Campbell Scientific HFT3 REBS User Manual

INSTRUCTION MANUAL

HFT3 Soil Heat Flux Plate

Revision: 10/03

Copyright (c) 1987-2003

Campbell Scientific, Inc.

Warranty and Assistance

The HFT3 SOIL HEAT FLUX PLATE is warranted by CAMPBELL
SCIENTIFIC, INC. to be free from defects in materials and workmanship under
normal use and ser vice for twelve (1 2) months from date of shipment unless
specified otherwise. Batteries have no warranty. CAMPBELL SCIENTIFIC,
INC.'s obligation under this warranty is limited to repairing or replacing (at
CAMPBELL SCIENTIFIC, INC.'s option) defective products. The customer
shall assume all costs of removing, reinstalling, and shipping defective products
to CAMPBELL SCIENTIFIC, INC. CAMPBELL SCIENTIFIC, INC. will
return such products by surface carrier prepaid. This warranty shall not apply
to any CAMPBELL SCIENTIFIC, INC. products which have been subjected to
modification, misuse, neglect, accidents of nature, or shipping damage. This
warranty is in lieu of all other warranties, expressed or implied, including
warranties of merchantability or fitness for a particular purpose. CAMPBELL
SCIENTIFIC, INC. is not liable for special, indirect, incidental, or
consequential damages.

Products may not be returned without prior authorization. The following
contact information is for US and International customers residing in countries
served by Campbell Scientific, Inc. directly. Affiliate companies handle repairs
for customers within their territories. Please visit www.ca mpbellsci.co m to
determine which Campbell Scientific company serves your country. To obtain
a Returned Materials Authorization (RMA), contact CAMPBELL
SCIENTIFIC, INC., phone [phone]. After an applications engineer
determines the nature of the problem, an RMA number will be issued. Please
write this number clearly on the outside of the shipping container.
CAMPBELL SCIENTIFIC's shipping address is:

CAMPBELL SCIENTIFIC, INC.

RMA#_____
815 West 1800 North
Logan, Utah 84321-1784

CAMPBELL SCIENTIFIC, INC. does not accept collect calls.

HFT3 Table of Contents

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1. General Description ....................................................1

2. Specifications .............................................................1

3. Installation...................................................................2

4. Wiring ..........................................................................3

5. Example Programs .....................................................4

6. Soil Heat Flux and Storage.........................................5

7. Maintenance................................................................6

8. References ..................................................................6

Figures

1. Placement of Heat Flux Plates....................................................................2

2. HFT3 Plate to Datalogger Connections......................................................3

Tables

1. Datalogger Connections for a Single-Ended Measurement.........................3

2. Datalogger Connections for a Differential Measurement............................3

3. Wiring for Example 1.................................................................................4

4. Wiring for Example 2.................................................................................4

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Model HFT3 Soil Heat Flux Plate

1. General Description

The HFT3 Soil Heat Flux plate uses a thermopile to measure temperature
gradients across the plate. Each plate is individually calibrated to output flux.

In order to measure soil heat flux at the surface, several HFT3s are used to
measure the soil heat flux at a depth of eight cm. A TCAV Averaging Soil
Thermocouple is used to measure the temporal change in temperature of the
soil layer above the HFT3. Finally, a CS615 Water Content Reflectometer is
used to measure the soil water content. The temporal change in soil
temperature and soil water content are used to compute the soil storage term.

The -L option on the model HFT3 Soil Heat Flux plate (HFT3-L) indicates that
the cable length is user specified. This manual refers to the sensor as the
HFT3.

2. Specifications

Operating Temperature: -40°C to +55°C

Storage Temperature: -40°C to +55°C

Plate Thickness: 3.91 mm (0.154 in.)

Plate Diameter: 38.2 mm (1.5 in.)

Sensor: thermopile

Measurement Range: ±100 W m

Signal Range: ±2.4 mV for the above range

Accuracy: better than ±5% of reading

Thermal Conductivity: 1.22 W m

-2

-1 K-1

1

Model HFT3 Soil Heat Flux Plate

Up to 1 m

3. Installation

2.5 cm

6 cm

2 cm

Ground Surface

8 cm

Partial emplacement of the HFT3 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 1. Placement of Heat Flux Plates

The HFT3 Soil Heat Flux plates, the TCAV Averaging Soil Temperature
probes, and the CS615 Water Content Reflectometer are installed as shown in
Figure 1.

The location of the heat flux plates and thermocouples should be chosen to be
representative of the area under study. If the ground cover is extremely varied,
it may be necessary to have additional sensors to provide a valid average of soil
heat flux.

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 dept hs from the top of the hole. With a small knife, make a horizontal
cut eight cm below the surface into the undisturbed face of the hole. Insert the
heat flux plate into the horizontal cut.

NOTE

Install the HFT3 in the soil such that the side with the white dot is
facing the sky.

CAUTION

In order for the HFT3 to make quality soil heat flux
measurements, the plate must be in full contact with the
soil.

2

White Dot
on Top

Model HFT3 Soil Heat Flux Plate

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.

Signal Black

Signal Reference Whi t e

Shield Clear

4. Wiring

FIGURE 2. HFT3 Plate to Datalogger Connections

TABLE 1. Datalogger Connectio ns f or a Single-Ended Measurement

Description Color CR10(X), CR510 CR23X, 21X, CR7

Signal Black Single-Ended Input Single-Ended Input

Signal Reference White AG

Shield Clear G

TABLE 2. Datalogger Connections for a Differential Measurement

Description Color CR10(X), CR510 CR23X, 21X, CR7

Signal Black Differential Input (H) Differential Input (H)

Signal Reference White Differential Input (L) Differential Input (L)

Shield Clear G

Connections to Campbell Scientific dataloggers are given in Tables 1 and 2.
The output of the HFT3 can be measured using a single-ended analog
measurement (Instruction 1) or a differential analog measurement (Instruction

2).

3

Model HFT3 Soil Heat Flux Plate

The wiring convention is that the black wire is positive with respect to the other
insulated wire, when energy is flowing through the transducer from the side
with white dot to the side without the white dot.

5. Example Programs

This section is for users who write their own dataloger programs. A datalogger
program to measure the sensor can be created using Campbell Scientific’s Short
Cut Program Builder software. You do not need to read this section if you use
Short Cut.

The HFT3 has a nominal calibration of 42 W m
accompanied with a calibration certificate. Each sensor has a unique
calibration label on it. The label is located on the pigtail end of the sensor
leads.

TABLE 3. Wiring for Exa mple 1

Description Color CR10(X)

Signal Black SE 5 ( 3H)

Signal Reference White AG

Shield Clear G

-2

mV-1. Each sensor is

Example 1 Sample CR10(X) Program using a Single-Ended Measurement Instruction

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 ;Black wire (SE 5), White wire (AG)
4: 1 Loc [ HFT3 ]
5: 1 Mult ;Enter Calibration
6: 0 Offset

TABLE 4. Wiring for Exa mple 2

Description Color CR23X

Signal Black 9H

Signal Reference White 9L

Shield Clear

4

Example 2 Sample CR23X Program using a Differential Measurement Instruction

(

∆

;Measure the HFT3 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 ;Black wire (9H); White wire (9L)
4: 1 Loc [ HFT3 ]
5: 1 Mult ;Enter Calibration
6: 0 Offset

6. 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, ∆T
interval, t, are required to calculate the stored energy.

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:

Model HFT3 Soil Heat Flux Plate

, over the output

s

CCCC C

=+ =+ρθ ρθρ (1)

sbdmw bdvww

ρ

w

θ

m

where C
of water, C

θ

= (2)

v

ρ

b

is the heat capacity of moist soil, ρb is bulk density, ρw is the density

S

is the heat capacity of a dry mineral soil, θm is soil water content

d

on a mass basis,

)

θ

is soil water content on a volume basis, and Cw is the heat

v

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
CS615 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).

TCd

ss

S

=

(3)

t

GG S

=+

sfc cm

8

(4)

5

Model HFT3 Soil Heat Flux Plate

7. Maintenance

8. References

The HFT3 requires minimal maintenance. Check the sensor leads monthly for
rodent damage.

Recalibrate the HFT3 every two years of continuous use. Obtain an RMA
number before returning the HFT3 to Campbell Scientific for calibration.

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.

6

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