Application of Hopcalite in Carbon Monoxide Alarm Sensors

I. The role played by hopcalite 

catalytic reaction sensors, hopcalite is precisely integrated as a key catalytic material into a detection circuit known as a "Wheatstone bridge".

The core structure of a sensor typically includes components with two resistance wire coils.

Detection component:

Its coil is coated with a hopcalite catalyst , which, when exposed to the ambient air, can catalyze a reaction with carbon monoxide .

Compensation components:

Their coils are coated with an inert material . This component is usually sealed or physically isolated so that it does not react with carbon monoxide in the environment .

These two components, together with two other precision resistors, form a Wheatstone bridge.

II. Catalytic reactions induce changes in electrical resistance

Catalytic reaction:

When air containing carbon monoxide diffuses to the surface of the detection element, carbon monoxide and O₂ molecules are adsorbed onto the hopcalite catalyst. Under the action of the catalyst, carbon monoxide can be oxidized to carbon monoxide₂ at room temperature , releasing heat.

Resistance change:

Platinum wire has a positive temperature coefficient characteristic; as the temperature increases, its resistance increases linearly.

Wheatstone bridge imbalance:

Because the compensating element is coated with an inert material that does not undergo a catalytic reaction, its temperature is only affected by the ambient temperature, and its resistance changes slowly. When the sensing element heats up due to the carbon monoxide reaction, its resistance increase is greater than the resistance increase of the compensating element, causing the originally balanced Wheatstone bridge to become unbalanced.

Signal output:

An imbalance in the bridge circuit will generate a voltage difference signal at its output that is proportional to the change in resistance and, consequently, to the carbon monoxide concentration. This weak analog signal is fed into subsequent circuitry.

Processing and alarm:

After amplification, linearization, and temperature compensation, the signal is converted into a specific carbon monoxide concentration value (ppm) and displayed on the screen. When the concentration exceeds a preset safety threshold, the processor will activate an audible and visual alarm.

III. Design of the hopcalite Formula

In carbon monoxide sensors, the hopcalite formulation is highly optimized to meet specific requirements .

High and low temperature activity:

It must have extremely high catalytic activity under a wide range of room temperature conditions in order to detect low concentrations of carbon monoxide and respond rapidly.

Excellent selectivity:

It must primarily react with carbon monoxide , while reacting weakly with other common combustible gases in the environment (such as hydrogen, methane, alcohol vapor, etc.).

Long-term stability:

The catalyst activity needs to remain stable over a long period of time and should not be easily poisoned or deactivated.

Water resistance:

This is the biggest challenge for traditional hopcalite in sensor applications. Water vapor strongly adsorbs onto the catalyst surface, competing with carbon monoxide for active sites, leading to decreased sensitivity or even temporary failure. Therefore, the water resistance of the hopcalite formulation must be considered when applying it to carbon monoxide sensors .

IV. Advantages and disadvantages of applying the hopcalite formula 

Advantage:

01. It has a good linear response to carbon monoxide , and the output signal is proportional to the carbon monoxide concentration within a certain range, which facilitates accurate measurement.

02. Fast response speed, capable of quickly detecting carbon monoxide leaks.

03. Long service life and relatively stable catalytic materials.

limitation:

01. Zero-point drift exists and requires periodic calibration.

02. It is susceptible to interference from other gases, and its selectivity is not 100%.

03. High temperature and high humidity environments affect accuracy and lifespan.

04. The power consumption is relatively high because the platinum wire coil needs to be continuously heated to the operating temperature.

05. Due to limitations in usage conditions, its application scope is generally limited to non-densely populated spaces such as mines and tunnels.