Mining Contract: Remote Methane Sensors
This contract integrated a micro electro mechanical system (MEMS) component into specialized electronics to measure methane levels in underground coal mines. Unlike most infrared-based gas sensors that separate the infrared source and detector, this chip incorporated those components into a single unit. A photonic crystal generated an infrared source at a wavelength of 3.3 µm. The infrared radiation passed out of the package, through the gas being analyzed, off an external mirror, back through the gas being analyzed, and into the sensor. The amount of radiation that returns to the sensor was a function of the methane concentration sampled.
Contract Status & Impact
This contract is completed. To receive a copy of the final report, send a request to OMSHR@cdc.gov.
Major tasks focused on improving the precision of the sensor and improving device ruggedness for survivability in an underground coal mine environment, as follows:
- Excellent progress was made to improve the stability of the sensor while increasing its sensitivity. This device reduced cross talk to water vapor by a factor of at least ten. This contract succeeded in designing and fabricating an infrared filter that only passed the wavelengths near the 3.3 µm methane absorption band while blocking the water vapor bands. This work also decreased the vacuum inside the device from 40 milli-torr (mT) to 10 mT. The thermal isolation of the gasses surrounding the chip improved to the point that the convection losses were negligible, and this provided a 10% sensitivity improvement.
- Custom electronic circuitry was developed and tested to minimize the sensor drift. The constant bridge topology electronics selected for this device facilitated amplification and increased stability of the signal output.
- To test durability for use in underground coal mines, the sensor was subjected to the requirements of MIL-STD-750, which included constant acceleration and vibration and exposure to variations in temperature and humidity. Constant accelerations of 10,000 and 20,000 g and constant vibrations of 20 g at 100 Hz to 2 kHz were conducted. This sensor also passed a 96-hour test at 85ºC and 80% relative humidity. PTFE filters, with available pore sizes of 0.5, 1, 2, and 5 microns, were used to protect the sensor from mine dusts without compromising accuracy. PTFE is also hydrophobic and should protect the sensor from water droplets and other incidental exposures to water.
- The vacuum encapsulation and very high power conversion efficiency provided a low case temperature for use in coal mines.
- The output from the methane monitor is readable on a laboratory quality voltmeter. The voltage output is proportional to the methane concentration flowing through the optics cell.