The Neptune Systems Conductivity 447 Calibration Fluid is a calibration fluid (447.0 mS) for use with the conductivity probe and PM2 for Ro/Di (osmose) water monitoring. This calibration fluid is intended to calibrate the conductivity electrode for use in osmosis water. If you want to monitor the quality of osmosis water, you need this 447 mS liquid.
Do you want to monitor the quality of your salt water? Then you need the 53 mS of liquid!