User manual

Hukseflux Thermal Sensors

HFP01SC Manual v0811 page 5/33

Introduction

The HFP01SC self-calibrating heat flux sensor

is a sensor

intended for high accuracy measurement of soil heat flux.

Also it offers improved quality assurance of the measurement.

The on-line calibration (the Van den Bos-Hoeksema method)

automatically corrects for various common errors, in particular

those due to the non-perfect matching of the thermal

conductivity of sensor and soil, and due to variations of the

thermal conductivity of the soil caused by varying moisture

content.

The self calibration can be used only if the sensor is completely

surrounded by at least 40 mm of the object.

A typical measurement location is equipped with 2 sensors for

good spatial averaging.

HFP01SC is a combination of a heat flux sensor and a film

heater. The primary purpose is to estimate the heat flux through

the surrounding soil. The HFP01SC output is a voltage signal that

is proportional heat flux through the sensor. The film heater that

is mounted on top can be activated to perform a calibration (see

chapter 1.3.), resulting in a new calibration factor that

compensates for the errors made under the circumstances of

that moment. Implicitly also cable connection, data acquisition

and data processing are tested. Also errors due to temperature

dependence and instability of the sensor are eliminated. The

result is a much improved accuracy & quality assurance of the

measurement (relative to conventional models such as model

HFP01).

The self-calibrating method has been developed at Hukseflux.

The main sensor of HFP01SC is a normal heat flux sensor.

The self calibration is performed by activating the film heater. It

is temporarily switched on to perform a heating cycle of around 3

minutes. During this cycle the normal heat flux measurement is

disabled.