A system and method for estimating total dissolved solids (TDS) concentration in boiler water 10 comprising: a sensor 14 having a transmitter to transmit an RF or microwave signal into the boiler water and a receiver to receive a reflection of the signal by the boiler water; and an analysis module 16 to determine a conductivity for the boiler water based on the reflected signal and to determine a TDS concentration based on the conductivity. The analysis module may determine a reflection coefficient based on the reflected signal and to determine the conductivity value based on the reflection coefficient. The system may have a control module which determines if the TDS concentration exceeds a threshold, if exceeded, an alert can be output, and/or a blowback control valve 20 may be opened. The sensor may be an open-ended co-axial probe. The analysis module may comprise a vector network analyser.