MP406C Soil Moisture Sensor
4-20mA Output for Irrigation Control

Soil Moisture and Irrigation Control

Measuring soil moisture content helps determine when to irrigate, how much water to apply, depth of wetting, patterns of soil moisture extraction by roots, and trends in soil moisture content with time. Soil moisture determines the transport and storage of dissolved nutrients and pollutants and the availability of water to plants.

Soil Water Content

MP406 soil moisture sensor measures dielectric permittivity of the surrounding medium. In soil, dielectric permittivity is directly related to the water content.

The MP406C with integrated 4–20 mA output interface has an analogue current loop output of 4–20 mA which can be used directly by industry standard irrigation controllers and cabling power systems and hence is ideal for use in Parks and Gardens, Golf Courses and similar installations.

Irrigation Control at One or Two Depths

The combinations of one or two MP406C sensors and 4-20 mA output enables connection to an irrigation controller to control the switching of sprinklers on and off. A single MP406C enables control by measuring at one depth whereas the two MP406Cs enable control by measuring at two depths, the outputs of which go into two channels of an irrigation controller. The two channel option can enable switching ON when the upper sensor is dry and switching OFF when water reaches the lower sensor which would be placed at the bottom of the root zone.

A 4 mA output represents 0 mVolt or 0 VSW% and 20 mA represents 1200 mVolt or 100 VSW%.

The MP406C is available in two versions, one designed for 12V operation, the other for 24V.

Ordering Information

MP406C-12V	MP406 Probe with integrated 4-20mA output (12V operation)
MP406C-24V	MP406 Probe with integrated 4-20mA output (24V operation)

MP406C Wiring Information

The following table lists the wiring colours for the MP406C:

Model	Red	Yellow	Black	Blue
MP406C-12V	Supply, 11-16V DC	4-20 mA output	DC Ground (-ve supply)	Analogue signal ground
MP406C-24V	Supply 16-24V DC	4-20 mA output	DC Ground (-ve supply)	Analogue signal ground

The MP406C is supplied with 4.5m of cable. If you require an extension to this length, ICT recommend the use of Hartland HCG304 4-core cable (PDF catalogue) which consists of 4 x 16/0.2 conductors. Note that the conductor colours in this cable are red, blue, green and yellow. It is suggested that the same colours are connected where possible, so that only black changes to become green (DC Ground). If you prefer to source your own cable, you may need to devise your own scheme for mapping the colours.

The maximum cable length of the 4-20 mA loop is limited to a maximum of 250 ohms total loop resistance. This would equate to a maximum length of 3,200m for a 20 AWG wire with approximately 78 ohms per km loop resistance. (39 ohms on each side of the loop).

Polynomial Look-up Table

Linearization tables can be added to Data Loggers using the following data:

A: Converting Data Table for MP-406. From VSW% to mV & mA in mineral soil

VSW%	mV MP406	mA MP406C	VSW%	mV MP406	mA MP406C
0%	0	4.00	55%	1015	18.5
2%	120	5.71	60%	1025	18.64
5%	210	6.99	65%	1035	18.785
10%	310	8.43	70%	1045	18.93
15%	415	9.93	75%	1055	19.07
20%	510	11.285	80%	1065	19.21
25%	610	12.71	85%	1075	19.28
30%	720	14.285	90%	1085	19.43
35%	825	15.785	95%	1105	19.64
40%	895	16.785	100%	1160	20.00
45%	955	17.64
50%	1005	18.35

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MP406C Soil Moisture Sensor 4-20mA Output for Irrigation Control

MP406C Wiring Information

Polynomial Look-up Table

MP406C Soil Moisture Sensor
4-20mA Output for Irrigation Control