National Institute of Occupational Safety and Health (NIOSH) research physicist John Edwards, who has conducted extensive research into mine fire and the use of sensors, said multiple fire sensors were the way forward in mine fire detection and nuisance discrimination.

Reliable early fire detection can be hindered by false fire alarms produced by carbon monoxide (CO) and smoke particulate emissions from diesel equipment. Workers can learn to ignore repetitious false alarms associated with these emissions, which can jeopardize miners safety in the event of an actual mine fire.

“The increased use of diesel equipment in underground coal mines requires a reliable method to eliminate their nuisance emissions signatures, which include CO and diesel combustion particulates,” said Edwards.

“If the approach is to increase the fire sensor's alert or alarm value, the question lingers of how much should the value be increased.

“If rates of change of CO are monitored, low CO emissions from conveyor belt heating, which could have advanced to the production of visibly obscuring smoke concentrations, would not be detected in the smouldering combustion stage.

“The interference of hydrogen with chemical CO sensors can create indeterminacy in the true CO value. Rock dusting can create an atmosphere to which a smoke sensor would respond.

“With these ambiguities implicit in a single type fire sensor, the safety of the miners could be placed in jeopardy.”

To combat this danger, NIOSH conducted a mine fire detection program to determine the effectiveness of combinations of fire sensors for early mine fire detection and nuisance signal discrimination.

The results of the tests showed to enhance miner safety, a smoke sensor should be used in a conveyor belt entry for early fire detection and at a battery charging station to eliminate false chemical cell CO alarms. With the operation of diesel equipment in underground mines, a nitric oxide-sensitive metal-oxide semiconductor (MOS) sensor can be used to discriminate diesel emissions and prove a positive indication of fire.

A combination of a CO sensor, smoke sensor and MOS nitric oxide sensitive sensor is one possible sensor combination for successful early detection and nuisance distinction.

To date, the use of multiple type sensors in US mines for elimination of nuisance alarms is currently limited to a combination CO-NO (carbon monoxide-nitric oxide) sensor, which is primarily used to discriminate against diesel emissions.

“Multiple type fire sensors are currently used for protection of residential and commercial buildings. The concepts behind these applications for early and reliable mine fire detection are equally valid for in-mine use,” said Edwards.