I gather on all 4 sensors you get a single output connection which is referred to power supply negative? Or if there is a - output connection, it is the same as power supply negative?
The best way is to run a single wire from negative on the 24VDC supply to SGND on the U3.
To avoid errors due to the resistance of SGND, you might need to use GND instead. Tell us more about the 24VDC supply. Is it an AC/DC supply? Does it connect to anything besides these 4 sensors? Do you think the negative of the supply is connected to earth/chassis/wall ground?
Ok so I've just checked, the 0V output from each of the sensors is connected to the 0V of the 24VDC power supply, therefore I only need to connect the +10V output from the sensors to the Labjack (I assume AIN0-3 are the best inputs to use?) And then I will also connect the 0VDC from the power supply to either the SGND or GND of the Labjack.
The 24VDC power supply only powers the sensors, and will be ran from through a transformer which is connected to the same 230V AC power supply as the computer that we are using to power the Labjack.
Just out of interest what is the difference between SGND and GND on the Labjack? I notice that there are 2 SGND connections, and 5 GND? Are all GND's connected together internally, or are they there to provide separate GND's for signals etc?