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.
| Parameter | Type | Default | Description |
| :--- | :--- | :--- | :--- |
| scaleN, scaleP, scaleK | float | 1.0 | Multiplier for the raw reading (use 0.1 if the sensor returns values in 10x magnitude). |
| offsetN, offsetP, offsetK | float | 0.0 | Additive constant to correct systematic bias (e.g., if the sensor always reads 2 units too high, set to -2.0). |
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.