Model OH121 from Siemens Industry ? Fire Safety is a photo / thermal smoke detector that contains a thermal sensor. When the sensor is utilized, an Alarm condition will trigger when the temperature reaches 135°F ? at this point, the detector will also lock into Alarm condition. The magnet-test feature allows for Model OH121 to be tested without the use of a heat gun, providing quick testing for installers and test personnel to prove the integrity of the wiring and proper connection to the fire alarm control panel (FACP).