Measurement Accuracy Factors

What affects WET150 VWC and EC accuracy, and how to avoid common errors.

Installation drives accuracy. Most “bad data” issues come from placement and poor substrate contact.

Use VWC Sensor Setup as the baseline. Then use this page as a checklist.

Quick accuracy checklist

• Prongs fully embedded. No exposed metal.
• No air gaps around prongs.
• Sensor is in the main root zone.
• At least 30 mm from container edges.
• At least 100 mm from other sensors (body and prongs).
• Correct calibration profile selected.

Installation factors

Positioning (height and orientation)

Water is not uniform in a pot or slab.

• Lower placement often reads wetter. Drainage accumulates at the bottom.
• Higher placement often reads drier. The top dries fastest.
• Side (horizontal) installation best matches typical profile assumptions.

If you change height or orientation, expect the absolute VWC to shift. Compare trends, not single values.

Edge effects and nearby sensors

Edges dry and heat faster. They also have different root density.

Keep the sensor away from edges and from other probes.

Follow the separation rule from setup: 100 mm minimum between sensors. Measure between both bodies and prongs.

Air gaps (the #1 failure mode)

Any prong exposure to air will make VWC read too low. It can also destabilize ECp.

Common causes:

• Reusing the same holes after reinstall.
• Substrate settling after moving pots.
• Cable weight pulling the sensor.

Support the cable. Reinsert in a fresh spot. Re-wet to runoff after reinstallation.

Substrate homogeneity

Install in a uniform section. Avoid:

• loose pockets
• debris
• poor mixing
• extreme compaction changes

This matters most in potting soils and mixes.

Roots, stones, and metal

Nearby objects can distort the sensor field. Keep clearance around the prongs.

Avoid placing prongs directly against rocks, dense roots, or metal. Aim for 25 mm of clearance when possible.

Configuration factors

Calibration profile selection

Using the wrong profile is the fastest way to get wrong absolute VWC.

Use Base Station and Aranet Cloud Configuration to select a profile that matches your substrate.

If your substrate is unusual, consider custom calibration. It is often the only way to improve absolute accuracy.

ECp model sensitivity at low VWC

ECp is calculated from ECb and VWC. It becomes sensitive when VWC is very low.

If the sensor is in air or has poor contact, ECp can spike or go negative. See Viewing Sensor Data in Aranet Cloud.

Expected variability between sensors

Two sensors in the “same” pot can disagree. That can still be normal.

Reasons:

• unit-to-unit tolerance
• micro-differences in contact and packing
• real moisture gradients inside the container

Use multiple sensors to learn the facility pattern. Use one sensor as a local reference. Focus on changes over time.

For exact accuracy specs, use the WET150 datasheet. Specs depend on substrate and calibration quality.

Temperature effects (mainly EC)

DP, ECb, and ECp are temperature-dependent.

As a rule of thumb, conductivity in a NaCl solution increases by about 2% per °C.

Permittivity also shifts with temperature. Water permittivity decreases by roughly 0.37 per °C. Expect some drift with temperature swings.