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
- Obtain a Reference: Use a professional soil test kit or a laboratory-analyzed soil sample as your "Ground Truth" value.
- Measure Raw Data: Set your sketch offsets to
0.0and scales to1.0. Take several readings of your reference soil and calculate the average. - Calculate Adjustments:
- Offset: If the sensor reads
10 mg/kgbut the lab says12 mg/kg, set your offset to2.0. - Scale: If the sensor reads
50 mg/kgbut the lab says100 mg/kg, set your scale to2.0.
- Offset: If the sensor reads
- Update the Sketch: Locate the configuration variables at the top of your
.inofile 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).