How to appropriately connect the FS1012-1002-C01-LQ to a Opamp and to the arduino?

Hello, I recently acquired the FS1012-1002-C01-LQ flow rate sensors for my thesis experimentation setup and I have been having troubles with the output voltage readings in my Arduino. I was wondering if I could be helped with this. I will provide more information. My experimental setup consists on 2 12V DC water pumps and the 2 flow rate sensors to monitor the flow rate these pumps are supplying and I plan to create a simple P-controller using the information of the sensors on flow rate to control the speed of the motor through voltage. In my circuit I have two things going on. The first part of the circuit, consists on having a N-type MOSFET transistor that is used to control the amount of voltage i am supplying to the 12V DC motors and I am using PWM to control the gate of the transistor. The second part of my circuit I have these FS1012-1002-C01-LQ flow rate sensors connected to an Opamp named Opa244 and I am using the single ended circuit (using the non inverting configuration) with the TP2 since I only want to measure fluid. Here I share my circuit configuration.

The problem that I am having is the following:

1. The arduino is reading nonsense values when reading the voltage at the Opamp output but when I disconnect the ground of the arduino that is connected to the transistor circuit, the voltage values that the arduino is reading gets "fixed" and reads a constant value. What is even more strange is that if i send a signal to the gate of the transistor, the voltage readings of the sensor lowers as if there is flow passing by but because the ground is disconnected from the arduino to the transistor circuit the motors do not activate therefore no flow is actually. happening.

2. In the diagram, I use 2 1K ohm resistors for the Opamp circuit but it is just an illustration. I am wondering what is the best way to set the Gain if the behavior of the sensor is not fully linear as from 0 to 0.1 SLPM it has a different slope than from 0.1 to 1 SLMP. Initially I set a gain of 50x because the range that I will be operating is from 0 to 0.1 SLPM but any tips on how to appropriately find the correct gain is also much appreciated. 

I tried connected a grounding cable to both the transistor circuit and Opamp and I still get the same result of nonsense voltage readings. I also tried using a different power source than the arduino 5V to power both the sensor and Opamp and still the same.

This is my first time using Opamps so I am still learning how to use them. In theory it seems simple enough but I am having these issues that I do not know how to solve. If more information is needed please let me know.

Here is my code and the spreadsheet of the Opamp.

//Motor 1 digtal pins

int motor1_Controlpin = 3;

//Motor 2 digtal pins

int motor2_Controlpin = 5;

//pulsation delay
int Motor_1_delayTime = 25;
int Motor_2_delayTime = 25;


//Voltage reading
int readPin = A0;
int readBits1;
float Volt1 = 0;
int PrintdelayTime = 50;


void setup() {
  // put your setup code here, to run once:

  pinMode(readPin, INPUT);
  Serial.begin(9600);

  pinMode(motor1_Controlpin, OUTPUT);
 
  pinMode(motor2_Controlpin, OUTPUT);

}

void loop() {
  // put your main code here, to run repeatedly:

  readBits1 = analogRead(readPin);
  Volt1 = (5./1023.) * readBits1;
  Serial.println(Volt1);
  delay(PrintdelayTime);
  
  analogWrite(motor1_Controlpin, 70); // analogWrite (pin, 0) = 0V i.o.w off! 
  delay(Motor_1_delayTime);
  analogWrite(motor1_Controlpin, 0); // analogWrite (pin, 0) = 0V i.o.w off! 
  delay(Motor_1_delayTime);// analogWrite (pin, 255) = 12V i.o.w max speed!

  analogWrite(motor2_Controlpin, 90); // analogWrite (pin, 0) = 0V i.o.w off! 
  delay(Motor_2_delayTime);
  analogWrite(motor2_Controlpin, 0); // analogWrite (pin, 0) = 0V i.o.w off! 
  delay(Motor_2_delayTime);// analogWrite (pin, 255) = 12V i.o.w max speed!
  
  
}