User manual

Hukseflux Thermal Sensors
HFP01SC Manual v0811 page 15/33
1.4 HFP01SC calculation
The heater generally is switched on every three or six hours. The
totoal calibration takes about 6 minutes. During 3 of these 6
minutes, a current is lead through the film resistor for self test,
in order to generate a well-known heat flux. The sensor output
signal, V
sen
, is measured. The difference in voltage output of the
sensor, V
amp
, when heating and not heating multiplied by 2
(because only half of the flux passes the sensor) divided by the
heat flux, ϕ
sen
, is the new sensor sensitivity, E
sen2
. Typically
measurements are done at 0, 180 and 360 seconds.
ϕ
sen
= (V
2
cur
.R
self
) / (R
2
cur
.A
self
) 1.4.1
V
amp
= ABS [ {V
sen
(0) + V
sen
(360)}/2 - V
sen
(180) ] 1.4.2
ABS stands for the absolute value.
E
sen2
= 2V
amp
/ ϕ
sen
1.4.3
Concluding:
E
sen 2
= 2V
amp
(R
2
cur
.A
self
)/(V
2
cur
.R
self
) 1.4.4
For HFP01SC’s heater a typical value is A
self
=38.85 10
-4
m
2
, an
R
cur
is recommended 10 Ω and an R
self
is sensor specific, around
100Ω. With the mentioned values ϕ
sen
= 257.V
2
cur
. Please mind
that V
cur
in this case is about 0.1 times the voltage that is
applied across the total circuit. At a 12-Volt power supply, V
cur
would be 1.09 Volt; the heat flux would be 305 W/m
2
, half of
which would pass the heat flux sensor. Power would be around
0.02 Watt (when switched on every 3 hours).
For type HFP01SC with R
cur
of 10 Ω and an R
self
of 100Ω :
E
sen2
= V
amp
/ 129 V
2
cur
1.4.5
The soil heat flux is calculated as:
ϕ = V
sen
/ E
sen2
1.4.6