Voltage from water sensor

Other Hardware General Electronics
1

I am trying to make a battery powered water sensor similar to this project. I have the basic functionality working fine but it is behaving a little odd and I don't have enough knowledge of electronics to know why.

The setup is basically a voltage divider with one of the resistors variable dependent on the water detected. This voltage is read by an analogue pin. The other ends of the divider I have attached to digital pins, I set one high and one low and alternate them.

What I cannot understand is that when I read with one pin high and the other low I get one value but when I switch to the other pin being high after about a second delay then the voltage is a fair bit different, consistently so. However, if I put the Arduino to sleep for a few seconds then switch polarity and read then the readings are very similar. Any ideas on why this could be?

Thanks

2

Just to make things a bit clearer, this is the basic schematic:

This is the code I am using to test:

#include <LowPower.h>

const int rPin = 6;
const int wPin = 8;
const int sPin = A0;
int wHigh = HIGH;

void setup() {
  pinMode(wPin, OUTPUT);
  pinMode(rPin, OUTPUT);                 
  pinMode(sPin, INPUT);
}

void loop(){
  int waterLevelRead;

  digitalWrite(wPin, wHigh);
  digitalWrite(rPin, !wHigh);
  delay(200);

  waterLevelRead = (wHigh == HIGH ? analogRead(sPin) : 1023 - analogRead(sPin));    
  
  digitalWrite(wPin, LOW);
  digitalWrite(rPin, LOW);

  if(keepGoing) {
    delay(250);
  else {
    LowPower.powerDown(SLEEP_250MS, ADC_OFF, BOD_OFF);
  }

  wHigh = !wHigh;
}

When keepGoing == true, I get the following values for waterLevelRead from A0:

pin set HIGH Level 0 Level 1 Level 2
wPin 760 680 525
rPin 720 650 500

When keepGoing == false, I get the following values for waterLevelRead from A0:

pin set HIGH Level 0 Level 1 Level 2
wPin 766 685 510
rPin 760 680 510

The voltages when the Arduino goes to sleep between each read are very close to expected readings. The readings when there is simply a delay between readings are ok when wPin is set HIGH but consistently lower than expected when rPin is set HIGH.