Mining Contract: Development of Non-regulatory Runtime Respirable Coal and Silica Dust Monitor Using Quantum Cascade Laser-based Ringdown Spectroscopy

Contract # 75D30120C09163
Start Date 9/1/2020
Research Concept

This contract proposes to enable a new laser-based infrared spectroscopic modality, runtime cavity ringdown spectroscopy, for the development of a highly sensitive coal and silica dust sensor. The quantum cascade laser (QCL)-based direct absorption spectroscopy will be used to develop the first mid-infrared runtime mining dust monitor capable of measuring ambient concentrations of non-regulatory silica and coal dust present in the mining environment. The device is capable of measuring concentrations of silica and coal dust and provides instant results allowing workers to take immediate actions to prevent overexposure. In this development, RingIR modifies their patented gas tracking technology to build a simple, low cost, wearable, highly sensitive and reliable dust monitoring device.

Contract Status & Impact

This contract is ongoing. For more information on this contract, send a request to

Laser spectroscopy has been the basis of numerous field portable gas sensors and using quantum cascade lasers (QCLs) as sources has greatly advanced infrared spectrometers. This contract proposes to utilize a cavity-enhanced direct optical absorption method coupled with QCLs. Laser-based optical methods are proven to be sensitive and efficient due to their accuracy, specificity, and linearity. RingIR’s patented direct optical absorption technology utilizes broadly tuning mid-infrared QCLs to produce a field-deployable runtime cavity ringdown (rtCRD) spectrometer for gas tracking and leak detection. rtCRD is an emerging technology that has shown its potential in many disciplines such as military, security, environmental and civilian. The systems RingIR has developed reach high parts-per-trillion (ppt) (> 0.1 ppt) sensitivity with low false positives (<0.01%). RingIR has to date developed prototype gas trackers operating uninterruptedly over the 7 to 11 μm wavelength range. RingIR technology has been demonstrated that it can determine the silica content in coal dust in either real-time or as a cumulative quantity.

The key technical objectives of this proposed personal dust monitor development include:

  1. Developing a QCL-based silica and coal dust monitor utilizing a novel sensing modality: Runtime Cavity Ringdown Spectroscopy (rtCRDS).
  2. Miniaturizing and reducing power need to develop a dust monitor that can be wearable and operate in an underground environment.
  3. Calibration and determining sensitivity limits.
  4. Enabling runtime response, audible and/or visual information, and warning alerts.

The capability of RingIR’s patented gas tracking technology will be used to manufacture a wearable personal dust monitor for runtime dust monitoring with the collaboration of researchers at New Mexico Tech.

To develop the next generation personal dust monitor, the contractor will develop a miniaturized system that mineworkers can wear on their belts. The system will continuously pull air from the environment with an adjustable airflow rate. The quantities of both silica and coal will be measured by the unit via data processing in an onboard computer and displayed instantly. Initially, it is proposed to utilize an advanced spectroscopic approach, rtCRDS with midIR QCL as the light source. This is an optical spectroscopic technique that measures the photon loss due to the interaction of gas molecules inside an optical cavity as light is passed through it. In this technology, the gaseous/aerosol sample from the environment pulled into the optical cavity using a pump. The samples travel through the optical cavity which contains two input coupling mirrors and two high reflecting mirrors. To measure the levels of both silica and coal, the contractor plans to use two different laser wavelengths, 9.34 μm and 7.4 μm. 9.34 μm is the optimal wavelength for silica absorption, however, coal also shows absorption in this wavelength. To overcome this issue, the contractor selected 7.4 μm, where coal shows the same absorption to 9.34 μm.

To calculate silica absorption, the contractor will subtract 7.4 μm from 9.34 μm. If the absorption at 9.34 μm is due to coal dust it should result in 0 absorption and otherwise the absorption is due to silica dust particles. As the light passes through a gaseous sample, based on the absorption capacities of the gaseous molecules inside the cavity, it is possible to measure the rate of decay of light within the cavity. This decay rate can be used to uniquely identify the gaseous molecules, hence the name “molecular fingerprinting.” During this process, the light intensity inside the optical cavity needs to be replenished to its resonant frequency (ring-up) and then allowed to decay (ring-down) in the presence of the sample. The contractor notes that this technique is independent of the light source intensity (I0) and the photodetector DC offset, both of which are required for the traditional CRDS method. This work has confirmed that the gas tracking technology proposed here is capable of detecting respirable coal and silica dust fractions.

If the detected silica or coal level is higher than the set exposure limit, the device will send audio or visual warning alerts to the user. All data will be logged while the unit is operational and can be added to the network using Wi-Fi or Bluetooth. The data can also be downloaded via the same system or to a USB drive as the unit is returned to the surface and for analysis. All devices will be designed to have a unique identifier when the units are issued. This will allow for cumulative data log for individual monitors. 

During the project, the intention is to develop three prototype units and to deliver one unit to NIOSH upon completion of the project, for testing purposes only. Two units will be used for testing purposes and the third unit will be developed as a final dust monitor.

Page last reviewed: 6/25/2021 Page last updated: 6/23/2021