soil moisture sensor 4 20ma

4-20mA is an analog current with strong anti-noise ability, suitable for long distance and simple wiring situations, especially suitable for direct reading of existing PLC/DCS systems. RS485 is digital communication and can transmit more data (such as temperature, conductivity), but it requires communication protocol and address configuration, and the bus length is limited. The core of reliability is not the output type, but installation and protection. Kingmach's 4-20mA sensor is IP68 sealed. As long as the terminals are waterproof, there is no problem in long-term operation in the field. If the system has already adopted an analog acquisition module, it is easiest to use 4-20mA.

Common probe lengths range from 6cm to 15cm. Kingmach provides customized services and can lengthen the probe as needed to monitor deeper soil. When using, the probe needs to be fully inserted into the area to be measured to avoid leaving any gaps. For high-saline-alkali or clay soils, the sensor will have a universal calibration when leaving the factory, but on-site soil type-specific calibration can improve accuracy. This requires taking a representative soil sample, using the drying method to make several points, and then inputting it into a secondary instrument or PLC for remapping. Kingmach technical support can guide this process.

The common 4-20mA sensor is a two-wire system, that is, the power supply and the signal share two wires. The wiring is simple, but the analog module of the PLC needs to support loop power supply. If the PLC only provides voltage input, an external 24V power supply is needed in series. The four-wire system mainly separates power supply and signal, and is usually used in situations with its own power output. Kingmach provides a two-wire system by default, which can be adjusted if there are special requirements on site. The power supply voltage must be stable. For outdoor solar systems, pay attention to whether the voltage drops below the minimum working voltage of the sensor on rainy days.

Each 4-20mA sensor needs to occupy an analog input channel of the PLC because they are independent current sources. If there are not enough channels, a cost-effective approach is to poll with multiple strobes, but this introduces switching delays and additional points of failure. Another idea is that some nodes use RS485 bus sensors, so that multiple devices can be hung on one bus. Kingmach has sensors with two output modes, which can be flexibly combined according to channel resources.

The electronic part of the sensor itself is very stable and generally does not require frequent calibration. However, after long-term use, the surface of the probe may be scaled or corroded, affecting the readings. For a quick on-site inspection, you can use a multimeter to measure the loop current, and then compare the moisture content of several points with the drying method to see if the deviation is within the acceptable range. If the deviation is large, the probe may need to be cleaned or calibrated on site. Kingmach recommends that in important projects, undisturbed soil samples be taken every six months to one year for verification as preventive maintenance.