So, just took a peak at a LM series regulator (who knows what your using) here:
https://www.google.com/url?sa=t&rct...uHDK-7j9MtCjWpQ&bvm=bv.74649129,d.aWw&cad=rja
You have the initial voltage or probably what you calibrated with, 4.8 to 5.2 or whatever. Drift is 1 mV/C, ripple is all over the map depending on load up to 100 mV and junction temperature effects.
Those can also be applied to your 12 V? supply.
Now take a look at a reference IC (REF-02):
https://www.google.com/url?sa=t&rct...-9M0KfIT3UWPzaCeDX_w69A&bvm=bv.74649129,d.aWw
There is a BIG difference.
It may not be enough to matter, e.g. If the 12 V regulator is basically only powering the sensor. Then your only real effect is regulator temperature. The sensor supposedly is temperature compensated,
All I'm trying to do is make you aware as to where the error sources are.
In some respect, a way to know is to get another sensor and make the output/display ratiometric. Maybe a panel meter with an external reference? In this way, you would have something to compare.
e.g.
https://www.google.com/url?sa=t&rct...u9WqPrF8avaX6jQ&bvm=bv.74649129,d.cWc&cad=rja
These meters used to be made by DATEL and now Murata. When i used DATEL meters they were very easy to use. They say to App not 3, for ratiometric applications.
I used them for a 6 channel mass flow meter, We had to calibrate full scale, so there was a switch that either selected the input from the MFC or 5V and a calibrate pot/attenuator and decimal point adjust,
Each MFC required a +- supply and 5V and the cables/connector had all of the required signals. because of the differnential inputs and the way the setpoint was implemented everything worked fin for manual control. When it came to computer control, there were ground loops I failed to anticipate. I was going to fix it with a circuit, but was voted down becaus eof time, so we went with a 4 channel commercial solution,
The box was developed in the first place because, the OEM's box was cumbersome to use. They suppled MFC's calibrated to specific gasses and had displays such as 20, 10, 100 etc set by plug in resistors. We did our own calibrations because we had mixes of gasses. So, my mods attempted to add easy calibration and computer control to an existing in "in house" box.
In another case, I adopted another manufacturer's MFC to our "standard" readout by putting an IC in the connector.
It now looks as if all of the MFC's are obsolete, but that MFC was "SO EASY" to fix and maintain. The parts that dies were the thermal valve and the thermal sensor for the most part. The only controller that had component changes was for Hydrogen.
So, you have a ratiometric sensor with a non-ratiometric output, so any regulator tempeerature effects and ripple will contribute.
You have a reference that uses a different 'regulator", so temperature effects and ripple matter.
You could make a meter with another sensor to track those effects (i.e. read the pressure meter properly) and you would not have to redesign anything. Just maybe add a "tweek control".