| Timestamp | Sensor reading | Action | Reason |
|---|---|---|---|
| 08:00 | 17.4 % VWC | Start | Below start threshold, irrigation begins. |
| 08:15 | 19.1 % VWC | Hold | Rising but not yet at stop threshold. |
| 08:35 | 29.2 % VWC | Stop | At or above stop threshold, irrigation ends. |
| 11:30 | 26.8 % VWC | Hold | Within control band, no switching. |
| 15:10 | 16.9 % VWC | Start | Drying trend crosses start threshold again. |
This calculator builds a start/stop band around your target range using hysteresis, noise buffering, and a strategy margin.
hyst_eff = hysteresis × hyst_mult
where hyst_mult depends on the chosen strategy.
start = target_min − (hyst_eff ÷ 2) − noise − marginstop = target_max + (hyst_eff ÷ 2) + noise + margin
low_alert = start − alert_marginhigh_alert = stop + alert_margin
m = (VWC₂ − VWC₁) ÷ (raw₂ − raw₁)b = VWC₁ − m × raw₁VWC = m × raw + b
- Pick a measurement mode that matches your sensor output.
- Select soil texture and crop type to load suggested targets.
- Enter a target minimum and maximum that you want to maintain.
- Add hysteresis and noise buffering to prevent rapid switching.
- Press Calculate thresholds and copy values into your controller.
- Export CSV or PDF to keep records and compare seasons.
Sensor signal behavior and sampling
Soil sensors can drift with temperature, salinity, and installation depth. Log readings at consistent intervals and avoid reacting to single spikes. A rolling average, such as five to ten samples, stabilizes automation. Place the probe at root-zone depth, away from emitters, so the threshold represents plant availability rather than a local wet spot. Label zones clearly.
Target band selection by soil and crop
The target range should reflect your soil’s water-holding capacity and the crop’s tolerance for stress. Sandy profiles drain quickly, so the minimum should be higher and checks more frequent. Clay retains water longer, so the maximum should be lower to limit saturation. For seedlings, keep the minimum wetter until roots expand. Use the built-in presets as a starting point, then refine after observing midday wilt and recovery.
Hysteresis and noise buffering
Hysteresis separates the start and stop decisions so pumps do not chatter near a boundary. Noise buffering adds an extra cushion for sensor jitter, wiring interference, and rapid surface changes. If switching occurs more than once per hour, increase hysteresis first, then noise. For slow systems, such as drip lines, a slightly wider band improves stability and reduces overshoot.
Alerts for maintenance and risk
Low alerts help you detect clogged emitters, broken lines, or unexpected heat load. High alerts identify overwatering, poor drainage, or stuck valves. Set alert margins outside the control band so alarms are meaningful. When an alert occurs, compare sensor readings with soil feel and plant appearance, then inspect filters, pressure, and the irrigation schedule. Recheck alerts monthly and recalibrate whenever you relocate probes.
Runtime estimation and records
The runtime estimate converts the moisture band into a replenishment volume using root depth and irrigated area. It is most useful for benchmarking and seasonal adjustments, not for exact dosing. Save CSV or PDF exports after each tuning session and note weather changes. Over time, consistent records make it easier to standardize thresholds across beds and zones.
What do the start and stop thresholds mean?
Start is the dry point where irrigation turns on. Stop is the wet point where irrigation turns off. Keeping a band between them reduces cycling and helps the root zone stay within your chosen target range.
Should I use VWC or kPa mode?
Use VWC when your sensor reports volumetric water content directly or after calibration. Use kPa when you read soil tension. Choose the mode your device outputs most reliably, then validate it against plant response.
How do I pick a good hysteresis value?
Begin with a small band and increase it until switching becomes stable. If you see rapid on/off events, raise hysteresis before changing targets. Slow irrigation systems usually benefit from a slightly wider band.
What if my sensor readings move in the opposite direction?
Some sensors report higher numbers when soil is drier. In that case, invert your control logic in the controller, or swap your target minimum and maximum definitions. Confirm behavior by testing in dry and wet soil.
How often should I recalibrate raw-to-VWC conversion?
Recalibrate when you change sensor placement, soil mix, or salinity conditions, or when seasonal temperatures shift significantly. A quick two-point check at dry and saturated conditions is usually enough for field accuracy.
Can I reuse one threshold set for multiple beds?
Only if soil texture, irrigation layout, and crop stage are similar. Different beds often have different drainage and emitter coverage. Use one zone as a reference, then fine-tune thresholds for each bed using logged readings.