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Ultrasonic convergence measurements
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    "Tests during pillar robbing, longwall operations, and other mining activities indicate that roof-floor convergence rates can be used as an indicator of imminent roof failure. A large number of electrical- mechanical devices have been built to measure convergence as well as convergence rates, but most obstruct mine traffic and are costly and time consuming to install. The Bureau of Mines has designed and is evaluating three types of small ultrasonic units to make these measurements. The new instruments provide nonobstructing measurements up to 35 ft away. The ultrasonic transducers can be attached to a roof bolt, placed in an unsupported area (by means of a toss basket), or handheld. Total distance to the target and rate of closure are displayed digitally to 0.001 Ft and 0.01 Ft/min, respectively." - NIOSHTIC-2

    NIOSHTIC no. 10004011

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