For combustible and toxic gases Automatic zero correction function, self-diagnosis Explosion proof housing, IP65 ingress class A simple magnetic control key is placed over clear marked positions on the surface plate to test and adjust the instrumentwithout opening the transmitter/detector head Dual alarm contacts and trouble alarm contact are provided for activation of external buzzer or alarm light. Powered by 24 VDC, the unit has a 4-20mA output for communication to outside controllers, for external readouts or alarming devices