help with significant figures on percentages

Projects Programming
1

I made a salt content probe for a project at school and for the percent salt content I need to get at least two significant digits, but I only get whole number percents. if it is not possible to get more than 1 it's okay but I want as many as possible.

// include the library code:
#include <SoftwareSerial.h>

// initialize the library with the numbers of the interface pins
SoftwareSerial mySerial(3,2);

// variables for input pin and control LED and known resistance value
  int analogInput = 1;
  int analogInput1=0;
  float vout = 0.0;
  float vout1 = 0.0;
  float vin1 = 0.0;
  float vin = 0.0;
  float R1 = 50000.0;    // !! resistance of R1 !!
  float R2 = 4400.0;     // !! resistance of R2 !!
  float R3 = 1186.0;

// variable to store the value of voltage resistance and conductance
  int value = 0.00;
  int value1 = 0.00;
  int final_output = 0.00;
  int ohm_value = 0.00;
  int mseimval = 0.00;
  int tds = 0.00;

void setup(){
  // declaration of pin modes
  pinMode(analogInput, INPUT);
  pinMode(analogInput1, INPUT);

  // set up the LCD's number of columns and rows:
  mySerial.begin(9600);
  delay(500);
}

void loop(){
  value = analogRead(1);
  value1 = analogRead(0);
  vout1 = (value1 * 5.0) / 1024.0;
  vout = (value * 5.0) / 1024.0;
  vin1 = vout1 / (R2/(R1+R2));
  vin = vout / (R2/(R1+R2));
  ohm_value = (vin * R3)/(vin1 - vin); //takes the two voltages and converts into an ohm value
  mseimval = (ohm_value ^ -1);  //takes the ohm value and converts it into conductance
  tds = (mseimval * 0.497);   //takes to conductance value times to tds constant of NaCl to get PPM
  final_output = (tds / 10000.0); //divides the PPM by ten thousand to get a percent
  mySerial.write(254);
  mySerial.write(128);
  mySerial.print(final_output); //displays percentage
  mySerial.print("% salinity");
  mySerial.write(254);
  mySerial.write(128);
  delay(500);
  mySerial.print("                ");
}
2

Your list of statements:

  int value = 0.00;
  int value1 = 0.00;
  int final_output = 0.00;
  int ohm_value = 0.00;
  int mseimval = 0.00;
  int tds = 0.00;

make no sense, because an int data type cannot have a fractional component. Also, since R2 and R1 are known at compile time, figure out a constant that equals R2 / (R1 + R2). (The divide operation is pretty slow.) Also, you have expressions like:

ohm_value = (vin * R3)/(vin1 - vin);

where you are using 4-byte floats in the right-hand expression, but assigning it into a 2-byte int in the left expression. This is like taking a four gallon bucket of information and trying to pour it into a 2 gallon pail...you're going to lose information. It would be best to make ohm_value a float, too. At the very least, cast the righthand expression to a float first:

ohm_value = (float)(vin * R3)/(vin1 - vin);
3

ok i changed the code but I still get a 1 significant figure answer any other changes you can think of to change this?
thanks forthe help!

4

mySerial.print(final_output,2); //displays percentage

Pete

6

If you want more sig figs coming out then you have to give more sig figs going in.

Why?
To two significant figures 1/3 is .33
To four sig figs it is .3333
etc.
Same input.

Pete

7

mseimval = (ohm_value ^ -1); //takes the ohm value and converts it into conductance

what is that line supposed to be doing?

just a style comment,

  • some empty lines can improve readability
  • some small comments added
// include the library code:
#include <SoftwareSerial.h>

// initialize the library with the numbers of the interface pins
SoftwareSerial mySerial(3,2);

// variables for input pin and control LED and known resistance value
// CONST VALUES
  const int analogInput = 1;
  const int analogInput1 = 0;
  const float R1 = 50000.0;    // !! resistance of R1 !!
  const float R2 = 4400.0;     // !! resistance of R2 !!
  const float R3 = 1186.0;

// VARIABLE VALUES
  float vout = 0.0;
  float vout1 = 0.0;
  float vin1 = 0.0;
  float vin = 0.0;


// variable to store the value of voltage resistance and conductance
  int value = 0.00;
  int value1 = 0.00;
  int final_output = 0.00;
  int ohm_value = 0.00;
  int mseimval = 0.00;
  int tds = 0.00;

void setup()
{
  // declaration of pin modes
  pinMode(analogInput, INPUT);
  pinMode(analogInput1, INPUT);

  // set up the LCD's number of columns and rows:
  mySerial.begin(9600);
  delay(500);
}

void loop()
{
  // MEASUREMENTS
  value = analogRead(1);  // why not use analogInput   here ?
  value1 = analogRead(0);

  // PROCESSING
  // convert adc to volts
  vout1 = (value1 * 5.0) / 1024.0;
  vout = (value * 5.0) / 1024.0;
  vin1 = vout1 / (R2/(R1+R2));
  vin = vout / (R2/(R1+R2));
  ohm_value = (vin * R3)/(vin1 - vin); //takes the two voltages and converts into an ohm value
  mseimval = (ohm_value ^ -1);  //takes the ohm value and converts it into conductance
  tds = (mseimval * 0.497);   //takes to conductance value times to tds constant of NaCl to get PPM
  final_output = (tds / 10000.0); //divides the PPM by ten thousand to get a percent

  // OUTPUT
  mySerial.write(254);
  mySerial.write(128);
  mySerial.print(final_output); //displays percentage
  mySerial.print("% salinity");
  mySerial.write(254);
  mySerial.write(128);

  delay(500);
  mySerial.print("                ");
}