Semi-Conductor Method
Sensor: SG

Stationary sensor
Example: SGF-8562

Stationary sensor
Example: SG-8581

1. Brief description

Category

This sensor uses a metal oxide semiconductor, which changes in resistance when it
comes into contact with a detectable gas. The sensor detects this change in resistanceas the gas concentration. It is a general-purpose sensor that detects all types of gases
ranging from toxic gases to combustible gases.

2. Structure and principles[Structure]

The sensor consists of a heater coil and a metal oxide
semiconductor (SnO2) formed on an alumina tube. The tube isequipped with two Au electrodes at its ends to measure theresistance of the semiconductor.

[Principles]

The heater coil heats the surface of the metal oxide semiconductor to350 to 400℃. With atmospheric oxygen adsorbed on this surface informs of O-and O2-, the semiconductor keeps a constant resistance.
Then, methane gas or the like comes into contact with the surface andbecomes chemisorbed by it, which is in turn oxidized by O2- ions andseparated. The reaction occurring on the surface of the sensor isrepresented as follows:
CH 4 ＋ 4O 2--> CO 2 ＋ 2H 2O ＋ 8e In short, methane gas adsorbs on the surface of the sensor and takes
the absorbed oxygen away; this increases free electrons inside thesensor, reducing the resistance. By measuring the change inresistance, the sensor determines the gas concentration.

Detectable gasCombustible

Solid[Conceptual rendering ofthe sensor elements]

Metal oxide
semiconductor

Au electrode

Heater coil

[Drive circuit]

Alumina tube

Lead wire

Zero adjuster
DCpower

Detectorelement

H2OIndicator

O2eHeatervoltage
regulator

Signalamplifier

Span adjuster

H2Load resistanceH2 ＋ O2- -> H2O ＋ 2e-

3. Features (of the sensor SG-8521 as an example)○ Output characteristics
The sensor detects changes in
the resistance of thesemiconductor, meaning that it
detects even low concentrations(ppm level) that cannot bedetected by new ceramic-basedsensors.
The sensor is highly sensitive
with a high sensor output level
for low concentrations.

○ Detection of toxic gases

100Since, in principle, the resistance changes according tochanges in the number of electrons and the electronmobility, the sensor detects a variety of gases, including
toxic gases, which produce less combustion heat.

Outputratio(%)
005000Gas concentration(ppm)○ Aging characteristics

The sensor maintains stability over the long term with a longlife. Compared with the catalytic combustion-based sensor,
this type sensor is highly resistant to toxicity and severe
atmosphere.

○ Gas selectivity

Adding an impurity to the semiconductor material changes
the interference effect. This characteristic allows thesensor to selectively detect some gases.

4. Detectable gas, molecular formula, model, and detection
range (examples)
Detectable gas

Molecularformula

Model #Detection
range

Hydrogen sulfide

H 2S

SGF-8562

0-100ppm

Ethylene oxide

C2H4O

SGF-8563

0-100ppm

Solvents
Combustible
gases in general-Hydrogen

H2

SG-8541

MethaneCH4

SG-8581

SG-8511
SG-85210-5000 ppm
0-200 ppm

5. Products of this type (examples)○ Stationary products... SD-3GH, SD-3DGH, GD-84D-EX,GD-A80V, GD-A80DV, GD-70D,SD-1GH, SD-D58･DC･GH

GD-70D

Toxic