Electronic – Load cells in series

If the scales are AC powered, the 0V in the scales maybe connected back to the AC earth and you are experiencing some problems there. Alternatively, when you measure the 5V on the scales, you might be connecting to -5V and 0V and convincing yourself it is +5V and 0V - this will work fine when the arduino powers it but give nonesense when you use the power from the scales.

What you should do is connect a common ground (0V) between the scales and the arduino and measure what the voltages on the scale are again. If they read negative values or are not how you expected them to be there is likely a mismatch in power supplies. Check also that you still measure about 2.5V on the midpoints.

It's a bit hit-and-miss doing stuff like this and you may get different values but if they are different to what you expect (with the common 0V) then there is a mismatch.

Another possibility is how the scales (when connected to loadcell) do their measurement. You can't rule out that they are doing an AC measurment with dc superimposed - you measure 5V but in fact there maybe a significant AC signal (probabaly between 100Hz and 10kHz) superimposed that your meter is oblivious to.

I've used pressure sensors from Kulite that have been recommended to run at 10V. I contacted them because I wanted to run at 3V with a precision PSU and they said "no problem" but they weren't prepared to "reduce" what the offset accuracy would be.

At 10V they said it's offset was +/-5mV of nominal. Full-scale was +/-100mV and they confirmed that running from 3V would have a full-scale of +/-30mV but, the offset would still be +/-5mV. I could live with this and maybe you can.

A lot of bridges are run with AC excitation in order to avoid DC errors in amplifiers and maybe you could ask them this too because, if it's capable of running on AC then it truly is ratiometric. One word of warning, the temperature compensation techniques used in some bridges may not be linear - I've heard this mentioned but not in connection with strain bridges I've used so it's definitely worth speaking with the supplier.

Regarding noise, what is your bandwidth? If it is low then digital or analogue filtering should help a lot. Regarding excitation, put some kind of current limit into your excitation - you don't want a short on the leads to the bridge dragging the whole supply down.