Measurement & Automation FAQs
RESOURCES & SUPPORT
Capacitive sensors react to liquid water in the soil. When there is frost, the liquid water turns to ice. However, ice cannot be measured correctly by the sensor. Soil moisture measurement only works up to the freezing point. However, irrigation at temperatures below the freezing point is normally not useful anyway.
The threshold value for switching on the irrigation depends on many factors, especially on the type of soil (sand, clay, loam) and the plants to be irrigated. Therefore, it is not possible to give a universal threshold. It is best to observe the plants after an irrigation. For example, you can see the onset of drought stress from the leaves and color changes and readjust the threshold. With continuous monitoring of soil moisture and graphing, one can also determine the threshold. After irrigation, the water content usually decreases sharply and then slowly falls to a plateau. In the transition area one can put the threshold value. Another approximation method is used for turf. At the location of the sensor, saturate the soil with water and then wait for 24 h. From the measured soil moisture value after 24 h, one takes approx. 60% as the threshold value. In horticulture and agriculture, several soil moisture sensors are sometimes used at different depths. By means of the temporal changes of the soil moisture one can follow the water transport in the soil. The aim here is to avoid too much water flowing into the lower soil layers, as this may also lead to nitrate entering the groundwater.
Small variations in soil moisture readings are caused by the temperature-dependent dielectric constant of water. Soil consists of water, air and soil grains. The sensor measures the dielectric constant of this mixture and calculates the water content from it. Temperature compensation is not generally possible for natural soils. This is partly because the water in the soil is present as so-called free water and as bound water, which have different temperature dependencies of the dielectric constant. In practical applications, the temperature-dependent variations are negligible.
The volumetric water content in most natural soils can range from 0 to slightly above 50%. At saturation, all air voids are filled with water. I.e. the air void ratio determines the maximum possible water content.
The maximum volumetric water content in other substrates may be higher. In horticulture, substrates with very low density and a large proportion of air voids are sometimes used. Tomatoes are often grown on rockwool and supplied with a nutrient solution. The rock wool can absorb much more water than a natural soil. Similar properties can be observed with humus. Humus has a very high storage capacity and can store 3-5 times its own weight in water.
The SMT50 has an output resistance of 10 kOhm. Most controllers have a much larger internal resistance at their analog voltage input, so that the measured voltage corresponds to the output voltage of the SMT50. In the Loxone controller, however, the input resistance is only 10 kOhm. Therefore a voltage divider is formed and the Loxone controller displays only half the voltage value. As a remedy, one simply multiplies the measured voltage with the factor 2 in the Loxone software and can then use the calibration characteristic curve in the data sheet of the SMT50 again.
The simplest connection to the Loxone control succeeds with the SMT100 analog 0 - 10 V. However, the disadvantage is that you need a separate analog input for each sensor. If you want to measure the temperature in addition to the humidity, you need 2 analog inputs. If there are several sensors, a bus system is more suitable. From Loxone there is a so called Modbus Extension, to which up to 32 sensors can be connected. For this you need the SMT100 RS-485 Modbus version.
There are a variety of options for irrigation control with soil moisture sensors:
• Controls from the field of building automation, e.g. Loxone or KNX
• Controls from the general automation technology (e.g. PLC from various manufacturers)
• Professional irrigation controllers with standard interfaces like 0 - 10 V voltage input
• DIY solutions based on Arduino or with other microcontrollers
• Raspberry Pi based systems (e.g. OpenSprinkler)
• Radio based systems e.g. with the radio standard LoRa
The choice depends on both the requirements and the budget.