dioxide formed by the decomposition of the silicon compound, which will inhibit the sensitivity of the sensor and cannot be restored.
1.2 Highly corrosive environment
1.3 Alkali, alkali metal salts, halogens When the sensor is exposed to high concentrations of corrosive gases (such as HS, SO,, Cl, HC1, etc.), it will not only cause corrosion or damage to the heating material and sensor leads, but also cause irreversible deterioration of the performance of sensitive materials. When the sensor is contaminated by alkali metals, especially salt water spray, or exposed to halogens such as Freon, it will also cause performance deterioration.
1.4 Contact with water Splashing or immersing the sensor in water will cause the sensor’s sensitivity to decrease.
1.5 Freezing water freezing on the surface of the sensor’s sensitive material will cause the sensitive layer to break and lose its sensitive properties.
1.6 Applied voltage is too high
If the voltage applied to the sensor or heater is higher than the specified value, even if the sensor is not physically damaged or destroyed, it may cause damage to the leads and/or heater and cause the sensor’s sensitivity characteristics to deteriorate.
1.7 Voltage added to wrong pin
As shown in Figure 8, the sensor pins 1 and 2 are connected to the heating circuit, and the pins 3 and 4 are connected to the measurement circuit;
Under the premise of heating and measurement, the same power circuit can be shared.
Note: Please pay attention to the protruding mark on the sensor. The two pins next to the mark are heating electrodes.
Figure 6: Pinout diagram (not included in this document)
2. Situations to avoid as much as possible
2.1 Condensation
Under indoor use conditions, slight condensation will have a slight effect on sensor performance. However, if water condenses on the surface of the sensitive layer and remains for a period of time, the sensor characteristics will deteriorate.
2.2 In high concentration gas
Regardless of whether the sensor is powered on or not, placing it in high-concentration gas for a long time will affect the sensor characteristics. For example, spraying lighter gas directly at the sensor will cause great damage to the sensor.
2.3 Long-term storage
If the sensor is stored for a long time without power, its resistance will have a reversible drift, which is related to the storage environment. The sensor should be stored in a sealed bag that does not contain volatile silicon compounds. Before using the sensor that has been stored for a long time, it needs to be powered on for a longer time to stabilize it.
The storage time and corresponding aging time are recommended as follows:
Storage time
Recommended aging time
Under 1 month
No less than 48 hours
1-6 months
No less than 72hours
6 months and above
No less than 168 hours
Table-2
2.4 Long-term exposure to extreme environments
Regardless of whether the sensor is powered or not, prolonged exposure to extreme conditions such as high humidity, high temperature or high pollution will seriously affect sensor performance.
2.5 Vibration
Frequent, excessive vibration can cause the sensor’s internal leads to resonate and break. This type of vibration can be generated during transportation and by using pneumatic screwdrivers/ultrasonic welders on the assembly line.
2.6 Impact
If the sensor is subjected to a strong impact or falls, its lead wires may break.
2.7 Conditions of Use:
2.7.1 Manual welding is the most ideal welding method for sensors. The recommended welding conditions are as follows:
Flux: Rosin flux with minimal chlorine content
Constant temperature soldering iron
Temperature: ≤350℃
Time: no more than 3 seconds
Violation of the above usage conditions will degrade the sensor characteristics.
