Watermark 200SS Soil Moisture Sensors

The Watermark 200SS from Watermark/Irrometer Company, Inc. is a robust, permanent soil moisture sensor designed to measure soil water tension (matric potential) by using a resistive granular matrix sensing element. When installed in the field, the sensor exchanges water with surrounding soil until equilibrium is reached; as the soil dries, resistance increases, as it wets, resistance decreases.

It offers stable calibration, minimal maintenance, and durability (including resistance to freezing and soil salinity) — making it ideal for long term monitoring of soil moisture status in agriculture, irrigation scheduling, environmental monitoring and more.

Key Features / Highlights

• • Proven stable factory calibration — no site specific calibration needed for standard installations.
• • Wide measurement range of soil water tension: up to approx ~0–239 centibar (cb) (~0 239 kPa) depending on reading device.
• • Durable stainless steel body plus ABS plastic end caps and hydrophilic fabric covered granular matrix — ensures longevity in soil.
• • Not affected by freezing soils; designed for permanent installation in soil throughout seasons.
• • Internally compensated for common soil salinity levels (buffering via internal gypsum) — improves accuracy in irrigated/saline soils.
• • Low cost, minimal maintenance; sensors can be used with handheld meters, data loggers or integrated into automated systems.

Technical Specification

Application

• • Irrigation scheduling: Monitoring soil water tension to determine when to irrigate, thereby optimising water use in agriculture and landscaping.
• • Agronomy & crop research: Tracking soil moisture status and water availability for plants in research plots, orchards and fields.
• • Environmental & monitoring networks: Long term installations in soil profiles for water balance, hydrology and ecosystem studies.
• • Leak detection & soil moisture change detection: Sensor can detect changes in soil moisture for infrastructure applications.
• • Permanent monitoring in landscapes, lawns, sports turf etc where a simple, reliable soil moisture sensor is needed.

Benefits / User Advantages

• • Reliable and proven technology (since 1978) — trusted by researchers and practitioners.
• • Permanent installation: no need to remove seasonally, saving labour.
• • Low maintenance and durable: suitable for harsh environments including freezing soils and salinity.
• • Cost effective: compared to more complex sensors, offers good value for long term monitoring.
• • Easy integration: can be used with handheld meters, data loggers, or adapted for system integration (e.g., analog output adapters).
• • Helps improve irrigation decisions, conserve water, understand plant water availability.

Best Practice & Considerations

• • Ensure proper installation: place sensor at the correct depth and in good soil contact; avoid large air gaps.
• • Use a compatible read out device or adapter: because the sensor gives resistance which needs to be converted via a meter or logger.
• • If using for automated monitoring, consider the voltage adapter (e.g., model 200SS VA) to convert sensor output to voltage (0–3 V) for easier interface.
• • Although calibration is stable, ensure reading device is configured properly (units in cb or kPa) and account for soil type conditions.
• • For very dry soils (tension beyond sensor range) or special media, check range suitability (0 239 cb).
• • Periodically inspect wiring/cables for damage, especially in outdoor installations.
• • In saline soils, though internal compensation exists, check performance over time to ensure accuracy.