Recent technological developments of sweat sensor systems offer new opportunities to unobtrusively monitor athletes, to optimize their performance and minimize injury risk. However, from physiological literature it can be concluded that for most sweat constituents, little is known yet about how these constituents change over time and how they relate to the physical status of an athlete. A parameter of interest is ammonium in sweat. Sweat [NH₄⁺] can potentially be used as a marker for muscle fatigue. To investigate the use of NH₄⁺ as a biomarker, real-time NH₄⁺ monitoring systems are required. NH₄⁺ will quickly evaporate from sweat, which makes it challenging to measure from sweat samples. Therefore, we present a new approach of monitoring NH₃ that is evaporated from sweat during exercise. A metal oxide gas sensor is placed in a capsule that is connected to the skin. This capsule is ventilated with dry air at low flow rates (0.2 -1.2 l/min). The capsule also contains a temperature and humidity sensor, to compensate for temperature and humidity effects. Experimental results prove that NH₃ sensors show good sensitivity from 27 mV/ppm to 1.1 mV/ppm in the desired measurement range of 1 to 30 ppm respectively. As expected, the baseline resistance varies between each gas sensor. Humidity changes will influence baseline resistance and sensitivity significantly. This confirms the design choice to measure humidity and temperature changes in the capsule. In future experiments, the setup will be tested in-situ, to validate if NH₃ levels can be measured in an exercise setting.