User manual

Hukseflux Thermal Sensors
HFP01SC Manual v0811 page 9/33
1 Theory of Self-Calibration
1.1 Introduction
This chapter is written for users that are familiar with the general
theory of heat flux sensors and error sources in the heat flux
measurement. In case more information on this subject is
required, literature (most notably the HFP01 manual) is available
free of charge via e-mail as a PDF file. Please request at
[e-mail].
The self calibrating principle can only be used if the sensor is
surrounded by at least 40 mm of soil. It is typically used in
meteorological applications for measurements of the soil heat
flux.
HFP01SC is a combination of a heat flux sensor and a film
heater. The heat flux sensor can, as usual, measure the heat flux
through the surrounding medium. The output is a voltage signal
that is proportional to the local heat flux.
This means that in case of emergency one can use HFP01SC as a
normal HFP01.
By activation of the heater, at reguar intervals an updated
calibration factor is determined.
Every new calibration results in an updated calibration factor
E
sen2
.
The main reasons for this update are changes in soil thermal
conductivity (deflection error) and temperature (temperature
dependence).
When performing this calibration, typically once every 3 or 6
hours (depending on available power), implicitly also cable
connection, data acquisition and data processing are tested.
Temperature dependence is eliminated. The result is a
dramatically improved accuracy & quality assurance of the
measurement (relative to using conventional sensors). The
method is generally referred to as the Van den Bos-Hoeksema
method.
More detailed explanation follows below.