Campbell HFP01 User manual

Model HFP01
Soil Heat Flux Plate
Revision: 7/12
Copyright © 2002-2012
Campbell Scientific, Inc.

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HFP01 Table of Contents
PDF viewers: These page numbers refer to the printed version of this document. Use the
PDF reader bookmarks tab for links to specific sections.
1. Introduction..................................................................1
2. Cautionary Statements................................................1
3. Initial Inspection ..........................................................1
4. Overview.......................................................................1
5. Specifications ..............................................................2
6. Installation....................................................................3
6.1 Placement in Soil......................................................................................3
6.2 Wiring.......................................................................................................5
6.3 Programming ............................................................................................5
6.4 Soil Heat Flux and Storage.......................................................................7
7. Maintenance.................................................................8
8. References ...................................................................8
Appendix
A. General Theory of Heat Flux Sensors...................A-1
A.1 General Theory................................................................................... A-1
A.2 Extended Theory................................................................................. A-2
Figures
6-1. Placement of heat flux plates...................................................................3
6-2. HFP01 plate to datalogger connections...................................................4
A-1. General characteristics of a heat flux sensor ..................................... A-1
A-2. Deflection error ................................................................................. A-2
Tables
6-1. Datalogger Connections for a Single-Ended Measurement ....................5
6-2. Wiring for Example 1..............................................................................5
6-3. Wiring for Example 2..............................................................................6
6-4. Wiring for Example 3..............................................................................7
i

Model HFP01 Soil Heat Flux Plate
1. Introduction
The HFP01 outputs a voltage signal that is proportional to the heat flux of the
surrounding medium (usually soil). It is typically used for energy-balance or
Bowen-ratio flux systems. At least two sensors are required for each site to
provide spatial averaging. Sites with heterogeneous media may require
additional sensors.
Before installing the HFP01, please study
•Section 2, Cautionary Statements
•Section 3, Initial Inspection
The installation procedure is provided in Section 6.
2. Cautionary Statements
•Care should be taken when opening the shipping package to not damage
or cut the cable jacket. If damage to the cable is suspected, consult with a
Campbell Scientific applications engineer.
•Although the HFP01 is rugged, it should be handled as a precision
scientific instrument.
3. Initial Inspection
•Upon receipt of the HFP01, inspect the packaging and contents for
damage. File damage claims with the shipping company.
•The model number and cable length are printed on a label at the
connection end of the cable. Check this information against the shipping
documents to ensure the correct product and cable length are received.
•The HFP01 is shipped with a calibration sheet and an instruction manual
or a ResourceDVD.
4. Overview
The HFP01 Soil Heat Flux Plate uses a thermopile to measure temperature
gradients across its plate. Operating in a completely passive way, it generates
a small output voltage that is proportional to this differential temperature.
Assuming that the heat flux is steady, that the thermal conductivity of the body
is constant, and that the sensor has negligible influence on the thermal flow
pattern, the signal of the HFP01 is directly proportional to the local heat flux.
1

Model HFP01 Soil Heat Flux Plate
The HFP01’s output is in millivolts. To convert this measured voltage to heat
flux, it must be divided by the plate’s calibration constant. A unique
calibration constant is supplied with each sensor.
To get the soil heat flux at the surface, use at least two HFP01s to measure soil
heat flux at a depth of 8 cm; a TCAV Averaging Soil Thermocouple to
measure the temporal change in temperature of the soil layer above the HFP01;
and a CS616, CS650, or CS655 water content reflectometer to measure soil
water content (see Figure 6-1). The temporal change in soil temperature and
soil water content are used to compute the soil storage term.
The -L extension in the model name (i.e., HFP01-L) indicates a user-specified
cable length. This manual refers to the sensor as the HFP01.
The sensor's cable can terminate in:
•Pigtails that connect directly to a Campbell Scientific datalogger
(option –PT).
•Connector that attaches to a prewired enclosure (option –PW). Refer
to www.campbellsci.com/prewired-enclosures for more information.
•Connector that attaches to a CWS900 Wireless Sensor Interface
(option –CWS). The CWS900 allows the probe to be used in a
wireless sensor network. Refer to www.campbellsci.com/cws900 for
more information.
A general discussion about the characteristics and principles of
operation of heat flux sensors is shown in Appendix A.
NOTE
5. Specifications
Features:
•Compatible with most of our contemporary and retired dataloggers
•Compatible with the CWS900-series interfaces, allowing it to be used
in a wireless sensor network
Compatibility
Dataloggers: CR800 series
CR1000
CR3000
CR5000
CR9000(X)
CR7X
CR510
CR10(X)
CR23X
21X
Operating Temperature: -30°to +70°C
Plate Thickness: 5.0 mm (0.20 in)
Plate Diameter: 80.0 mm (3.15 in)
2

Model HFP01 Soil Heat Flux Plate
Weight (w/o cable): 200 g (7.05 oz)
Sensor: Thermopile
Measurement Range: ±2000 W m-2
Sensitivity (nominal): 50 μV W-1 m-2
Expected Typical Accuracy
(12 hour totals): within -15% to +5% in most common soils
Nominal Resistance: 2 W
Sensor Thermal Resistance: <6.25 x 10-3 Km2W-1
2 cm
6 cm
8 cm
Ground Surface
Up to 1 m
2.5 cm
Partial emplacement of the HFP01 and the TCAV
sensors is shown for illustration purposes. All
sensors must be completely inserted into the soil face
before the hole is backfilled.
FIGURE 6-1. Placement of heat flux plates
6. Installation
6.1 Placement in Soil
The standard set of sensors for measuring soil heat flux includes two HF01
Soil Heat Flux Plates, one TCAV Averaging Soil Thermocouple, and one
CS616, CS650, or CS655 water content reflectometer. These sensors are
installed as shown in Figure 6-1.
The location of the heat flux plates and thermocouple should be chosen to be
representative of the area under study. If the ground cover is extremely varied,
an additional set of sensors is required to provide a valid soil heat flux average.
3

Model HFP01 Soil Heat Flux Plate
Use a small shovel to make a vertical slice in the soil. Excavate the soil to one
side of the slice. Keep this soil intact so that is can be replaced with minimal
disruption.
The sensors are installed in the undisturbed face of the hole. Measure the
sensor depths from the top of the hole. With a small knife, make a horizontal
cut 8 cm below the surface into the undisturbed face of the hole. Insert the
heat flux plate into the horizontal cut.
Install the HFP01 in the soil such that the side with the red label
is facing the sky and the side with a blue label facing the soil.
NOTE
In order for the HFP01 to make quality soil heat flux
measurements, the plate must be in full contact with the
soil.
CAUTION
Never run the sensors leads directly to the surface. Rather, bury the sensor
leads a short distance back from the hole to minimize thermal conduction on
the lead wire. Replace the excavated soil back into it original position after all
the sensors are installed.
To protect sensor cables from damage caused by rodents, it is
recommended to bury them inside of flexible electrical tubing.
NOTE
Signal (White)
Signal Reference (Green)
Shield (Clear)
FIGURE 6-2. HFP01 plate to datalogger connections
4

Model HFP01 Soil Heat Flux Plate
6.2 Wiring
Connections to Campbell Scientific dataloggers are given in Table 6-1 and
Figure 6-2. The output of the HF01 can be measured using a single-ended
analog measurement (VoltSE() in CRBasic or Instruction 1 in Edlog) or a
differential analog measurement (VoltDiff() in CRBasic or Instruction 2 in
Edlog).
TABLE 6-1. Datalogger Connections for a
Single-Ended Measurement
Description Color
CR10(X), CR510,
CR500
CR800, CR850,
CR1000, CR3000,
CR5000, CR23X,
21X, CR7,
Signal White Single-Ended Input Single-Ended Input
Signal Reference Green AG
Shield Clear G
6.3 Programming
To calculate the calibration multiplier, divide 1000 by the nominal calibration
sensitivity (i.e., 1000/sensitivity). The nominal calibration sensitivity is unique
for each HFP01. Each sensor is accompanied with a calibration certificate and
has a label on it with the calibration values. The label is located on the pigtail
end of the sensor leads.
In example 1, the nominal calibration sensitivity is 67.1 μV/W-2 for the
HFP01#1 and 67.0 μV/W-2 for the HFP01#2. In examples 2 and 3, the
nominal calibration sensitivity is 67.1 μV/W-2.
TABLE 6-2. Wiring for Example 1
Description Color CR3000
Sensor Signal #1 White 7H
Sensor Signal Reference #1 Green 7L
Shield #1 Clear
Sensor Signal #2 White 8H
Sensor Signal Reference #2 Green 8L
Shield #2 Clear
5

Model HFP01 Soil Heat Flux Plate
Example 1. Sample CR3000 Program using Differential Measurement Instructions
'CR3000 Series Datalogger
'Copyright (c) 2007, Campbell Scientific, Inc. All rights reserved.
'This datalogger program measures two HFP01 Hukseflux soil heat flux sensors
'*** Wiring ***
'7H HFP01 #1 signal (white)
'7L HFP01 #1 signal reference (green)
'gnd HFP01 #1 shield (clear)
'8H HFP01 #2 signal (white)
'8L HFP01 #2 signal reference (green)
'gnd HFP01 #2 shield (clear)
'*** Constants ***
Const HFP01_CAL_1 = 14.90 'Unique multiplier for HFP01 #1. 1000/sensitivity (1000/67.1)
Const HFP01_CAL_2 = 14.92 'Unique multiplier for HFP01 #2. 1000/sensitivity (1000/67.0)
'*** Variables ***
'Energy balance sensors.
Public shf(2)
Dim shf_cal(2) 'Soil heat flux plate calibration coefficients.
Units shf = W/m^2
'*** Final Output Data Tables ***
DataTable (soil_hf,TRUE,-1)
DataInterval (0,30,Min,10)
Average (2,shf(1),IEEE4,FALSE)
EndTable
'*** Program ***
BeginProg
'Load the HFP01 factory calibration.
shf_cal(1) = HFP01_CAL_1
shf_cal(2) = HFP01_CAL_2
Scan (1,Sec,3,0)
'Measure the HFP01 soil heat flux plates.
VoltDiff (shf(1),2,mV50C,7,TRUE,200,250,shf_cal(),0)
CallTable soil_hf
NextScan
EndProg
TABLE 6-3. Wiring for Example 2
Description Color CR10(X)
Signal White SE 5 (3H)
Signal Reference Green AG
Shield Clear G
6

Model HFP01 Soil Heat Flux Plate
Example 2. Portion of CR10(X) Program using the Single-Ended Measurement Instruction
The instruction below does not store data in final storage. P92,
P77, and an output processing instruction are required to store
the data permanently.
NOTE
01: Volt (SE) (P1)
1: 1 Reps
2: 2 7.5 mV Slow Range ;CR510 (7.5 mV);CR23X (10 mV); 21X, CR7 (5 mV)
3: 5 SE Channel ;White wire (SE 5), Green wire (AG)
4: 1 Loc [ HFP01 ]
5: 14.90 Mult ;Enter Calibration
6: 0 Offset
TABLE 6-4. Wiring for Example 3
Description Color CR23X
Signal White 9H
Signal Reference Green 9L
Shield Clear
Example 3. Portion of CR23X Program using the Differential Measurement Instruction
The instruction below does not store data in final storage. P92,
P77, and an output processing instruction are required to store
the data permanently.
NOTE
;Measure the HFP01 Soil Heat Flux plate.
;
01: Volt (Diff) (P2)
1: 1 Reps
2: 21 10 mV, 60 Hz Reject, Slow Range ;CR510, CR10(X) (7.5 mV); 21X, CR7 (5 mV)
3: 9 DIFF Channel ;White wire (9H); Green wire (9L)
4: 1 Loc [ HFP01 ]
5: 14.90 Mult ;Enter Calibration
6: 0 Offset
6.4 Soil Heat Flux and Storage
The soil heat flux at the surface is calculated by adding the measured flux at a
fixed depth, d, to the energy stored in the layer above the heat flux plates. The
specific heat of the soil and the change in soil temperature, ΔTs, over the
output interval, t, are required to calculate the stored energy.
7

Model HFP01 Soil Heat Flux Plate
The heat capacity of the soil is calculated by adding the specific heat of the dry
soil to that of the soil water. The values used for specific heat of dry soil and
water are on a mass basis. The heat capacity of the moist soil is given by:
8
C
dmw bdvww
()
CCCC=+ =+ρθ ρθρ (1)
sb
θρθ
mw
bv
=ρ(2)
where CSis the heat capacity of moist soil, ρbis bulk density, ρwis the density
of water, Cdis the heat capacity of a dry mineral soil, θmis soil water content
on a mass basis, θvis soil water content on a volume basis, and Cwis the heat
capacity of water.
This calculation requires site specific inputs for bulk density, mass basis soil
water content or volume basis soil water content, and the specific heat of the
dry soil. Bulk density and mass basis soil water content can be found by
sampling (Klute, 1986). The volumetric soil water content is measured by the
CS616 water content reflectometer. A value of 840 J kg-1 K-1 for the heat
capacity of dry soil is a reasonable value for most mineral soils (Hanks and
Ashcroft, 1980).
The storage term is then given by Eq. (3) and the soil heat flux at the surface is
given by Eq. (4).
STCd
t
ss
=Δ
GG S
sfc cm
=
(3)
(4)
+
8
7. Maintenance
The HFP01 requires minimal maintenance. Check the sensor leads monthly
for rodent damage.
Recalibrate the HFP01 for every two years of continuous use. Obtain an RMA
number before returning the HFP01 to Campbell Scientific for calibration.
8. References
Hanks, R. J., and G. L. Ashcroft, 1980: Applied Soil Physics: Soil Water and
Temperature Application. Springer-Verlag, 159 pp.
Klute, A., 1986: Method of Soil Analysis. No. 9, Part 1, Sections 13 and 21,
American Society of Agronomy, Inc., Soil Science Society of America,
Inc.

Appendix A. General Theory of Heat
Flux Sensors
This Appendix discusses the general theory and characteristics of heat flux sensors
similar to the HFP01.
A.1 General Theory
constantan
filler
FIGURE A-1. General characteristics of a heat flux sensor
When heat is flowing through the sensor in the indicated direction, the
filling material will act as a thermal resistance. Consequently the heat
flow, ϕ, will follow a temperature gradient across the sensor, flowing
from the hot to the cold side. Most heat flux sensors are based on a
thermopile—a number of thermocouples connected in series. A single
thermocouple will generate an output voltage that is proportional to
the temperature difference between the joints (copper-constantan and
constantan-copper). Provided that errors are avoided, the temperature
difference is proportional to the heat flux—depending only on the
thickness and the thermal conductivity of the sensor. Using more
thermocouples in series will enhance the output signal. In Figure A-1,
the joints of a copper-constantan thermopile are alternatively placed
on the hot and the cold side of the sensor.
The thermopile is embedded in a filling material, usually a plastic.
Each individual sensor will have its own sensitivity, Esen, usually
expressed in Volts output, Vsen, per Watt per square metre heat flux, ϕ.
The flux is calculated:
ϕ=Vsen/Esen
The sensitivity is determined by the manufacturer, and is found on the
calibration certificate that is supplied with each sensor.
When used for measuring soil heat flux, heat flux sensors such as the
HFP01 reach a limited level of accuracy. This has to do with the fact
that thermal parameters of soil are constantly changing (soil moisture
content) and with the fact that the ambient temperature is not fixed. A
realistic estimate of the error range is ±20% over a thermal
conductivity range from 0.1 to 1.7 W/mK (dry sand to water-saturated
sand) across the temperature range of -30° to +70°C. The accuracy is
A-1

Appendix A. General Theory of Heat Flux Sensors
better if the soil conditions are closer to the reference conditions (see
the sensor specifications) and, in an actual experiment, the expected
error range will probably be ±10%.
The reference conditions for the calibration are a thermal conductivity
of 0.8 W/mK and a nominal temperature of 20°C.
A.2 Extended Theory
It is obvious that there is the possibility that the sensor itself can
significantly disturb the phenomenon that it is supposed to measure.
By adding a sensor to the material under observation, you can add
additional, and sometimes differing, thermal resistances.
The deflection error, as shown in Figure A-2, represents the effect
that, as a result of differing resistances, the flow pattern will change,
especially at the edges of the heat flux sensor. The order of
magnitude of this error for strongly different thermal conductivity
values between the sensor and its environment (for example 0.6 for a
typical sensor and 0.03 for an insulating wall) is about 40%.
FIGURE A-2. Deflection error
The heat flux is deflected at the edges of the sensor. As a result, the
heat flow at the edges is not representative.
Apart from the sensor thermal resistance, the contact resistance
between the sensor and surrounding material require special attention.
The conductivity of air is approximately 0.02W/mK which is ten
times smaller than that of the heat flux sensor. It follows, therefore,
that air gaps can form major contact resistances. In all cases the
contact between sensor and surrounding material should be as close
and as stable as possible, so that it will not influence the
measurements.
The aspects of differing thermal properties between sensor and its
environment can also be dealt with during the measurement using a
higher accuracy self-calibrating type of heat flux sensor.
A-2

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