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Air-Quality Meter For Checking CO & CO2 Levels

This Air Quality Monitor indicates carbon dioxide (CO2) and carbon monoxide (CO) levels on a dual bargraph and sounds an alarm when either level reaches a preset concentration. You should use it if you have an unflued heater in your home, boat, caravan or any indoor space.

By John Clarke

Main Features & Specifications


• Detects carbon monoxide (CO) and carbon dioxide (CO2) levels

• 15-level LED bargraph display for each gas

• Three-stage alarm ranging from initial warning through to urgent

• Internal fan replenishes air for sensors

• Automatic display dimming in low light levels


CO2 Range: 0.03% to 1% (300-10,000ppm) with recommended calibration.

CO Range: .003% to .03% (30-300ppm) with recommended calibration.

Bargraph Displays: separate bargraphs to show CO and CO2 concentrations, each consisting of eight LEDs with 15 display levels.

Alarm Modes: 16ms chirp every 16s (third top LED), 32ms chirp every 4s (second top LED) and 64ms chirp every 0.5s (top LED).

CO2 Sensor Heating: continuous at 200mA.

CO Sensor Heating: 60s heating at 150mA; 90s reading period at 42mA.

Readings Update: CO = 2.5 minutes; CO2 = after an initial 60s then with a nominal 5s lag due to sensor response.

Dimming Range: 205 brightness levels.

Diagnostic Display: CO sensor only when VR4 is set to give 0V on TP4. The top two LEDs are lit during the heating cycle, while the third top LED and LEDs below light for the measurement cycle with these LEDs extinguishing successively every 15s.

Power Supply: 12VDC 500mA plugpack.

All combustion heaters, including those using wood, coal, coke, kerosene, methylated spirits and gas, draw oxygen from the air as the fuel is burnt. If used indoors, such as inside a house, this gradually reduces the oxygen concentration in the air unless there is sufficient ventilation to the outside. However, judging how much room ventilation is needed to keep the air safe is almost impossible and it’s all too easy to provide insufficient outside air. After all, you do want to keep warm.

If you don’t have sufficient fresh air in the room, there is the immediate danger that the deadly gas carbon monoxide will be produced. This is much more likely if the heater is unflued, whereby the combustion gases are released into the room. Unfortunately, most gas heaters used in Australia are unflued and every one of these is a potential source of carbon monoxide and other noxious gases.

Some gas heater designs attempt to get around this problem by employing an oxygen depletion sensor. These extinguish the heater if the oxygen concentration in the room is reduced by 20%. While better than having no sensor at all, this definitely should not be regarded as a safe answer. Why? Because regardless of whether the oxygen depletion sensor, a pretty crude device, is working, the heater may still produce some carbon monoxide as well as the normal combustion products of carbon dioxide, water vapour, nitrogen oxides, sulphur dioxide and formaldehydes.

An oxygen depletion sensor does not detect or react to any of these noxious and potentially harmful gases – it only detects a reduction in oxygen concentration.

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