Catalytic Combustion Method
Sensor: HWStationary sensor
Example: HW-6239

1. Brief description
This sensor detects gas based on heat generated by combustible gas burning on anoxidation catalyst. It is the most widely used gas sensor designed specifically for
combustible gases.

Category

Detectable gas

SolidCombustible

2. Structure and principles[Conceptual rendering of the sensor elements][Structure]

This sensor consists of a detector element and a compensation element.
The detector element consists of a coil of a precious-metal (e.g.,platinum) wire and an oxidant catalyst—a substance active against
combustible gas—sintered on the coil along with an alumina support.
The element burns in reaction to any detectable gas. The compensation
element consists of a coil of a precious-metal wire and glass—asubstance inactive against combustible gas—sintered on the coil alongwith an alumina support. This element corrects the effect of theatmosphere.

[Principles]

The precious-metal wire coil heats the detector element to 300℃ to450℃. Then, a combustible gas burns on the surface of the detector
element, increasing the temperature of the element. With changes intemperature, the precious-metal wire coil, a component of the element,changes in resistance. The resistance changes almost in proportion tothe concentration of the gas. The bridge circuit shown in the right figure
allows the sensor to recognize the change in resistance as the voltage
to determine the concentration of the gas.

Precious-metal
wire coil

Oxidation
catalyst +alumina
supportGlass +
alumina support

Precious-metal
wire coil
0.5-1.4 mm

0.5-1.4 mm

0.8-1.4 mm

0.8-1.4 mm

Detector element

Compensation element[Bridge circuit]

Increased element temperature=> Increased resistance

Decreased element temperature=> Decreased resistanceCompensation
element＋Detector element－Sensor output

VSideresistance RSensor voltage Vs

3. Features (of the sensor HW-6239 as an example)○ Output characteristics

The precious-metal wire coil, the heatsource, linearly changes intemperature resistance coefficient. In
the lower-explosion-limit (LEL)
concentration region, the burning
reaction is proportional to the gas
concentration. In this region, theoutput from the sensor slowly
changes according to the change ingas concentration.

○ Responsiveness○ Temperature andhumidity characteristics

100Outputratio(%)
00

Gas concentration(% LEL)

Temp char
Hum char

0-20

Temperature (℃)

60

Gas InTime (sec.)

Molecularformula

Model #Combustible
gases in general
Methane-HW-6211

CH4HW-6239

Vinyl chloride

C2H3Cl

HW-6214-HW-6228

High-boiler gases

Reading(% LEL)

The detector element uses a catalyst that promotes
burning reaction. Having been developed in-house forgas sensors, this catalyst makes use of our proprietaryknow-how, providing long-term stability.

100

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

90

CH4 0100○ Catalyst development

Gas Out

100

The combustion heat produced on thesurface of the detector element
Outputtransfers to the precious-metal wireratio
coil, changes the resistance of the(%)
bridge circuit, and then transforms intosignals. With a high reaction rate, this0sensor excels in responsiveness,
0accuracy, and reproducibility.

100

The materials used in theelements have high electricalresistances and are less likely
to be affected by thetemperature and humidity inthe use environment, allowing
the reading to stay almost
constant.

Humidity (% RH) (40℃atmosphere)

Detectionrange

5. Products of this type (examples)○Stationary products... GD-A80, GD-A80D, SD-1 (TYPE GP),SD-D58･DC (TYPE GP), SD-2500○Portable products
0-100% LEL... GP-1000

SD-1 (TYPE GP)