moisture sensor for soil

Both measure the soil dielectric constant, which changes with water content. TDR (time domain reflectometry) sends a fast pulse along a probe and times its reflection; it tends to be more accurate in salty or high‑clay soils but costs more. FDR (frequency domain reflectometry) uses an oscillator and measures the frequency shift; it is simpler, lower power, and works well in typical mineral soils. Kingmach primarily manufactures FDR sensors because they offer a good balance between cost and long‑term reliability for routine geotechnical and agricultural monitoring.

The sensor body is typically 10–20 cm long, and you install it at the depth of interest—whether that is 20 cm for turf root zones or 2 m for landslide shear zones. The depth limit usually comes from the cable and the installation method, not the sensor itself. We supply cables up to 100 m with reinforced jackets, and for deep boreholes we recommend using a rigid PVC conduit to guide the sensor into place and protect the cable during backfilling.

Factory calibration works well for many mineral soils, giving volumetric water content with about ±3% error. But if your site has high organic matter, heavy clay, or very coarse sand, we suggest a site‑specific calibration using the gravimetric method. Our team can provide a simple procedure: take a few disturbed samples at different moisture levels, oven‑dry them, and we will help you work out a polynomial correction. Many data loggers let you store these coefficients so that readings are adjusted on the fly.

Yes, the electronics are potted and the housing is rated IP68, so the sensor can handle freeze‑thaw cycles. What changes is that liquid water turns to ice, so the dielectric reading drops sharply—the sensor is measuring the unfrozen water content. This can actually be useful for detecting frost depth. Just be sure to use a cable with a cold‑flex jacket, which we offer as an option, to prevent jacket cracking.

It depends on the output type. With 4‑20 mA sensors, you need one analog input per sensor, so you are limited by the logger’s channel count. With SDI‑12 digital sensors, you can connect up to 62 sensors on a single bus, each with a unique address. For large‑scale monitoring networks, we recommend SDI‑12 because it simplifies wiring and lowers overall cost. Most of our field technicians can set up a 10‑sensor array with SDI‑12 in under an hour using off‑the‑shelf loggers.