Sensor Calibration

Sensor Calibration

While the NPK sensor is factory-calibrated, soil composition varies significantly by region. To achieve professional-grade accuracy, it is recommended to fine-tune the sensor readings using Offset and Scale factors within the Arduino sketch.

Calibration Parameters

The adjustment logic follows a standard linear calibration formula: Corrected Value = (Raw Sensor Reading * Scale) + Offset

| Parameter | Description | Impact | | :--- | :--- | :--- | | Offset (offsetN, offsetP, offsetK) | A constant value added or subtracted from the reading. | Fixes "zero-point" errors or consistent baseline shifts. | | Scale (scaleN, scaleP, scaleK) | A multiplier applied to the raw reading. | Fixes proportional errors (e.g., the sensor is always 10% too low). |

Step-by-Step Calibration Procedure

1. Obtain a Reference: Use a professional soil test kit or a laboratory-analyzed soil sample as your "Ground Truth" value.
2. Measure Raw Data: Set your sketch offsets to 0.0 and scales to 1.0. Take several readings of your reference soil and calculate the average.
3. Calculate Adjustments:
   • Offset: If the sensor reads 10 mg/kg but the lab says 12 mg/kg, set your offset to 2.0.
   • Scale: If the sensor reads 50 mg/kg but the lab says 100 mg/kg, set your scale to 2.0.
4. Update the Sketch: Locate the configuration variables at the top of your .ino file and update them:

// --- Calibration Settings ---
// Adjust these values based on your soil test comparisons

float offsetN = 0.0;  // Add/Subtract for Nitrogen
float scaleN  = 1.0;  // Multiplier for Nitrogen

float offsetP = 5.2;  // Example: consistent 5.2 mg/kg under-read
float scaleP  = 1.0; 

float offsetK = 0.0;
float scaleK  = 0.95; // Example: 5% over-read correction

Best Practices for Accuracy

• Stable Insertion: Ensure the sensor probes are fully submerged in the soil and that the soil is packed firmly around the stainless steel prongs. Air gaps will cause erratic readings.
• Moisture Levels: NPK sensors rely on soil moisture to facilitate electrical conductivity. Calibrate your sensor in soil with the same moisture level you expect during normal operation.
• Averaging: The code outputs values in real-time. For calibration, take the average of 5–10 readings over a minute to account for any minor signal noise in the RS485 bus.
• Multi-Point Check: For the highest precision, perform a two-point calibration by testing a low-nutrient soil sample and a high-nutrient soil sample to find the most accurate linear slope (Scale).