In LabCOM's app/software/web application, the active chlorine calculator is based on J. O'Brien's work, which also is used by the US Environmental Protection Agency (USEPA). Detailed information about this calculation methodology can be found in the USEPA calculator documentation at:  • https://usepaord.shinyapps.io/cyanuric/ The listed formulas in the documentation are all derived from O'Brien's work and have been extensively reprocessed. This system provides an almost identical concentration of HOCl as the LabCOM active chlorine calculator, but the cyanuric acid (CYA) setting of the USEPA calculator is limited to a maximum of 10mg/l. To better understand the subject of cyanuric acid setting it is recommended to read the following article which covers the subject in a well understandable manner:  • https://blog.orendatech.com/minimal-cya-pillar-4