User manual

Hukseflux Thermal Sensors
HFP01SC Manual v0811 page 6/33
The self calibration results in an improved sensitivity estimate for
the heat flux measurement. The HFP01SC design dates from
1998. In its first years of existence the HFP01SC has rapidly
been accepted as the state-of-the-art method for measurement
of soil heat flux in meteorology.
In meteorological applications there are two reasons for the
popularity of HFP01SC. The first reason is that it offers a higher
than usual level of accuracy, the second is that it offers a higher
than usual level of quality assurance of the measurement.
The accuracy of soil heat flux measurements very much depends
on the matching of the sensor thermal conductivity to that of the
surrounding medium. A typical heat flux sensor (Hukseflux
HFP01) has a thermal conductivity of 0.8 W/mK, while soils can
vary between extremes of 0.2 and 4 W/mK. Sand in relatively
dry condition can have a thermal conductivity of 0.3 W/mK
(perfectly dry 0.2) while the same sand when saturated with
water reaches 2.5 W/mK. A typical sensor performing a correct
measurement in dry sand will make a – 16% error in wet sand.
As in wet sand the heat tends to travel around the badly
conducting sensor, the flux will be underestimated by 16%.
This example serves to illustrate that in soils where conditions
vary the so-called “thermal conductivity dependence” leads to
large deflection errors.
The second important error is temperature dependence.
Over the entire temperature range from -30 to + 70 degrees C,
the temperature error is +/- 5%. Taking the worst case soil,
pure sand, for the conventional heat flux measurement in
meteorology the overall worst case accuracy is estimated to be
+8 /- 24%. This is rounded off to +10 / -25 %.
HFP01SC, by using the possibility to calibrate itself, eliminates
both errors. For the HFP01SC we estimate +/- 3%, which
essentially equals the initial calibration accuracy.