Why is my pressure always negative?

[psycleri]

Hello, in the past few weeks I've been working on a project about measuring the wind speed using the sensor MPX10DP and the amplifier INA125 (pieces given by my teacher to make this project possible) however I've been strugling with the measure of pressure necessary for the wind speed formula. This pressure is always negative and I dont know why. I calibrated the sensor from 0 to 1 kPa (I know it isn't the proper equipment but it's what i have) and I think it has something to do with this. Also when amplifying the signal i notice somthing odd, i get diferent values for the same experiment as you can see in the images below.

void setup() {
  Serial.begin(9600);
}

void loop() {  
  int value=analogRead(A0);
  float Vout=value*4.9; //Voltage in mV
  
  float Pressao=0.2142*(Vout/57.097) - 5.7304; //after calibration 
  float root=2*Pressure*1000/1.225; 
  float Velocidade=((float)sqrt(raiz)*3600/1000);

  Serial.println(Pressao*1000);//i only print the pressure in Pa because if its positive i will have some velocity
  
  delay(100);
}

The results below are both for the Rg of 1kohm
Btw to make it clear what i'm asking is for guidance in what to do next due to this results i'm having.
Thx for your time

[Railroader]

What does Your teacher say?

[DVDdoug]

The only thing I see "negative" is "- 5.7304".

There could be something wrong with your amplifier circuit, or it could be your formula/algorithm.

You might look (print-out) at the raw readings (either "value" or "Vout") to see if they are correct at known pressures. And/or, you can check the voltage with a multimeter. If you have a multimeter that's sensitive enough, check the unamplified voltage out of the sensor.

If those readings are bad you've got a hardware/amplifier problem and if they are correct you've got a software problem

[srturner]

Can you post a diagram of your setup?

If you're trying to measure air speed by the Venturi effect, the measured pressure should be negative (relative to atmospheric).

S.

[JCA34F]

What is your ADC value with zero pressure on BOTH ports of the MPX10DP?

[dlloyd]

Some things to check
What you should calibrate ... the raw A0 value or pressure value?
What happens to your numbers as you go from the raw A0 value to your final printed result
What happens to your final resolution ... each chart shows about 20 Pa? per step

Some things to consider
What is the Min and Max values you'll ever need to work with?
Do you need to contstrain, scale or shift readings prior to calibration?

Something to try
Use a for loop to cycle through all possible values to see what results you get.

[psycleri]

Thanks for all the awnsers. A few things:
I am trying to measure the air speed by a pitot tube (in this case a prundtl tube)
The ADC value on both of my exits of the sensor are 3.5mV (positive) and 3.2/3.3mV(negative) wich is okay (i cant measure more accurate because my multimeter cant read more)
When i calibrate the pressure sensor i do it by using water coluns of around 0 to 5 cm (i know again ins't the rigth sensor for this porpose) that means i get pressure from 0.05 Pa to 0.25 Pa, however because of the linearity of the equation i can calculate speed up to 60km/h (37.28mph).

[dave-in-nj]

Sensor span is 10kpa
Undefined Accuracy is 1%

Combined accuracy (error)
Is the sum of sensitivity hysteresys, repeatability.... etc

It appears that the sensor is not capable of offering the results you seek.

You cannot fix a hardware problem in software.

[JCA34F]

You said you were amplifying the sensor signal with an INA125 with a Rg of 1K, an amplification factor of 64. The sensor datasheet says, with zero pressure the offset voltage is about 20mV, 0.02 * 64 = 1.28V. So, with zero pressure on both sensor ports, what is the INA125 output voltage?
BTW: A 5cm water column is 490Pa, 0.49kPa.