So, my common mode voltage on the Y and W is 4V. Raising pin 5 voltage to also be ~4V required the use of a 1Mohm resistor.
4 volts common mode is too much for a 5 volt supply.
But you had eight...that's more headroom for you.
Now to pin 5... observe there's going to be some small current out of it that'll give a drop across any external resistance in series with pin5.
You'd do better to raise pin 5 by use of a lower ohm voltage divider - say a couple of 1K resistors in series across the supply, will give you Vsupply/2 at their junction. better yet would be to use an opamp buffer between the resistive divider and pin 5 for effectively zero ohms in series with pin 5. Here's what they say about resistance in series with pin 5...
page 7, "Applications information"
The output is referred to the output reference (Ref) terminal which is normally grounded. This must be a low-impedance connection to ensure good common-mode rejection. A resistance of 8 Ω in series with the Ref pin will cause a typical device to degrade to approximately 80dB CMR.
LM324 is a garden variety opamp that's cheap and useful for such tasks. Its common mode input voltage goes from zero to about 1.5 volts short of positive rail.
Question, does the graph on page 4 scale in a linear way for supply voltages other than ±5?
It is not intuitive what those graphs are showing you.
Best way to answer your question is:
Take a look at the adjacent graph on page 4, the one for for +/- 15 volt supply.
It shows that input common mode, on left vertical axis, can swing from at least -10 to +10, with slight shift positive as output goes more positive.
As i said earlier, the opamp wants a few volts between its input pins and the supply pins (supply voltage is often called "Supply Rails" - ie a hard limit you cant't go outside of).
The area enclosed by that parallelogram is the operating range of your INA, ie locus of desirable inputs.
Now back to your right hand graph
With +/- 5 volt rails and pin5 halfway between them at 0, your input common mode needs to be between -3.8 to +2.8 , that range shifting more positiove as output goes more positive.
Interesting ; 4 volts is too much common mode for even +/5 volt supply. But you had eight/zero , so it at least sort of worked for you.
Now - the little rectangle for +5/0 shows how severely limited is your input CMV range for that small supply.
There's a better explanation 'why' than i could compose at bottom of page 8, paragraph "input common mode range".
So your next step is to get supply voltages that are suitable for your input CMV from the Quarry scale,
and place pin5 ~halfway between them through low impedance.
Can you steal power from the Quarry scale? They solved this problem somehow. Can you locate its power supply rails on the circuit board?
here's a gizmo that'll turn a single supply into dual.
http://intronics.com/products/pdf/dc200.pdf
Wall-warts are plentiful in thrift shops
perhaps one of these little dual supplies and a wall wart to power it would do. That's how i power my breadboard.
From my other post above the first push jumps up about 400 units on the graph, but with this setup it jumps up only 200. Is it fair to call that a lower resolution? Is that because of the lower range of input voltage (6 compared to 8)?
I strongly suspect you are running into limits because of your small supply voltage combined with the 4 volts common mode from that Quarry scale.
Were your two bridge input wires nearer zero you'd have done well with the +/-3.3 and pin 5 to common i think.
You are progressing remarkably here - you've got it almost working great, keep on pushing !
old jim
