Abstract: Apparatus for measuring or detecting a chemical entity in a liquid sample, the apparatus including a first module operationally connected to a second module, the first module having a flow passage for the sample, the flow passage including a sensor for measuring or detecting the chemical entity and pump means for advancing the sample along the flow passage, the second module including means for actuating the pump means, and the second module being connected to the first module via connecting means permitting disconnection of the first and second modules and connection of a replacement first module to the second module.