SCM134

Sensor haracteristics

Stability for long work periods

Long Life with a reliable stability

Components arrangement.

The figure shows the sensor dimensions and the components arrangement.

Sensor dimensions and the components arrangement

List of Materials

Item	Descriptions	Materials
1	gas sensing layer	SnO2
2	measurement electrode	Au
3	measurement electrode ignited line	Pt
4	heater	Ni-Cr alloy
5	tubular ceramic basic body	Al2 O3
6	anti-explosion network	dual layer of 100 stainless steel
7	clamp ring	Ni with vulcanized material
8	basic seat	Bakelite
9	tube foot	Ni with vulcanized material

The activated carbon filter reduces the interferences from NOx, Alkenes and other gases.

The SCM134 has 6 pins, 4 (A, B) are used to detection signals and the other 2 (H) are used for the heater circuit.

The electric measurement circuit.

Operation Conditions of the GAS sensor

As shown in Figure 2, the standard measuring circuit of SCM134 GAS sensor includes 2 parts. One is the heating circuit which has the time control function for the heater (the high voltage and the low voltage circuits.) The second is the signal output circuit; it can accurately respond to the changes of surface resistance of the sensor. The surface resistance of the sensor, Rs, is obtained through drop of voltage signal through load resistance RL.. The relationship between them as below:

Rs\RL = (Vc-VRL) / VRL

Based on Fig. 2 circuit, Fig. 4 is the graphical of the signal output curve when the sensor is moved form clean air to carbon monoxide (CO) gas environment. Those data was measured after heating period 2.5 minute from high voltage to low voltage.

The environment temperature will affect the sensitivity, which relationship curve is show in the Figure.5.

Work parameters of the sensor:

Symbol	Description	Operating Value	Remarks
VC	Input Voltage	5 V	either AC or DC
VH (H)	High Heating Voltage	4 V +/-0.1 either AC or DC
VH (L)	Low Heating Voltage	1.4 V+/-0.1	either AC or DC
RL	Load resistance	Adjustable	Ps < 25 mW
TH (H)	High Heating Time	50 +/-1 sec.
TH (L)	Low Heating Time	90 +/-1 sec.
RH	Heater resistance	33 Ohms +/-5%
Ps	Power Consumption	Less than 25 mW.

Environmental operating and storage conditions

Symbol	Description	Operating Value	Remarks
Tao	Operation temperature	De -20°C a 50°C
Tas	Storage Temperature	De -20°C a 50°C
RH	Relative humidity	Less than 95% Rh
O2	Oxygen concentration	21% (at standard conditions)	minimum value is above 2%

Sensibility characteristics of the GAS sensor

Symbol	Description	Operating Value	Remarks
Rs	Sensor resistance	2 to 20 KOhms (100 ppm of Carbon Monoxide)	Detection ranges: 20-1000 ppm of Carbon Monoxide
α (300/100 ppm)	Concentration slope rate	<= 0.5
Standard Operating Conditions	Temperature 20°C +/-2°C; RL 10 KOhms +/-5%; Relative Humidity 65% +/-5%; Vc 5V +/-0.1; VH (H) 4V +/-0.1; VL (L) 1.4V +/-0.1 V
Preheating Time	Greater than 48 Hours

Handling characteristics

Variable

Conditions

Remarks

Vibration

100 cpm of vertical vibration with an amplitude of 4 mm

Should be conformed to

normal sensitivity

Shock

100G five times

The SCM134 sensor relative sensibility for different gases

Name of gas	with active carbon filter Rs (100ppm)/Rs(100ppmCO)	without active carbon filter Rs(100ppm)/Rs(100ppmCO)
Methane	can not measure	can not measure
Propane	can not measure	6. 5 8
Isobutane	can not measure	can not measure
Hydrogen	6. 2 5
Alcohol	can not measure	2. 5 6
Isopropanol	can not measure	3. 4 8
Methyl benzene	can not measure	2 0. 9 8
Ethyl acetate	can not measure	3. 2 8
Benzene	can not measure	3 1. 2 0
Heptane	can not measure	3. 6 0
Carbon dioxide	can not measure	can not measure
Hydrogen sulfide	can not measure	0. 0 2

Figure 3.- VRL Relationship Vs. CO Concentration

Characteristic sensitivity curves

Figure 4.- This is de output signal when the sensor was moved from clean air into other with CO concentration. This output was obtained using the circuit shown in the figure 2

Effect of the temperature on the sensibility

The Sensitivity Adjustment

The resistance value of the SCM134 sensor changes with carbon monoxide concentrations. So, when using the sensor, it is necessary a sensitivity adjustm