The 20% comes from the ±10% accuracy of 1.1volt Aref (not factory calibrated).Meaning YOU have to calibrate (to 0%).
The resulting pressure value of that circuit could depend on supply voltage of the Arduino.If you power the Arduino with regulated 7-9volt on the DC socket, it might be stable.But if you power the Arduino via USB, pressure value might fluctuate.
These specifications tell us the Arduino is capable of measuring voltages to an accuracy of ± 2 LSB - so the maximum error is 2 bits (4 decimal) in 10 bits (1024 decimal)So the worst case accuracy of the converter is 4 / 1024 or 1 part in 256 i.e. 0.25%.
The Arduino has its own voltage references - but they are not very preciseThe chip - depending on type - is provided with SOME of the following reference voltagesDEFAULT: the default analog reference of 5 volts (on 5V Arduino boards) or 3.3 volts (on 3.3V Arduino boards)INTERNAL: a built-in reference, equal to 1.1 volts on the ATmega168 or ATmega328 and 2.56 volts on the ATmega8 (not available on the Arduino Mega)INTERNAL1V1: Vint1 - a built-in 1.1V reference (Arduino Mega only)INTERNAL2V56: Vint2 - a built-in 2.56V reference (Arduino Mega only)EXTERNAL: the voltage applied to the AREF pin (0 to 5V only).(3)However the accuracy of these "reference" voltages is very limited. For exampleDEFAULT: depends on your computer power supply (4: 7.3.2) USB2 Vbus = 4.4V - 5.25V (2: 31. Vint 1 = 1.10V actually 1.00 -- 1.20V(2: 31. Vint 2 = 2.56V actually 2.40 -- 2.80VThe accuracy of the reference against which the voltage is measured - using any of the above references - is at best only 5.25 - 5.0/5.0 * 100 = 5% - much worse than the 0.25%. the ADC provides. Clearly if we wish to measure voltages to the accuracy provided by the Arduino the on-chip references are not good enough. The diagram below shows these errors as compared to a 4.096v reference described below.
Keep the rest of the (protection) parts the same, but add a diode (1N4004) across the sense resistor (anode to ground) for added polarity reversal protection.Leo..