A device called "pill" is provided for in-line measurement of properties, such as pressure and temperature, in a fluid flow within a pipeline system. The device is small enough to travel in the medium of the pipeline system in an unrestricted manner. It has at least one sensor to scan properties such as pressure and temperature at a desired rate. Also, the device includes a microprocessor for logging data scanned by the sensor so that it can later be downloaded and analysed by a PC. Moreover, the device has a buoyancy compensator for adjusting the specific gravity of the device to make it compatible with the medium in which it travels. The novel device is particularly suitable for use in solids transport systems, such as mining backfill systems.