This report develops an online fluence meter. The meter is based on a UV-sensitive chemical that forms a photocatalytic product with a relation to the applied UV dose. The report proceeds to find naturally-occurring substances or chemicals to be dosed while reacting with UV-C light (200-285 nm) and produces measurable photocatalytic products in the 0-1,000 J/m2 dose range. To properly disinfect water, UV sensors enable the UV dose to be monitored. However, these sensors only detect 254 nm UV light, which is unsatisfactory for UV systems equipped with medium-pressure lamps. Moreover, these sensors are not very accurate, break down easily, and have to be recalibrated frequently. Therefore, the readings of each individual sensor have to be computed with Computational Fluid Dynamics (CFD) to be able to calculate the UV dose. Producing a clear and mathematically useful relation between the concentration of the photo product and the applied UV dose required that the system be dosed with either naturally-occurring compounds or toxicologically acceptable chemicals. The chemicals selected were investigated by performing collimated beam tests. Although several parameters affected the UV dose in UV disinfection systems, only the variation in water quality was taken into account in this research.