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Perspiration level monitoring enables numerous applications such as physical condition estimation, personal comfort monitoring, health/exercise monitoring, and inference of environmental conditions of the user. Prior works on perspiration (sweat) sensing require users to manually hold a device or attach adhesive sensors directly onto their skin, limiting user mobility and comfort. In this paper, we present a low-cost and novel wearable sensor system that is able to accurately estimate an individual's sweat level based on measuring moisture. The sensor is designed in a threadlike form factor, allowing it to be sewn into the seams of clothing, rather than having to act as a standalone sensor that the user must attach to their body. The system is comprised of multiple cotton-covered conductive threads that are braided into one sensor. When a person sweats, the resistance between the braided conductive threads changes as moisture becomes trapped in the cotton covering of the threads. The braided three-dimensional structure allows for robust estimation of perspiration level in the presence of external forces that may cause sensor distortion, such as motion. We characterize the relationship between the volume of sweat and measured resistance between the braided threads. Finally, we weave our sensors into the fabric of a shirt and conduct on-body experiments to study users' sweating level through various activities.