No, using liquid standards to calibrate values between 18 µS (55,555 Ω) and 0.05 µS (18,000,000 Ω) are not practical due to carbon dioxide contamination from the atmosphere.  A solution of 18 megohm-cm will degrade to approximately 1 megohm-cm if allowed to stand in open air.

A resistivity monitor/controller can have its electronics calibrated using a cell substitute.  This is a module that is comprised of accurate fixed resistors that are installed in place of the actual resistivity electrode.  Once the cell sub is installed the electronics can be calibrated very accurately.  Contact your Myron L distributor to obtain information regarding the cell sub.

Source: Myron L FAQ