A high sensitivity single-probe heat pulse soil moisture sensor based on a single npn junction transistor

• We present a new semiconductor based soil water content measurement sensor.
• The sensor uses only a bipolar transistor as heating and temperature sensing device.
• Temperature is measured using the very well-known characteristic of VBE(T).
• The sensor has a sensitivity which is better than all previous heat pulse sensors.
• The sensor is low-cost, robust, small, lightweight, and easy to manufacture.

The classic technique for measuring soil water content using heat dissipation properties was employed to develop a new single probe heat pulse sensor (SPHP). The novel sensor is comprised of only one element, a npn junction bipolar transistor, which works as both heating and temperature sensing elements. The variations of VBE(T) as a function of the temperature are used to measure the temperature of the sensor and, since the behaviour of VBE(T) is one of the most well studied expressions that describe the bipolar transistor, the sensor’s temperature can be easily measured, with high accuracy. A technique which increases simultaneously the collector-base voltage VCB and the collector current IC of the transistor allows for the application of the heat pulse with low level currents, in the order of 6 mA. The sensor developed in this work presented a ΔTmax range of 9.1 °C between wet and dry soil conditions (θv between 5% and 35%), while the current available heat pulse probe sensors present a typical ΔTmax range of only 1.5 °C, showing the potential benefits of using a single bipolar junction transistor as a high sensitivity soil water content measurement sensor.