User manual

Hukseflux Thermal Sensors
HFP01SC Manual v0811 page 11/33
Figure 1.2.2 The resistance error: a heat flux sensor (2)
increases or decreases the total thermal resistance of the object
on which it is mounted or in which it is incorporated. An
otherwise uniform flux (1) is locally disturbed (3).
A first order correction of the measurement is:
ϕ = (R
thobj
+R
thsen
) V
sen
/ E
sen
R
thobj
1.2.2
In addition to the resistance error, the fact that the medium
thermal conductivity differs from the sensor thermal conductivity
causes the heat flux to deflect. The resulting error is called the
deflection error. The deflection error is determined in media of
different thermal conductivity by experiments or using
theoretical approximations. The result of these experiments is
laid down as the so-called thermal conductivity dependence E
λ
.
The order of magnitude of E
λ
is constant for one sensor type. For
HFP01 and HFP01SC, E
λ
is given in the list of specifications
E
sen
= E
sen, cal
(1+E
λ
(λ
cal
- λ
med
)) 1.2.3
Note: this correction can only be applied when there is a
substantial amount of (at least 40 mm) medium on both sides of
the sensor. In soils λ
med
usually is not known. The value of λ
cal
typically is zero.