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A heat flux plate for use in soil
J . agric, Engng Res. (1971) 16, 420-422
RESEARCH NOTES
A Heat Flux Plate for Use In Soil K. GARZOLl;*
r, BLACKWELL;*
E. S. TRICKE1T*
1. Introduction The rate of heat flow through soil is influenced by variations in thermal conductivity arising from the differences in size, shape and composition of the individual particles , the size of the spaces between these particles, and the presence of air and /or water (in either liquid or vapour phase) in the spaces. In addition to the transfer of heat by conduction, there is often a simultaneous heat and mass transfer due to the movement of water through the soil. This movement is caused by gravity, capillary suction or the existence of temperature gradients, moisture tending to move down the temperature gradient (Hadley & Eisenstadt, 1953). Various instruments have been developed for the measurement of thermal conductivity of soil (e.g. Mann & Forsyth, 1956; de Vries & Peck, 1958; lanse & Borel, 1965). Such instruments allow the thermal conductivity to be calculated from the measured rate of rise of temperature of an electrically heated wire buried in the soil. While this technique is accurate to about ±5 % for the measurement of thermal conductivity, it does not provide a direct and continuous measurement of heat flux. Further, the presence of heated wire upsets the normal flow of moisture through the soil. 2. The heat flux plate In many applications, particularly in agricultural research, it is desirable to measure the heat flux directly, often to an accurac y of no better than about ± 10%. At this level of accuracy the transport of sensible heat due to the movement of water can safely be ignored in most pract ical applications (de Vries, 1963). However, it is important that the instrument should not interfere with the free movement of water through the soil, otherwise a source of error is introduced due to the formation of a film of moisture on the surface of the instrument in the path of the migrating moisture. The instrument described here was developed to satisfy these conditions. Other features that were considered necessary include : (a) simplicity and robustness to enable reliable operation over a long period; (b) an electrical output that could be recorded continuously and , by previous calibration, be related directly to the heat flux. 3. Principle of operation The instrument that has been developed is based on a similar principle to that of the flux plate described by Morris, Trickett & Mousley (1955). This principle involves the sensing of the temperature difference between 2 parallel layers of soil perpendicular to the direction of heat flow. Neither these actual temperatures, nor the temperature difference need to be known since the calibration allows the heat flux to be determined directly. This method also eliminates the need to know the thermal conductivity of the soil and the length of the heat flow path along which the temperature difference occurs. 4. Description of the instrument The instrument consists of a piece of resin-bonded matrix board, l6 in thick (as used in electrical circuits) with a series of thermocouple junctions connected electrically in series and thermally · C .S.I. R .O., D ivision of Irrigati on Research , Gr iffith, New Sou th Wales 2680, Australia
420
421
K. GARZOLI; J. BLACKWELL; E. S. TRICKETT
in parallel i.e., the "hot" junctions are on one side and the "cold" junctions on the other. This means that, for a particular heat flux, the e.m.f. produced is directly proportional to the number of thermocouple junctions. Proprietary thermocouple wires were used (British Driver Harris T.I. and T.2. alloys). Short lengths were threaded through the holes and the junctions formed by spot welding. The assembly was then degreased in an ultrasonic bath with a small quantity of detergent and the junctions bonded to the matrix board with araldite in order to fix them rigidly to the board, to give added strength and rigidity, and to protect the instrument from chemical and bacterial attack while in the soil. The plate was then laid on the bench and covered with a sheet of polyethylene and weighted in order to obtain the best possible adhesion, and minimise projections. Resin-bonded matrix board was chosen for the mounting of the junctions for the following reasons: (1) The holes allow free passage of water; (2) the thickness of the instrument is small, thus enabling the greatest possible accuracy to be obtained (Philip, 1961); (3) the material has a thermal conductivity similar to that of soil. This results in a minimum of disturbance to the heat flow through the soil (Morris, Trickett & Moulsley, 1955). The flux plate is shown in Fig. 1.
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Fig. 1
5. Calibration Each flux plate was calibrated by measuring its output at a known heat flux produced by a flat heat source with a variable power supply. The heat source consisted of a spiral of resistance wire glued to a sheet of resin-bonded paper board. In order to prevent loss of heat downwards or sideways, separate guard spirals were fixed beneath the main heating spiral and around its circumference. The bottom guard spiral was glued to a separate sheet of resin-bonded paper board and separated from that containing the main heating spiral by a sheet of perspex. The side guard spiral was wound on the same board as the main heating spiral, separated by an annular gap of ! in. Each spiral had its own variable power supply and was covered by a thin sheet of copper (insulated electrically by a sheet of fibre glass) to provide uniformly heated surface. Thermocouples were soldered to the copper covering each spiral and the whole assembly was enclosed in foam polystyrene leaving only the upper surface, or main heat source, exposed. For calibrating, the surface of the heat source was covered with soil in which was embedded the flux plate under test with its output connected to a D.C. amplifier which could be monitored continuously. The desired level of power was supplied to the heat source and the supply to the guard spirals was adjusted so that the thermocouples registered no difference in temperature. Under these conditions, all the power to the main heat source is dissipated as heat uniformly over its upper surface. When stability had been achieved for I h, readings of the power supply
422
A HEAT FLUX PLATE FOR USE IN SOIL
to the heat source (converted to B.T.U/ft 2/h) and the output from the flux plate were recorded. Since the output of a thermocouple increases linearly with temperature, the calibration curve is, theoretically, a straight line. However, the calibrating procedure should be repeated, at least once, to check that a straight line relationship in fact exists, and to verify its slope. REFERENCES
Hadley, W. A.; Eisenstadt, R. Moisture movement in soils due to temperature difference. Heat Pip. Air Condit. 1953, 111-114 Janse, A. R. P.; Borel, G. Measurement of thermal conductivity in situ in mixed materials, e.g. soils. Neth. J. Agric. Sci., 1965,13, 57-62 Mann, G.; Forsyth, F. G. E. Measurement of the thermal conductivity of samples of thermal insulating materials and of insulation in situ by the "heated probe" method. Mod. Refrig. 1956,50: 188-191 Morris, L. G.; Trickett, E. S.; Moulsley, L. J. The measurement of heat flow in soil with reference to glasshouse heating. N.LA.E. Tech. Memo No. 111, 1965 Philip, J. R. The theory of heat flux meters. J. geophys. Res. 1961,66: 571-579 de Vries, D. A. Thermal Properties of Soils. In Physics ofPlant Environment (Ed. W. R. van Wijk). North Holland Publishing Company, Amsterdam, 1963 de Vries, D. A.; Peck, A. J. On the cylindrical probe method of measuring thermal conductivity with special reference to soils. 1. Extension of theory and discussion of probe characteristics. Aust. J. Phys. 1958, 11: 255-271,
RESEARCH NOTES
A Heat Flux Plate for Use In Soil K. GARZOLl;*
r, BLACKWELL;*
E. S. TRICKE1T*
1. Introduction The rate of heat flow through soil is influenced by variations in thermal conductivity arising from the differences in size, shape and composition of the individual particles , the size of the spaces between these particles, and the presence of air and /or water (in either liquid or vapour phase) in the spaces. In addition to the transfer of heat by conduction, there is often a simultaneous heat and mass transfer due to the movement of water through the soil. This movement is caused by gravity, capillary suction or the existence of temperature gradients, moisture tending to move down the temperature gradient (Hadley & Eisenstadt, 1953). Various instruments have been developed for the measurement of thermal conductivity of soil (e.g. Mann & Forsyth, 1956; de Vries & Peck, 1958; lanse & Borel, 1965). Such instruments allow the thermal conductivity to be calculated from the measured rate of rise of temperature of an electrically heated wire buried in the soil. While this technique is accurate to about ±5 % for the measurement of thermal conductivity, it does not provide a direct and continuous measurement of heat flux. Further, the presence of heated wire upsets the normal flow of moisture through the soil. 2. The heat flux plate In many applications, particularly in agricultural research, it is desirable to measure the heat flux directly, often to an accurac y of no better than about ± 10%. At this level of accuracy the transport of sensible heat due to the movement of water can safely be ignored in most pract ical applications (de Vries, 1963). However, it is important that the instrument should not interfere with the free movement of water through the soil, otherwise a source of error is introduced due to the formation of a film of moisture on the surface of the instrument in the path of the migrating moisture. The instrument described here was developed to satisfy these conditions. Other features that were considered necessary include : (a) simplicity and robustness to enable reliable operation over a long period; (b) an electrical output that could be recorded continuously and , by previous calibration, be related directly to the heat flux. 3. Principle of operation The instrument that has been developed is based on a similar principle to that of the flux plate described by Morris, Trickett & Mousley (1955). This principle involves the sensing of the temperature difference between 2 parallel layers of soil perpendicular to the direction of heat flow. Neither these actual temperatures, nor the temperature difference need to be known since the calibration allows the heat flux to be determined directly. This method also eliminates the need to know the thermal conductivity of the soil and the length of the heat flow path along which the temperature difference occurs. 4. Description of the instrument The instrument consists of a piece of resin-bonded matrix board, l6 in thick (as used in electrical circuits) with a series of thermocouple junctions connected electrically in series and thermally · C .S.I. R .O., D ivision of Irrigati on Research , Gr iffith, New Sou th Wales 2680, Australia
420
421
K. GARZOLI; J. BLACKWELL; E. S. TRICKETT
in parallel i.e., the "hot" junctions are on one side and the "cold" junctions on the other. This means that, for a particular heat flux, the e.m.f. produced is directly proportional to the number of thermocouple junctions. Proprietary thermocouple wires were used (British Driver Harris T.I. and T.2. alloys). Short lengths were threaded through the holes and the junctions formed by spot welding. The assembly was then degreased in an ultrasonic bath with a small quantity of detergent and the junctions bonded to the matrix board with araldite in order to fix them rigidly to the board, to give added strength and rigidity, and to protect the instrument from chemical and bacterial attack while in the soil. The plate was then laid on the bench and covered with a sheet of polyethylene and weighted in order to obtain the best possible adhesion, and minimise projections. Resin-bonded matrix board was chosen for the mounting of the junctions for the following reasons: (1) The holes allow free passage of water; (2) the thickness of the instrument is small, thus enabling the greatest possible accuracy to be obtained (Philip, 1961); (3) the material has a thermal conductivity similar to that of soil. This results in a minimum of disturbance to the heat flow through the soil (Morris, Trickett & Moulsley, 1955). The flux plate is shown in Fig. 1.
];1
. •e •• •• • • . -.... .,• • •co •• '" Q
to
.
~
0
••• • .. • • • •••• • .. a • • ., • • • • • .. ••••••• a
Ii
6'
~
G
~
•,.
fO
a
~
••
~
•
" .e
• •• • • • • ... • • • • • • ., •••••• co
I)
0)
0
io
~
0
1::
Fig. 1
5. Calibration Each flux plate was calibrated by measuring its output at a known heat flux produced by a flat heat source with a variable power supply. The heat source consisted of a spiral of resistance wire glued to a sheet of resin-bonded paper board. In order to prevent loss of heat downwards or sideways, separate guard spirals were fixed beneath the main heating spiral and around its circumference. The bottom guard spiral was glued to a separate sheet of resin-bonded paper board and separated from that containing the main heating spiral by a sheet of perspex. The side guard spiral was wound on the same board as the main heating spiral, separated by an annular gap of ! in. Each spiral had its own variable power supply and was covered by a thin sheet of copper (insulated electrically by a sheet of fibre glass) to provide uniformly heated surface. Thermocouples were soldered to the copper covering each spiral and the whole assembly was enclosed in foam polystyrene leaving only the upper surface, or main heat source, exposed. For calibrating, the surface of the heat source was covered with soil in which was embedded the flux plate under test with its output connected to a D.C. amplifier which could be monitored continuously. The desired level of power was supplied to the heat source and the supply to the guard spirals was adjusted so that the thermocouples registered no difference in temperature. Under these conditions, all the power to the main heat source is dissipated as heat uniformly over its upper surface. When stability had been achieved for I h, readings of the power supply
422
A HEAT FLUX PLATE FOR USE IN SOIL
to the heat source (converted to B.T.U/ft 2/h) and the output from the flux plate were recorded. Since the output of a thermocouple increases linearly with temperature, the calibration curve is, theoretically, a straight line. However, the calibrating procedure should be repeated, at least once, to check that a straight line relationship in fact exists, and to verify its slope. REFERENCES
Hadley, W. A.; Eisenstadt, R. Moisture movement in soils due to temperature difference. Heat Pip. Air Condit. 1953, 111-114 Janse, A. R. P.; Borel, G. Measurement of thermal conductivity in situ in mixed materials, e.g. soils. Neth. J. Agric. Sci., 1965,13, 57-62 Mann, G.; Forsyth, F. G. E. Measurement of the thermal conductivity of samples of thermal insulating materials and of insulation in situ by the "heated probe" method. Mod. Refrig. 1956,50: 188-191 Morris, L. G.; Trickett, E. S.; Moulsley, L. J. The measurement of heat flow in soil with reference to glasshouse heating. N.LA.E. Tech. Memo No. 111, 1965 Philip, J. R. The theory of heat flux meters. J. geophys. Res. 1961,66: 571-579 de Vries, D. A. Thermal Properties of Soils. In Physics ofPlant Environment (Ed. W. R. van Wijk). North Holland Publishing Company, Amsterdam, 1963 de Vries, D. A.; Peck, A. J. On the cylindrical probe method of measuring thermal conductivity with special reference to soils. 1. Extension of theory and discussion of probe characteristics. Aust. J. Phys. 1958, 11: 255-271,