ADC conversion factor

Projects Programming
1

Hi,

I am trying to convert an analog pressure readings into bar values using Arduino Uno.

Reading the pressure with a digital gauge and DMM I found:
Pressure range: 0.00bar to 1.50bar
Voltage range: 0.70V to 1.40V

The voltage conversion result is correct while the bar result is not as you can see in the attached result.
correctV_wrongB
Surely the conversion used to get the bar is not correct.

bar = (avrageAdc - minAdc) * maxBar / (maxAdc - minAdc);

Thanks

2

You simply solve y = mx + c equation; as a result, the raw pressure will come out exactly in the calibrated form.

3

it depend on wich µC you use, due to voltage and ADC bits

4

Thanks both.
I went on averaging the ADC from 50 samples instead of 10 and get better results.

5

@frfn
This is the correct equation
float voltage = analogRead (A0) * 5.0/1023.0
pressure = ((1.5/0.7) * voltage) - 1.5

6 
float pressure=( analogRead(A0)  - 143 ) / 95.333 ;
1 Like
7

Try this sketch with Sensor connected at A0-pin of UNO:

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

void loop()
{
      float digitalBar = (7.5/716.1)*analogRead(A0) - 1.5;
      Serial.print("Pressure: ");
      Serial.print(digitalBar, 1); Serial.prinln(" Bar);
      delay(1000);
}
8

If analogRead is zero you would get a negative pressure, Is that possible?

9

OP will answer to this question as he has clearly given the following two points responses of his sensor based on which the equation is derived by me and by @kolaha of post #6.

A(0.7 V, 0.00 Bar)
B(1.40 V, 1.50 Bar)
Q(V, Bar)
===> Bar = (7.5/716.1)*analogRead(A0) - 1.5
10

Is that a YES or a NO?

11

Theoretically, yes!

12

not correct. because:

a(0.0V, -1.50 Bar)
A(0.7 V, 0.00 Bar)
B(1.40 V, 1.50 Bar)

(7.5/716.1)* 0 is not -1.5

13

Sorry for the mistake!

===> Bar = (7.5/716.1)*analogRead(A0) - 1.5

15

Couple of points .

  1. If your transducer is not a ratiometric device then you will get inaccuracy using the default ADC reference .
  2. The map function will sort your y=mx+c
  3. Beware maths ! You are using small portion of the ADC range ( default ref ) so your resolution will not be great and multiply/dividing by high resolution numbers is meaningless and. Can mislead you ( eg thinking you have 1.005bar when it’s somewhere between 0.9 and 1.1 from ADC Resolution )
  4. Really you need to calibrate with known pressures and read the corresponding ADC numbers and use those numbers in map.
  5. Work in integers , say in 1/10’bar to keep in whole numbers and correct at the output .
1 Like
16

It's not exactly a straight line so my equation will be a little off on the high end

17

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