Non-dispersive infra-red (NDIR) measurement of carbon dioxide at 4.2 μm in a compact and optically efficient sensor

• New design of non-dispersive infra-red (NDIR) sensor for CO2 and other gases.
• Standard miniature format for gas sensors – a cylinder 20 mm D × 16.5 mm H.
• Raytrace – estimated 32 mm pathlength.
• Manufactured in volume using gold-coated injection-moulded reflective optics.
• Short-term limit of detection 1 ppm CO2, noise equivalent absorbance 3 × 10−5 AU.

Non-dispersive infra-red (NDIR) gas detection has enjoyed widespread uptake as a result of development of devices in the standard miniature format for gas sensors, consisting of a cylinder with external dimensions of 20 mm diameter × 16.5 mm height. We present a new design for such a sensor, making use of low-cost injection moulding technology. The design pays particular attention to the problem of maintaining a high optical throughput while providing an acceptable optical pathlength for gas detection. A detailed analysis of the design is presented, with the results of optical raytracing, showing a raytrace estimate for 4% of the total emitted radiation reaching each of two separated detector elements and a pathlength of 32 mm. Finally, we show experimental results obtained with as-manufactured devices, with a short-term limit of detection for carbon dioxide (CO2) estimated at 1 ppm or a noise equivalent absorption (NEA) of 3 × 10−5 AU.