Classification of sensor - pressure transducer, maf sensor

[Testing_Manager]

Hi all,

I have a test bench with pressure transducers and maf sensors.  The pressure transducers are 3 wire, connected to a DIN rail 12 V PSU + and -, the third wire is the output fed to a 6363 AI card with screw terminal block.  The maf sensors are 4 wire: 12 V + and -, ECU ground, signal.   The ECU ground and signal wire are routed through shielded twin core cable, the shield is connected only at the 6363 end to AI GND, the same connection as the ECU ground wire and the signal wire to an AI terminal.  The 12V+ and - are routed on separate unshielded wires to the maf.  The 12V PSU has a common ground with a 24V PSU used for relays (which switch at start of test then stay closed) and a +/-15V PSU powering some hall effect LEM sensors.

I think there are some ground loops in the circuitry as the pressure readings have all shifted slightly after some wires were moved around (although same connection philosophy.)

Question 1: is a pressure transducer classed as a floating signal source, not connected to building ground OR ground-reference Signal Source?  What about a maf sensor?

I'm referencing the DAQ X Series User Manual and have also read the "Field Wiring and Noise Considerations for Analog Signals" paper.  The DIN Rail 12V PSU is powered off the building mains and I presume the neutral is grounded at the building main consumer unit (can't think why we'd have an isolated ring main)

Question 2: are all the AI GND screw terminals linked to each other?  Are they connected internally within the PXI Chassis to the PXI Chassis power supply -ve?  I'm getting up to 300 mV difference between various AI GND points within the terminal block and can't understand how, if they are internally connected.

thanks