Devices and Methods for sensing UV dosage of a fluid stream are described. In a first aspect, a device 22 has a first sensor arrangement 44 for measuring a flow speed of the fluid and a second sensor arrangement 50 for measuring an intensity of UV light radiation. A dosage calculation unit 36 calculates a dosage value from measurements of the sensor arrangements 44, 50. The first sensor arrangement includes a resistive electrical component 32 in thermal contact with the fluid. The electrical component 32 may be heated by an electrical current and has a temperature dependent electrical resistance. Thus, the first sensor arrangement may be used to determine the flow speed of the fluid from a cooling rate of the resistive electrical component 32. According to a second aspect, a device 70 includes at least one UV sensor component 76 for measuring an intensity of UV light irradiation. A data storage 82 for storing intensity or dosage values and an electrical power storage 88 are provided. The device is adapted to freely float within the fluid stream. As such, the device 70 may be inserted into a fluid flowing through a channel, and a dosage value may be determined from data stored in the data storage after the device 70 has passed through the channel.