FAQ sensor & cable

Yes, the cable can be extended. However, great care must be taken to ensure that the contact point of the extension is protected against moisture. Under no circumstances may the contact point be in the ground. Insulating tape and heat shrink tubing do not provide sufficient protection against ground moisture!

Instructions for extending the sensor connection cable can be found here

The TRUEBNER TrueFlex cable is particularly suitable for cable extension. It is designed for buried installation. You can also use other four-core cables (SDI-12 also three-core) that are suitable for underground installation and have a minimum cross-section of 0.25 mm² per core.

The maximum recommended cable length for the SMT50 is about 50 m. If possible, no power lines should be routed in the vicinity of the sensor cable, which could lead to interference. The TRUEBNER TrueFlex cable or a suitable cable with a larger copper cross-section can be used for the extension. If the cable length is long, a small voltage drop occurs on the ground line. This can lead to measured value fluctuations, which, however, are usually insignificant in the practice of irrigation. A larger copper cross-section or averaging of the measured values suppresses the fluctuations.

The maximum recommended cable length for the SMT100 / AquaFlex with analog output signal is about 100 m. If possible, no power lines should be routed in the vicinity of the sensor cable, which could lead to interference. For the extension, the TRUEBNER TrueFlex cable or a suitable cable with a larger copper cross-section can be used. If the cable length is long, a small voltage drop occurs on the ground line. This can lead to measured value fluctuations, which, however, are usually insignificant in the practice of irrigation. A larger copper cross-section or averaging of the measured values suppresses the fluctuations.

RS-485 is a so-called bus system, i.e. several sensors can be connected to one cable harness. If you add up all partial cable lengths, you should not exceed 300 - 400 m if possible. Longer cable lengths can be achieved with active distributors from TRUEBNER.

SDI-12 is a so-called bus system, i.e. several sensors can be connected to one cable harness. If you add up all partial cable lengths, you should not exceed 600 m (typ. 10 sensors à 60 m).

In most cases, the colored wires in the cable are incorrectly connected, the voltage supply is not properly present or there is a cable interruption (animal bite!). Please check the pin assignment and the entire connection cable of the sensor. For sensors with digital interface, check the correct address.

Connect e.g. a 9 V battery to the SMT50 (brown: +, white -). Measure the voltage between green and white (temperature) and between yellow and white (humidity) with a multimeter. At room temperature you should measure about 0.6-0.7 V (see characteristic curve in the SMT50 data sheet). To measure humidity, grip the sensor firmly with your hand. Then there is a clear voltage increase depending on the individual hand humidity.

Connect e.g. a 12 V battery to the SMT100 (brown: +, white -). Measure the voltage between green and white (temperature) and between yellow and white (humidity) with a multimeter. At room temperature you should measure about 6 - 7 V (see characteristic curve in the SMT100 data sheet). To measure humidity, grip the sensor firmly with your hand. Then there is a clear voltage increase depending on the individual hand humidity.

No. The sensor requires a sufficiently large measuring volume. The main measuring volume of the sensor extends approximately in a radius of 5 cm around the sensor. However, the measuring field does not suddenly stop there. The calibration assumes an infinitely extended measuring volume. Therefore, take at least a 30 l bucket of water, with which you can achieve just under 100% moisture display.

Yes. The SMT100 is a material moisture sensor and can determine the water content in many materials. The digital versions of the SMT100 (RS-485 and SDI-12 interface) can output not only the water content but also the raw data and the relative dielectric constant of the material. The conversion from dielectric constant to water content is done with the so-called Topp formula, which is commonly used in soil science. For a material other than soil, a different conversion function is usually required. This function can be determined experimentally. The SMT100 is used for many other materials such as building materials and agricultural products.

The lifetime of the sensors is basically not limited. Long-term experience has existed since 2012, when over a thousand SMT100s were installed in long-term hydrological experiments and are still working today.

The sensors are waterproof and can remain permanently in the ground. The electronics are completely enclosed in epoxy resin. The special cable has a polyurethane matel, which is waterproof, microbe resistant and suitable for underground installation.

The temperature sensor is embedded waterproof in epoxy resin. Due to the heat capacity, it takes a while until it assumes the ambient temperature. This is unproblematic for the application in the ground, because the ground temperature changes only slowly anyway.

The TRUEBNER sensors are maintenance-free. Recalibration is not necessary.