Mining Contract: Remote Methane Sensors

Contract Status & Impact

This contract is complete. To receive a copy of the final report, send a request to mining@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.