If the system-mounted TDS meter's display is blank:
First, check to make sure the unit is powered on. It will turn off after a period of inactivity to conserve battery.
If the TDS meter will still not power on, its batteries will need replaced. Replacement batteries for meters should be ordered locally. The Dual TDS Meter and the Handheld TDS Monitor for the VistaPure take two (2) of the following battery: 357A (LR44) Alkaline Button Cell - 1.5V 110mAh