Real-time emotion sensing is now a reality due to computing power of mobile phones and portability of sensors. Sensing emotions in real-time supports a multitude of fields as health care, driving, fitness, decision support systems, among others. This paper presents a prototype of a low-cost mobile Galvanic Skin Response (GSR) sensor and two case studies exploring its use, namely: (1) a cloud-based support application designed to help speakers during talks when a stress event is identified; (2) a quantitative User eXperience (UX) analysis during the gameplay of a popular tower defense multiplayer mobile game (Clash Royale), comparing arousal events of winning and losing matches. The proposed mobile GSR sensor enables the detection of signal peaks indicating stress/arousal events during interaction and opens multiple possibilities of quantitatively measuring UX in mobile contexts. With the presented results, one expects to motivate Human-Computer Interaction (HCI) practitioners to explore quantitative measures of situated UX or to design support systems considering the Biocybernetic Loop.