Sensing Spatial Coverage (SSC) is one of the main Quality of Service (QoS) measures in a Wireless Sensor Network (WSN). This quantifies the WSN monitoring ability. The initial phase of WSN implementation is to determine how many sensors with specific sensing ability are required for the intended application or scenario. For this purpose, the sensing profile of the sensors should be known. Most of the previous works considered a Boolean model for the sensors, limiting the sensing ability to a fixed radius known as the sensing range. The sensing ability of the sensors depends on the nature of the application they are used for. In this work fire detection is assumed to be the application and the sensor consists of a simple Infra-red (IR) photodiode. A realistic sensing model will be introduced and the detection ability of the sensor will be studied in detail and closed form formulas will be derived for the probability of fire detection in different distances.