Abstract: The integrity of mine roofs and rock bolts is tested by determining the natural frequencies of the roof and bolt. The roof is struck at a plurality of locations and the resulting vibrations sensed with accelerometers. The output of each accelerometer is separated into a plurality of signals of different frequencies by a comb filter having selected band passes. The amplitude of the outputs of each of the band passes of the filter are used to determine the condition of the roof and bolt adjacent each accelerometer. The high amplitude signal resulting from the initial impact of the striking implement is electrically gated from the accelerometer output.

Abstract: A fluid level indicator circuit is used in cooperation with a probe attached to a long wire where the probe has first and second electrodes for contacting the surface of the fluid and leads communicating to the indicator circuit. The water level indicator has a low current high gain transistor and a power source which has sufficient potential to not only properly reverse bias the transistor when the probes are out of contact with the fluid but also properly forward bias when the probes are in contact with conductive fluid. A meter and current adjusting device are provided in series with a collector and emitter to provide an indicator when the probes are in or out of the water. The circuit also has a light to give a visual indication.