Halogen Systems Inc. Receives DNV GL Type-Approval for Its TRO Sensor January 20, 2021 – Recently Halogen Systems Inc has gained more recognition and significant approvals on its Amperometric DPI (direct pipe insertion) TRO (total residual oxidant) sensor. They have just been awarded DNV GL type approval on their ballast water management system (BWMS) sensor that measures TRO in ballast water for applications of 0.2 PSU salinity and higher. President