In conclusion, in this publication we report a significant enhancement on the previously published low-cost inline detector from Höving et al. by changing the detector to one that can measure multiple wavelengths simultaneously and changing the irradiation source to a white LED. As a result, we were able to obtain real-time inline spectral data from a flow reactor operating in a slug flow regime, indicating when slugs pass the detector even when undetectable to the human eye. This device enables slug timing within a flow reactor to be determined in a quick and facile manner. Furthermore, the device could be used to monitor residence time distribution within a system based on shape of the slug peak, along with perhaps estimation of reaction conversion if the optical absorbance profile of starting material and product were known. Details to construct the device are provided in the supplementary information document, including links to purchase the printed circuit boards directly from the manufacturer we used. Source code, 3D print .stl files and circuit board Gerber files (if you wish to organise manufacture yourself) are all provided on our GitHub repository: