User manual

Hukseflux Thermal Sensors
HFP01SC Manual v0811 page 12/33
Figure 1.2.3 The deflection error. The heat flux (1) is deflected in
particular at the edges of the sensor. As a result the
measurement will contain an error; the so-called deflection error.
The magnitude of this error depends on the medium thermal
conductivity, sensor thermal properties as well as sensor design.
In addition, there is a temperature dependence TD reflects the
fact that the sensitivity changes with temperature:
E
sen
= E
sen, cal
(1+TD (T
cal
- T
sen
)) 1.2.4
Combining 1.2.3 and 1.2.4:
E
sen
=
E
sen, cal
{(1+E
λ
(λ
cal
- λ
med
))+ (1+TD (T
sen
- T
cal
))} 1.2.5
Apart from the sensor's own thermal resistance, also contact
resistances between sensor and surrounding material are
demanding special attention. Essentially any air gaps add to the
sensor thermal resistance, at the same time increasing the
deflection error in an unpredictable way. In all cases the contact
between sensor and surrounding material should be as well and
as stable as possible, so that it is not influencing the
measurement. It should be noted that the conductivity of air is
approximately 0.02 W/m.K, ten times smaller than that of the
heat flux sensor. It follows that air gaps form major contact
resistances, and that avoiding the occurrence of significant air
gaps should be a priority whenever heat flux sensors are
installed.