Data Scaling & Filtering

Data Scaling & Processing

The NPK sensor communicates via the Modbus RTU protocol, returning raw 16-bit integer values for Nitrogen (N), Phosphorus (P), and Potassium (K). These raw values must be scaled and adjusted to reflect accurate mg/kg (ppm) concentrations.

Raw to Physical Value Conversion

By default, the raw values received from the sensor registers are typically mapped at a 1:1 or 1:10 ratio depending on the sensor model. This project uses a scaling factor to convert these integers into floating-point numbers for higher precision.

The conversion formula is: $$\text{Final Value} = (\text{Raw Value} \times \text{Scale Factor}) + \text{Offset}$$

Configuration Parameters

Users can fine-tune the output by modifying the following variables in the configuration section of the sketch. These are essential for matching sensor readings with laboratory soil test results.

Example Usage

If your Nitrogen sensor returns a raw value of 142 but the actual concentration is 14.2 mg/kg, configure your scales as follows:

// Calibration settings in the sketch
const float scaleN = 0.1;  // Adjusts 142 to 14.2
const float offsetN = 0.0;

const float scaleP = 0.1;
const float offsetP = 0.5; // Example offset correction

const float scaleK = 1.0;  // Direct 1:1 mapping

Data Filtering (Noise Reduction)

To prevent erratic readings caused by electrical noise in the RS485 line or soil heterogeneity, a basic Simple Moving Average (SMA) filter is recommended if you are integrating this into a production environment.

While the basic sketch provides raw real-time output, you can implement a software filter by averaging multiple samples:

// Example: Implementation of a 5-sample average
float getFilteredN() {
  float sum = 0;
  int samples = 5;
  for(int i = 0; i < samples; i++) {
    sum += readSensorNitrogen(); // Internal function to fetch raw data
    delay(100); 
  }
  return (sum / samples) * scaleN + offsetN;
}

Validation

To ensure the scaling is working correctly, observe the Serial Monitor. The status should report OK. If values remain at 0 or 65535 (0xFFFF), check your RS485 wiring and the SENSOR_ADDRESS configuration, as these indicate communication timeouts or transmission errors rather than scaling issues.