Advantages and disadvantages of the heat dissipation sensor

CHAPTER 1 INTRODUCTION

1.7 Outline of document

3.4.5 Advantages and disadvantages of the heat dissipation sensor

The advantages of the heat dissipation sensor outweigh the disadvantages.

Advantages include:

• a matric potential range of -10000 to -10 J kg-I (Campbell et aI., undated), and an accurate operating range of -500 to -10 J kg-¹according to Scan10n et al.

(1999), -1200 to -10 J kg-I according to Reece (1996) and -1500 to 0 J kg-I according to Phene et al. (1971),

• long-term use of the sensor (Phene et al., 1971),

• reliability in sensor outputs (Phene et aI., 1971),

• independency of sensor to salinity (Phene et al., 1971; Jovanovic and Annanda1e, 1997),

• estimated temperature and matric potential (Phene et aI., 1971)

• ease of construction of sensor (Phene et al., 1971),

• moderate cost of sensor (Phene et aI., 1971),

• simplicity of sensor (Phene et aI., 1971),

• possible linear relationship between Pmand T_- To for the Pmrange -1500 to

o

^{J kg-1}(Phene et al., 1971), and

• a single calibration curve for sensors through normalising for thermal conductivity (Reece, 1996).

Disadvantages of the sensor include:

• a decrease in the accuracy ofmatric potential beyond the -100 to 0 J kg-I range, especially when applying an exponential function (Jovanovic and Annandale, 1997),

• a smaller sensitivity to decreasing matric potentials less than -500 to -10 J kg-·

resulting in predicted soil water potentials greater than actual potentials (Scanlon et al., 1999) as suggested by Reece (1996) and Jovanovic and Annandale (1997) for soil water potentials below -1200 J kg-¹and

• a slight over-prediction of matric potential for range -1200 to -400 J kg-\ when compared to soil water potentials measured with thermocouple psychrometers (Reece, 1996),

• large power requirements for frequent measurements (SOWACS, undated),

• required initial equilibrium time in soil before measurements,

• required measurement separation time, to allow heat from previous pulse to dissipate before next measurement,

• cracking of the sensor ceramic installed in swelling and shrinking soils, and

• required contact between soil and sensor.

3.5 Thermocouple psychrometric technique

The free energy difference between soil water and pure water per unit volume of water determines the soil water potential. Water moves from higher to lower potential areas and requires energy. The larger the difference in the water potential between two points, the more energy will be exchanged whilst moving the water. Thermocouple psychrometers measure the total soil water potential based on the energy exchange that result from the water potential differences. The thermocouple psychrometric technique is a highly specialised technique and requires instruments of extreme accuracy (Brown and Oosterhuis, 1992; Jovanovic and Annandale, 1997; Wescor Inc., 1998).

3.5.1 A description of the thermocouple psychrometry technique

Soil psychrometers or hygrometers measure the total water potential of a soil. This measurement is based on energy exchange to move water reversibly and isothermally from the soil under consideration to a reference state. The soil psychrometer or

hygrometer measures the relative humidity of a soil air sample that has equilibrated with the soil. The water potential(If/, inPa) of a soil is related to the relative humidity through the Kelvin equation:

RT

~

1fI=-ln e

~\' eo

3.13

whereRis the universal gas constant (8.3123 JmOrl K-\ Tthe absolute temperature in K, e/eothe fractional relative humidity, andVwthe partial molar volume of water (18 x 10-⁶m³mOrl) (Savageet al., 1981; Wescor Inc., 1998).

Soil psychrometers or hygrometers employ one of two methods (SOWACS, undated;

Wescor Inc., 1998):

• psychrometry through the wet bulb technique, or

• hygrometry through the dewpoint technique.

Of these, Baughn (1974) (cited by Savage et al., 1981) noted that psychrometry (wet bulb) is more popular than hygrometry (dewpoint). The dewpoint hygrometer is less sensitive to temperature changes and gradients than wet bulb psychrometer (Savageet

at.,

1981). The hygrometer is also much more sensitive to voltage changes than the psychrometer (-7.0 x 1O-31J.VkPa-¹vs 3.7 x 10-3 IJ.VkPa-¹at 25 QC). However, the accuracy of the hygrometer is highly dependent on the correct dewpoint cooling coefficient, especially at temperatures of less than about 15 QC (Savageet

at.,

^1981).

In thepsychrometricmethod, the total soil water potential is related to the wet bulb depression temperature of the thermocouple junction minus the ambient temperature.

Peltier cooling (with a current of between about 5 and 8mA) is used to cool the thermocouple below the dewpoint. Different cooling times can be used ranging between 60 s under dry conditions and 15 s under wet conditions. During this process very small droplets of water are condensed on the thermocouple junction surface. These small droplets are allowed to evaporate, and this evaporation process (cooling or release of latent heat) causes the temperature of the thermocouple junction to be reduced below the ambient temperature. The wet bulb depression continues until all the small water droplets have been evaporated. After this, the thermocouple junction temperature returns to ambient or the block temperature (Wescor Inc., 1998). Any temperature difference(-11), between the ambient temperature(Tb)and the thermocouple junction

surface temperature (1)), results in a voltage(V)as given by the Seebeck effect (Savage et aI., 1981)

3.14

whereSis Seebeck coefficient in IlV

°e

^l (S= 58.62+0.09 T for psychrometer temperatures Tbetween 0 and 50 0C).I

When evaporation eases, the voltage corresponding to the wet bulb temperature (or endpoint) is measured. After evaporation, the voltages decrease rapidly, or slow or stop at a plateau, and then decrease further to a reference voltage level. For high water potentials or long Peltier cooling times, the plateau is horizontal. However, for low water potentials (dry samples) or short cooling periods, the endpoint is less clear and can be quite subjective (Savage and Wiebe, 1987).

However, in the hygrometric method the total soil water potential is related to the dewpoint depression. Inthis method too, the thermocouple is cooled below the dewpoint. Here the thermocouple temperature is controlled by the heat of the condensing water. The thermocouple temperature converges to the dewpoint and remains there with a static amount of water (Wescor Inc., 1998). Therefore, if a wet thermocouple junction is held at dewpoint temperature, water will not be lost (through evaporation) or gained (by condensation) (Savage et al., 1981). This technique is found to be relatively unrelated to the wetting characteristics of the thermocouple junction and the size and shape of the water droplets formed on the junction (Neumann and Thurtell,

1972 cited by Savage and Cass, 1984).