You must pick a range module that covers your 2/3 of your operating range. If you pick a range module that is too broad, then your accuracy will suffer or it will not show a number on the display. For example, if your operating range is 100-150 microsiemens, a range module of 0-200 microsiemens (-115) would be a good choice.
A range module of 0-5,000 microsiemens (-123) would not be a good choice for this application.

Source: Myron L FAQ