This tester is used for monitoring the quality of the water produced by your MB Series filter. It's unit of measure is parts per million. The reading is an indication of mineral level in the water. All dissolved metals will conduct electricity in water and this tester takes the measurement of this conductance and converts it to parts per million. An MB Series filter should be serviced when the TDS tester reads 1 ppm. It can stay at zero for a long time. But once it reaches 1 ppm, it can rise quickly.
This tester is also a useful tool for people who own reverse osmosis water. Simply test the TDS of your tap water and then your R/O water. Subtract your R/O TDS from your Tap TDS. Then take that number and divide it into the tap water TDS. The result is your rejection rate. Most manufacturers recommend changing the membrane when the rejection rate drops to 70%