How to get PPM value from MQ-3?

Question

I tried to build project to detect alcohol with Arduino Uno and an MQ-3 Alcohol sensor.

I already have the R0 value, but I faced problem here: I don't know how to convert it to PPM or BAC. I already tried some code from tutorial for convert it to BAC but when I gave the sensor alcohol the code gives me a value of 0.00, and I don't know why.

Here is the code to find R0 value:

void setup()
{
 Serial.begin(9600);
}
void loop()
{
 float sensor_volt;
 float RS; // Get the value of RS via in a clear air
 float R0; // Get the value of R0 via in Alcohol
 float sensorValue;
 for(int i = 0 ; i < 100 ; i++)
 {
 sensorValue = sensorValue + analogRead(A0);
 }
 sensorValue = sensorValue/100.0; //get average of reading
 sensor_volt = sensorValue/1024*5.0;
 RS = (5.0-sensor_volt)/sensor_volt; //
 R0 = RS/60.0; // 60 is found using interpolation
 Serial.print("R0 = ");
 Serial.println(R0);
 delay(1000);
}

Here is the code to determine BAC:

float R0= 0.28;
void setup() {
 Serial.begin(9600);
}
void loop() {
 float sensor_volt;
 float RS_gas; // Get value of RS in a GAS
 float ratio; // Get ratio RS_GAS/RS_air
 float BAC;
 int sensorValue = analogRead(A0);
 sensor_volt=(float)sensorValue/1024*5.0;
 RS_gas = (5.0-sensor_volt)/sensor_volt; // omit *RL
 /*-Replace the name "R0" with the value of R0 in the demo of First Test -*/
 ratio = RS_gas/R0; // ratio = RS/R0
 float a = pow(ratio, 2); 
 float b = ratio/10;
 BAC = (0.1896*a) - (8.6178*b) + 1.0792 ; //BAC in mg/L
 Serial.print("BAC = ");
 Serial.println(BAC*0.0001); //convert to g/dL
 Serial.print("\n\n");
 delay(1000);
}

I would to know what's wrong with the code, and if you know how to convert the R0 values to PPM, please tell me.

• jsotola: figure 5 in the datasheet shows the relation between the output voltage and PPM value .... cdn.sparkfun.com/datasheets/Sensors/Biometric/… ........... you have to use a lookup table to convert the value

Answer 1

A series of if() statements and some simple division should be able to provide you with your PPM results.

From the datasheet, Alcohol Gas Sensor(Model:MQ-3), figure 5 shows the sensitivity curve:

Sensitivity curve

It can be seen here

Sensitivity curve - three regions

that there are three regions that the curve can be broken into:

1. The relationship is linearly proportional between 2.25 V and 3.25 V
2. Between 3.25 V and 3.8 V there is a curve
3. The relationship is linearly proportional between 3.8 V to 4.25 V

The slope of the straight line regions described in points 1 and 3 can be represented by the use of a constant and the PPM value by a simple division of the voltage by the constant.

The curve from the second part could be:

• approximated by a straight line (thus, once again, using a constant), for simplicity, or;
• more accurately, using a lookup table.

Using a straight line approximation for the curve (shown in lurid green), the graph becomes:

Straight line approximation

To determine the constant for the three lines simply take two points on each line and use:

(y₂ - y₁) / (x₂ - x₁)

So for region one, assuming (by approximation, 3 V corresponds to 100 ppm and 2.5 V corresponds to 85 ppm (my eye sight may be slightly off here), the slope is

(3 - 2.5)/(100-80) = 0.5/20 = 0.025

You can calculate the constants for regions two and three in a similar manner (if using a straight line approximation for region two).

Therefore the code would be alomg the lines of

const double kRegionOne = 0.025; // region one constant
const double kRegionTwo = ???; // region two constant
const double kRegionThree = ???; // region three constant
int ppm; // ppm result
/*
 * your code here
 * ...
 */
if ((sensor_volt < 3.25) && (sensor_volt >= 2.25)) {
 ppm = sensor_volt/kRegionOne; // we are in region 1
} else if ((sensor_volt < 3.8) && (sensor_volt >= 3.25)) {
 ppm = sensor_volt/kRegionTwo; // we are in region 2
} else if (sensor_volt >= 3.8) {
 ppm = sensor_volt/kRegionThree; // we are in region 3
}

If you wish to be more accurate then use a lookup table for region two, as jsotola states in their comment.