Patent ID: 8237576

Claim:
In combination with a manhole and a removable manhole cover closing the manhole; a barrier in the form of a dish removably mounted in said manhole below and separate from said cover and extending completely across said manhole separate from said manhole cover; said barrier completely closing said manhole, an alarm system mounted to the underside of said barrier and operable to detect the presence of a volatile gas in the manhole and to produce an alarm when the presence of a volatile gas is detected in the manhole, said system comprising: a sensor operable to monitor the presence of volatile gases in the manhole; an alarm actuated by the sensor when volatile gases are detected in the manhole; a station remote from the manhole; and said alarm system further comprising a transmitter actuated by said sensor to receive and transmit a signal to the remote station to indicate at a remote location that volatile gases are present in the manhole.