PPM stands for "parts per million", and it is a unit used to measure the concentration of gases in air.

**What Does PPM Mean?**
1 PPM = 1 part of gas per 1 million parts of air

Example:

100 PPM of CO (carbon monoxide) means 100 molecules of CO per 1,000,000 molecules of air

It’s like saying 1 mg of gas per 1 liter of air, though technically it’s a mole/mole ratio in gases.

**How Do Gas Sensors Estimate PPM?**
[Gas sensors](https://www.onzuu.com/category/gas-sensors) like MQ-2, MQ-135 do not directly output PPM. Instead, they measure changes in resistance (Rs) in response to gas exposure, and the PPM is estimated from that using mathematical models.

**1. Internal Working of an MQ Sensor**
* Sensing material: SnO₂ (tin dioxide)
* In clean air: high resistance
* In presence of gas: gas molecules react with adsorbed oxygen → reduces resistance (Rs)

The sensor outputs an analog voltage based on this resistance.

**2. Calculating PPM: From Resistance to Concentration**
**Step 1: Measure Rs (Sensor Resistance)**
Using voltage divider formula:

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Rs = RL × (Vc - Vout) / Vout

* RL = load resistor (e.g., 10kΩ)
* Vc = supply voltage (e.g., 5V)
* Vout = analog output voltage

**Step 2: Get R0 in clean air (calibration)**

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R0 = Rs_clean_air / known_ratio_from_datasheet

Example:

For MQ-2, clean air ratio is ≈ 9.83 →
R0 = Rs / 9.83

**Step 3: Use Rs/R0 Ratio to Estimate PPM**
From [sensor](https://www.ampheo.com/c/sensors) datasheets, you'll find log-log plots that relate:

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log(PPM) = a × log(Rs/R0) + b

Or:

c

PPM = pow(10, a * log10(Rs/R0) + b);

You derive a and b from the graph for each gas (e.g. LPG, CO, NH₃, etc.)

**Why It's an Estimate**
* MQ [sensors](https://www.ampheoelec.de/c/sensors) are not selective: multiple gases affect Rs
* Environmental factors (humidity, temp) can skew readings
* They are great for trend and threshold detection, not laboratory accuracy

**Example: Estimating LPG Concentration with MQ-2**

c

float RL = 10.0; // kΩ
float Vc = 5.0;  // volts
float Vout = analogRead(A0) * (5.0 / 1023.0);
float Rs = RL * (Vc - Vout) / Vout;

float R0 = 0.6;  // obtained from calibration
float ratio = Rs / R0;

// Approximate for LPG
float ppm = pow(10, -0.47 * log10(ratio) + 1.7);

**Summary**