An alarm device to detect a condition of ambient air, the alarm device comprising: a housing having a longitudinal axis and containing a sensor and having vents for access by ambient air to the sensor, a signal processing circuit with a processor linked with the sensor, a power supply for the circuit and the sensor, an obstacle detector for detecting presence of an unwanted obstacle in the flow of ambient air to said sensor, the obstacle detector comprising: an ultrasonic transducer mounted in or on the device having a field of emission outside of the device, a processor (18, 19) in the circuit and linked with the ultrasonic transducer to monitor ultrasonic return values and process the ultrasonic return values to determine if any of the ultrasonic return values have been reflected by an unwanted obstacle, wherein the processor is configured to generate an alert to indicate presence of such an obstacle which may affect access of ambient air to the sensor through the vents, and a guide mounted to the housing substantially symmetrically to the device longitudinal axis and comprising a guide element spaced-apart from the housing to reflect emitted ultrasonic waves in radial directions relative to the longitudinal axis, characterized in that, at least some of the vents (3(b)) are arranged so that at least some ultrasonic waves pass through the vents, the vents include vents (3(a)) which are primarily facing radially and vents (3(b)) which at least have a directional component facing axially, and the housing includes a barrier to render application of tape to the vents difficult, said barrier comprising a barrier element mounted by pillars so that the barrier is spaced apart from at least some of said vents. a housing having a longitudinal axis and containing a sensor and having vents for access by ambient air to the sensor, a signal processing circuit with a processor linked with the sensor, a power supply for the circuit and the sensor, an obstacle detector for detecting presence of an unwanted obstacle in the flow of ambient air to said sensor, the obstacle detector comprising: an ultrasonic transducer mounted in or on the device having a field of emission outside of the device, a processor (18, 19) in the circuit and linked with the ultrasonic transducer to monitor ultrasonic return values and process the ultrasonic return values to determine if any of the ultrasonic return values have been reflected by an unwanted obstacle, wherein the processor is configured to generate an alert to indicate presence of such an obstacle which may affect access of ambient air to the sensor through the vents, and a guide mounted to the housing substantially symmetrically to the device longitudinal axis and comprising a guide element spaced-apart from the housing to reflect emitted ultrasonic waves in radial directions relative to the longitudinal axis, characterized in that, at least some of the vents (3(b)) are arranged so that at least some ultrasonic waves pass through the vents, the vents include vents (3(a)) which are primarily facing radially and vents (3(b)) which at least have a directional component facing axially, and the housing includes a barrier to render application of tape to the vents difficult, said barrier comprising a barrier element mounted by pillars so that the barrier is spaced apart from at least some of said vents. an ultrasonic transducer mounted in or on the device having a field of emission outside of the device, a processor (18, 19) in the circuit and linked with the ultrasonic transducer to monitor ultrasonic return values and process the ultrasonic return values to determine if any of the ultrasonic return values have been reflected by an unwanted obstacle, wherein the processor is configured to generate an alert to indicate presence of such an obstacle which may affect access of ambient air to the sensor through the vents, and a guide mounted to the housing substantially symmetrically to the device longitudinal axis and comprising a guide element spaced-apart from the housing to reflect emitted ultrasonic waves in radial directions relative to the longitudinal axis, characterized in that, at least some of the vents (3(b)) are arranged so that at least some ultrasonic waves pass through the vents, the vents include vents (3(a)) which are primarily facing radially and vents (3(b)) which at least have a directional component facing axially, and the housing includes a barrier to render application of tape to the vents difficult, said barrier comprising a barrier element mounted by pillars so that the barrier is spaced apart from at least some of said vents.