NBL-S-HF Soil Heat Flux Sensor

The NiuBoL NBL-S-HF Soil Heat Flux Sensor, also known as a soil heat flux plate or heat flow sensor, is designed to measure the rate of heat transfer through soil, building surfaces and other materials.

The sensing element uses a thermopile structure. A temperature difference across the sensor produces a thermoelectric voltage proportional to the heat flux passing through the plate. Depending on the selected configuration, the measurement can be provided through RS485, 4–20 mA or 0–2 V output.

The NBL-S-HF is suitable for soil energy-balance studies, agricultural meteorology, evapotranspiration research, environmental monitoring, forestry and building heat-transfer measurements.

NBL-S-HF Soil Heat Flux Sensor Technical Specifications

How the NBL-S-HF Soil Heat Flux Sensor Works

The NBL-S-HF uses a thermopile consisting of junctions formed by different metallic materials. When heat flows through the sensor, a temperature difference develops across the plate and produces a thermoelectric voltage.

The generated signal is proportional to the heat flux passing through the sensor. In the basic sensing principle:

Heat Flux (W/m²) = Calibration Coefficient × Sensor Output

Transmitter versions convert the sensor signal into standard analog or digital outputs such as 4–20 mA, 0–2 V or RS485 for easier integration with monitoring systems.

What Does Soil Heat Flux Mean?

Soil heat flux describes the rate at which thermal energy moves through a unit area of soil. It is normally expressed in watts per square metre (W/m²).

The sign of the heat-flux value depends on sensor orientation and the sign convention used by the monitoring system. It should not automatically be interpreted as “summer positive” or “winter negative” without considering installation direction and local conditions.

Soil Heat Flux and Soil Energy Balance

Soil heat flux is an important component of the surface energy balance. In agricultural meteorology and evapotranspiration research, it is commonly evaluated together with net radiation, sensible heat flux and latent heat flux.

Does a Soil Heat Flux Sensor Measure Thermal Conductivity?

The NBL-S-HF directly measures heat flux. Thermal conductivity is a material property describing how easily heat passes through soil or another material.

Thermal conductivity can be studied by combining measured heat flux with an appropriate temperature gradient and experimental method. Therefore, the heat flux plate can be part of a thermal-conductivity measurement system, but it should not be described as directly measuring soil thermal conductivity by itself.

NBL-S-HF Product Features

How to Install the Soil Heat Flux Sensor in Soil

For soil measurement, the heat flux plate should be installed horizontally so that its surface is approximately perpendicular to the primary direction of vertical heat flow.

Why the Sensor Should Not Be Installed Directly on the Soil Surface

Direct surface installation is generally avoided because natural soil surfaces are uneven and exposed directly to solar radiation, wind and air-temperature changes. These conditions can make measurements less representative of actual soil heat transfer.

Installing the plate below the surface provides better contact with the soil and reduces direct radiative effects. The appropriate installation depth should still be selected according to the research method and project requirements.

Wall and Surface Heat Flux Measurement

The NBL-S-HF can also be used for suitable wall or building-surface heat-transfer measurements. The sensor should maintain close thermal contact with the measured surface.

For temporary surface measurement, a suitable thermal-contact material can be applied between the sensor and the measured surface to reduce air gaps. The sensor should be allowed to reach thermal equilibrium before measurements are evaluated.

RS485 Modbus Communication

NBL-S-HF RS485 Wiring

Wiring note: Output configuration varies between RS485, 4–20 mA and voltage versions. Always confirm the ordered model and wiring identification before connection.

Typical NBL-S-HF Soil Heat Flux Sensor Applications

Using NBL-S-HF in an Agricultural Weather Station

For agricultural research and evapotranspiration studies, the NBL-S-HF Soil Heat Flux Sensor can be combined with other meteorological and soil sensors.

Common Installation Errors

NBL-S-HF Soil Heat Flux Sensor FAQ

Q1. What does the NBL-S-HF Soil Heat Flux Sensor measure?

A1. It measures the rate of heat transfer through soil or another measured surface, expressed in W/m².

Q2. What is the measurement range?

A2. The current NBL-S-HF configuration has a measurement range of -200 to 200 W/m².

Q3. What does a negative heat flux value mean?

A3. A negative value indicates that heat is moving in the direction opposite to the sensor's defined positive direction. Interpretation depends on sensor orientation and system sign convention.

Q4. Does the sensor directly measure soil thermal conductivity?

A4. No. It directly measures heat flux. Thermal conductivity can be evaluated when heat-flux data is combined with temperature-gradient measurements and an appropriate calculation method.

Q5. How deep should the heat flux plate be buried?

A5. The instruction manual recommends a typical soil installation depth of approximately 3–10 cm. The exact depth should match the research method and monitoring objective.

Q6. Can the sensor be used on a wall?

A6. Yes. The sensor can be used for suitable wall or surface heat-transfer measurements when close thermal contact is maintained.

Q7. What outputs are available?

A7. Depending on the selected configuration, RS485, 4–20 mA and voltage output versions are available.

Q8. Can it connect to an agricultural IoT system?

A8. Yes. The RS485 version can connect to a compatible data logger, PLC or IoT gateway for local recording or remote transmission.

NBL-S-HF Soil Heat Flux Sensor Datasheet

NBL-S-HF-Soil-Heat-Flux-Sensor.pdf

NBL-S-HF for Soil Energy Balance Monitoring

The NBL-S-HF Soil Heat Flux Sensor provides heat-flow measurements for agricultural meteorology, environmental research, soil energy-balance studies and building heat-transfer monitoring. RS485 and analog output options allow the sensor to be integrated with both scientific data-acquisition equipment and IoT monitoring systems.