In response to this, NIOSH researchers have initiated a project investigating measurement procedures and instruments for calculating air velocity in larger opening mines.

There are up to 200 mines in the US with openings with a cross section of 500 square feet or more. These mines have very low air velocities and consequently experience ventilation difficulties.

“Large openings mean these mines have to put a tremendous amount of air to ensure that velocities are sufficient. In most underground areas the air velocity is below the minimum able to be measured by vane anemometers.” NIOSH monitoring section chief and project leader Robert Timko said.

“What we will do in this project is look at different types of instruments capable of measuring lower velocity air to get some idea how much air is ventilating these larger opening mines.”

The first instrument the NIOSH team looked at was the ultra-sonic anemometer. This instrument emits an ultrasonic pulse to a sensor and the transit time the pulse takes varies depending on air velocity.

At this stage NIOSH researchers are developing a procedure to accurately obtain average velocities in large opening mines. Normally a moving traverse is taken in an opening; however large openings pose a problem with this technique.

“Moving traverses work well in entries having less than 250 square feet cross-sectional area,” Timko said.

“A moving traverse doesn’t work in large opening entries so we are forced to perform fixed-point measurements that involve dividing the opening into equal-area segments. We are trying to determine how many measurements we have to make across a large opening to accurately determine what the total flow in that entry is.”

“We will determine what the minimum number of measurements that need to be taken to get an accurate result. Presently, eight to 12 divisions seem to work.”