Product Description
The gas-sensitive material used in the SJ21 gas sensor is tin dioxide (SnO₂), which has low conductivity in clean air. When there is combustible gas in the environment where the sensor is located, the conductivity of the sensor increases with the increase of the concentration of combustible gas in the air. A simple circuit can be used to convert the change in conductivity into an output signal corresponding to the gas concentration.
The SJ21 gas sensor has high sensitivity to butane, propane and methane, and can take both methane and propane into consideration. This sensor can detect a variety of combustible gases, especially liquefied gas (propane). It is a low-cost sensor suitable for a variety of applications.
Sensor Features
This product has good sensitivity to combustible gases (especially propane) in a wide concentration range and has the advantages of long life, low cost and simple driving circuit.
Main Applications
It is widely used in household gas leak alarms, industrial combustible gas alarms and portable gas detectors.
Technical Specifications Table 1
Product Model
SJ21
Product Type
Semiconductor gas sensor
Packaging Standards
Bakelite, metal cover
Detection gas
Liquefied gas, methane
Detection concentration
0-1000ppm (methane, propane)
Loop voltage
Vc
5.0V±0.1V DC
Standard circuit conditions
Heating voltage
Vh
5.0V±0.1V AC or DC
Load resistance
R
Adjustable
Heating resistor
Rh
55Ω±15Ω (room temperature)
Gas sensor charact-
eristics under standard test conditions
Heating power con-
sumption
Ph
≤450mW
Sensitivity
S
Rs(in air)/Rs(in 2000ppm C 2 H 8 ) ≥5
Output voltage
Vs
0.5-3.5V (in 2000ppm C 2 H 8 )
Concentr-
ation slope
α
≤0.6 (R3000ppm/R1 C 2 H 8 )
Temperature, humidity
20℃±2℃; 55%±5% RH
Standard test conditions
Standard test circuit
Vc:5.0V±0.1V, VH:5.0V±0.1V
Warm-up time
Not less than 48 hours
Oxygen content
21% (not less than 18%, (oxygen concentration will affect the initial value, sensitivity and repeatability of the sensor. Please consult if you use it under low oxygen concentration)
Service life
5 years
Note: Output voltage (Vs) refers to VRL in the test atmosphere.
Figure 1: Sensor structure diagram
Dimensions not marked in the figure Unit: mm Tolerance of unmarked dimensions: ±0.1mm
Basic Circuit
diagram 2: SJ21 test circuit
Description: The figure above is the basic test circuit of SJ 2 1 sensor. The sensor needs to apply 2 voltages: heater voltage (VH) and test voltage (VC). VH is used to provide a specific operating temperature for the sensor, which can be a DC power supply or an AC power supply. VRL is the voltage on the load resistor (RL) connected in series with the sensor. VC is the test voltage for the load resistor RL, which must be a DC power supply. The sensor resistance (Rs) can be calculated using the following formula: Rs=(Vc/VRL-1)×RL; the sensitive body power consumption (Ps) can be calculated using the following formula: Ps=Vc2×Rs/(Rs+RL) 2.
Sensor Characterization
Figure 3: Response recovery curve
Figure 3 shows the VRL values of the sensor in different propane concentrations. The load (RL) used was 4.7KO, and all tests in the figure were completed under standard test conditions.
Figure 4: Sensor linearity curve
Figure 4 shows the change in VRL value of the sensor when the sensor is first placed in the detection atmosphere and then removed from the atmosphere .
Cross-interference characteristics
Figure 5: Long-term stability observation curve
All tests in the figure were completed under standard test conditions. The horizontal axis is the observation time and the vertical axis is the VRL value.
Precautions
1 Situations that must be avoided
1.1 Exposure to volatile silicon compound vapors
The sensor should avoid being exposed to silicone adhesives, hair spray, silicone rubber, putty or other places where volatile silicone compounds exist. If the surface of the sensor is adsorbed with silicone compound vapor, the sensitive material of the sensor will be wrapped by silicon dioxide formed by the decomposition of the silicone compound, which will inhibit the sensitivity of the sensor and cannot be restored.
1.2 Highly corrosive environment
When the sensor is exposed to high concentration of corrosive gases (such as H2S, SOX, Cl2, HCl, 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.
1.3 Pollution by alkali, alkali metal salts and halogens
The performance of the sensor may also deteriorate if it is contaminated by alkali metals, especially salt water spray, or exposed to halogens such as Freon.
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 degrade.
1.7 Voltage is added to the wrong pin (only for the side-heated series) For the 6-pin sensor, 2 and 5 are heating electrodes, (1, 3)/(4, 6) are test electrodes, 1 and 3 are connected, and 4 and 6 are connected. If voltage is applied to pins 1, 3 or 4, 6, the lead will burn out, and if it is applied to pins 2 and 4, no signal will be obtained. (See Figure6 on the right)
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. After long-term storage, the sensor needs to be powered for a longer time before use to stabilize it. The storage time and corresponding aging time recommendations are shown in Table 2.
Table 2
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: lead-free and halogen-free flux
Constant temperature soldering iron
Temperature: ≤350℃
Time: no more than 3 seconds
Violation of the above usage conditions will degrade the sensor characteristics.
