HMS9000 Capacitive Moisture Sensor

HMS9000 Moisture Probe

MicroTerm hand-held display

Description

The HMS9000 Moisture Probe is a capacitance sensor which provides measurements of volumetric water content.  The sensor uses a capacitance technique to measure the electrical properties of the soil, which are related to and indicative of the soil-water content. 

The sensor was developed by researchers at I.N.R.A. (Institut Nacionale de Recherche Agronomique) in Avignon, France, and is used extensively throughout France and the rest of Europe.

Installation and Operation

The probe is normally installed permanently in the soil and connected to a datalogger (at a weather station, for example) for long-term, automated water-content measurements.  A small hole is augered to the desired depth of measurement, and the sensor is installed in the hole.  Several sensors may be installed at different depths at one location to monitor the water content throughout the profile. 

The HMS9000 measurements differ from those made by tensiometers and the AM400’s resistance sensors in that they indicate the volume of water held in the soil profile. Measurements are expressed on a volume basis, as a volume of water per volume of soil (m³_water/m³_soil). This is often simplified and expressed on a surface-area basis, as a depth of water per depth of soil (mm_water/mm_soil). 

The probe also measures the soil temperature, for use in sensor temperature-compensation and as a stand-alone soil-temperature measurement.  

Volumetric or depth measurements are used to 
•  monitor the absolute amount of water in the soil
•  examine the amount of water used by plants 

•  estimate the amount of irrigation water needed to refill the root zone 

•  estimate the volume of water draining below the root zone 

•  monitor soil-water storage in order to complete water-balance studies.

Calibration

A straightforward, two-step calibration is required by the user in order to obtain more accurate volumetric water content measurements for a specific location. 

The calibration consists of collecting a few soil samples of varying moisture contents, and determining their water contents gravimetrically (the commonly accepted standard method).  Sensor measurements taken at the same time and depths are then correlated with the known water contents.  The resulting linear calibration equation adjusts probe measurements for site– and soil-specific electrical properties, resulting in improved, accurate, and repeatable moisture measurements.

MicroTerm Portable Display

The HMS9000 can also be used portably with the optional MicroTerm handheld display unit.  The MicroTerm displays measurements in real time, allowing the probe to be used to examine the variation in moisture at different locations in a field 

•  determine the depth of wetting in the profile 

•  determine the lateral movement of water around

•  irrigation delivery points (such as drip emitters or sprinklers).

Using the data

The HMS9000 reports water contents on a volume basis, often expressed as a depth of water (mm_water/mm_soil or in_water/in_soil).  In this way, the amount of water in the profile or at different layers in the profile can be estimated.  A water content of 0.25 in_water/in_soil, for example, in the top 2 ft (24 in) of soil would indicate that 6 in of water was held in that 2 ft of soil.  

Water content measurements can be used to estimate the timing and quantity of irrigation water applications.  The grower establishes a critical moisture level for his specific soil and plant conditions, and irrigates when the field moisture level decreases to that point. 

Knowing the maximum amount of water that the soil can hold (its field capacity), the amount of water depleted at the time of irrigation, and the depth of the root zone, the amount of water needed can be calculated. Over-irrigations and deep drainage of water and nutrients can be avoided, and the actual water requirements of the plant determined. 

Care must be taken in using volumetric water content measurements. The amount of water detected by the sensor is the total amount present, but it may not all be available for plant use. Availability still depends on the plant’s ability to extract the water from the soil. If the water is held too tightly by the soil, it may be there but the plant roots cannot use it. Soil sampling and a water retention curve help determine availability.