Published online 21 June 2006
Published in Vadose Zone J 5:856-859 (2006)
DOI: 10.2136/vzj2005.0075
© 2006 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
NOTES
Design and Testing of a Low-Cost Soil-Drying Oven
Martha P. L. Whitakera,*,
Ty P. A. Ferréa,
Bart Nijssena,b and
Jim Washburnea
a Dep. of Hydrology and Water Resources, Univ. of Arizona, Tucson, AZ 85721-0011
b Dep. of Civil Engineering Mechanics, Univ. of Arizona, Tucson, AZ 85721-0011
* Corresponding author (mplw{at}hwr.arizona.edu)
Received 24 June 2005.
 |
ABSTRACT
|
|---|
Measuring gravimetric soil water content is a relatively elementary laboratory procedure; however, the inaccessibility of a laboratory oven makes this activity impractical for K–12 students. The Soil Moisture Campaign (SMC) Project of the Global Learning and Observation to Benefit the Environment (GLOBE) Program was developed to use gravimetric water content measurement as an introduction to earth science research for K–12 students. We have designed and tested a low-budget, low-technology "light-bulb oven" that is affordable enough for use in a classroom. The light-bulb oven was tested by drying replicate samples of four different soil types in the light-bulb oven and in a traditional laboratory oven. The results show that the performance of the light-bulb oven was comparable to that of a traditional laboratory oven.
|
INTRODUCTION
|
|---|
QUANTIFICATION of soil water content is important for constructing water budgets, defining the available water for agricultural crops and natural vegetation, and predicting land–atmosphere interactions. Several laboratory and field methods have been developed to measure soil water content across a range of scales. Most of these methods are indirect, correlating a more easily measured property to the volumetric water content. The gravimetric approach is the only direct water content measurement method. Methods can be further distinguished based on their sample volume, equipment cost, ease and cost of analysis, and applicability under specific conditions (see, e.g., Topp and Ferré [2002] for a more complete review of soil water content monitoring methods). Each water content measurement method has advantages and disadvantages. The choice of an optimal method depends on the specific measurement needs. Some key considerations for selection of a water content measurement method include cost, need and availability of trained personnel to calibrate and operate the instrumentation, the ability to collect automated data, sample size, ability to make nondestructive measurements, and the ability to make noninvasive measurements.
There is growing interest in the use of soil water content measurements made across large areas in global circulation models. An example of these activities is the coordinated Soil Moisture Experiments (SMEX) that were conducted in 2002, 2003, 2004, and 2005 (Jackson and Lettenmair, 2004; also http://www.ars.usda.gov/main/site_main.htm?modecode=12-65-06-00 [accessed 21 Sept. 2005; verified 9 Mar. 2006]). In these experiments, satellite remote sensing, aircraft remote sensing, and in situ measurements of soil moisture were collected synoptically for the purpose of comparison of methods, and to validate satellite soil moisture products from the Advanced Microwave Scanning Radiometer on the NASA Aqua satellite (see http://www.ars.usda.gov/main/site_main.htm?modecode=12-65-06-00 [accessed 21 Sept. 2005; verified 9 Mar. 2006]). This comparison has been conducted with the recognition that, while remotely sensed soil moisture measurements are important for use in hydrological models, global climate change models, and climatological analyses (among other uses), observational datasets of actual, in situ measurements remain valuable for model development and evaluation, and as ground truth for remote sensing (Robock et al., 2000). The GLOBE SMC is aimed at combining the collection of distributed soil water content measurements with providing K–12 educational opportunities. GLOBE is a worldwide, hands-on, primary- and secondary-school-based science and education program (see http://www.globe.gov [accessed 8 Dec. 2005; verified 9 Mar. 2006]). The GLOBE program is a cooperative effort of schools, federal agencies, universities, and nongovernmental organizations in partnership with 107 countries worldwide. The GLOBE SMC aims to recruit and mobilize GLOBE-participating students worldwide to collect near-surface (i.e., 0–5 and 8–12 cm below the soil surface) gravimetric soil moisture data for use in ongoing scientific investigations (see http://www.hwr.arizona.edu/globe/sci/SM/SMC [accessed 18 Dec. 2005; verified 9 Mar. 2006]). A major challenge for the SMC was to develop a method for collecting soil moisture content data that would meet the quality standards required for scientific data, using low-budget, low-technology instrumentation that could be made available in schools worldwide.
Many methods have been developed for small-scale, in situ measurement of water content; however, the gravimetric method remains the standard with which other, indirect, methods are compared. The gravimetric method has the following advantages: it is a direct measurement of water content that does not require calibration, it is inexpensive, it requires very little training, and it is portable. In its simplest form, the method only requires basic soil collection equipment (a shovel and sample containers), a balance, and a drying oven. For most scientific and engineering applications, this equipment is considered inexpensive and is often readily available. For community and school groups, however, especially in underdeveloped countries, traditional laboratory convection ovens are prohibitively expensive (>US$300). Microwave ovens can be more cost effective than convection ovens, but uneven soil heating and small capacities limit their use. Alternatively, samples can be air dried, but this approach typically requires very long drying times and may be adversely affected by differing relative humidities among measurement locations.
The relative simplicity of collecting gravimetric soil samples makes the thermogravimetric method of using convective oven drying particularly well suited for a student-based soil water content measurement campaign. In support of the GLOBE SMC, we developed a drying oven that is inexpensive, simple to use and construct, and large enough to dry multiple samples simultaneously for greater efficiency. The design and preliminary testing of this oven is presented with the hope that it will be useful for other scientific and educational projects that could make use of widely distributed gravimetric water content measurements. For this approach to be practical in K–12 settings, we designed a soil-drying oven that could be built and used by teachers and students. The specific objectives were to: (i) design a low-budget, low-technology soil-drying oven that achieves a steady target temperature of 100 to 110°C, has a capacity that is at least as large as a traditional laboratory oven, is simple enough in design to be built based on a simple set of design schematics, construction instructions, and a list of readily available parts, and is safe to operate under adult supervision; and (ii) compare the ability of the oven with that of a traditional laboratory oven to dry soil samples across a range of soil textures.
|
THE "LIGHT-BULB OVEN" DESIGN
|
|---|
The light-bulb oven is a 208-L (55-gallon) steel drum, cut in half lengthwise, covered externally with aluminum-backed fiberglass insulation, and placed on a ring of flat cement bricks (Fig. 1). The light-bulb oven can hold approximately 75 8-cm-diam. soil sample cans, which is comparable to the capacity of a typical laboratory drying oven. Heat is provided by four 100-W light bulbs. A glass thermometer placed in the oven allows manual temperature readings, and a gap in the ring of concrete bricks provides a vent for air circulation and temperature control. Adjusting the size of the air gap allows the user to regulate the oven temperature to a temperature near 105°C. Figure 2 shows an example of how the oven temperature was lowered by increasing the air gap. The brick that was used to adjust the air gap was simply moved incrementally to form a gap of approximately 1, 3, 5, and finally 8 cm, to achieve a final temperature of approximately 105°C. The size of the air gap would probably have to be determined experimentally for each oven to attain 105°C. Other factors may affect the oven temperature and air gap size that corresponds to 105°C. For example, a higher ambient laboratory temperature may require a larger air gap, while a lower laboratory temperature might require a small air gap, or perhaps no gap at all. The goal is to achieve a steady temperature between 100 and 110°C, as recommended by Topp and Ferré (2002). Figure 2 is thus presented as a demonstration of how well the light-bulb oven can achieve a specified temperature of 105°C. For the data shown in Fig. 2, the average ambient laboratory temperature was 27°C.
View larger version (20K):
[in this window]
[in a new window]
|
Fig. 2. Graph of temperature changes in the empty light-bulb oven in response to increases in the size of the air gap.
|
|
A detailed list of the materials required for building the oven is given in Table 1. The specific sizes of the bricks that we used are listed, but any similar brick could be used as long as the oven is elevated sufficiently to create an opening for air circulation. The bricks also contribute thermal mass and offer protection to the surface on which the oven is placed. Accordingly, the temperature stability may be impacted by the use of different support bricks. Note that Fig. 1 is a schematic, and that the number of bricks shown in the figure (seven bricks) appears different than the number listed in Table 1 (eight bricks). The exact number of bricks can be adjusted to create a continuous ring that elevates the oven and provides an air gap for temperature regulation.
The total cost of the light-bulb oven is US$110 or as little as US$92 if two groups split the cost of the steel drum (only half a drum is needed for each oven). This can be compared with a cost of US$335 for a low-end, laboratory convection oven.
|
TESTING THE LIGHT-BULB OVEN
|
|---|
The ability of the light-bulb oven to achieve a constant temperature in a range that is acceptable for soil drying was examined. Nine thermocouples were placed on a 3 x 3 grid in the center of the light-bulb oven. The same grid of thermocouples was also used to measure the temperature distribution on the center rack of a standard drying oven (Yamato DX-400, Yamato Scientific Co., Japan). A datalogger was used to record temperatures at 15-min intervals for 24 h, which corresponds to the typical minimum length of time specified for drying soil samples for gravimetric analysis (Topp and Ferré, 2002). For this initial experiment, there were no soil samples present and there was no air gap between the concrete blocks. The empty light-bulb oven heated gradually, achieving a constant temperature in approximately 12 h (Fig. 3). The desired temperature near 105°C was achieved within 2 d by sequential adjustment of the air gap (Fig. 2). From Days 8 through 14, the light-bulb oven maintained an average temperature of 104.8°C. During that time, the standard deviation was 0.28°C, and the temperature varied as much as 0.67°C. In comparison, the convection oven achieved the target temperature of 105°C within 1 h, then the temperature varied as much 8°C (Fig. 3). The temperature variation may be an idiosyncrasy of this particular laboratory oven; other oven designs may minimize temperature oscillations. Although the temperature in the traditional oven frequently exceeded 105°C, the average temperature was 
107°C, well within the desired range of 100 to 110°C.
View larger version (18K):
[in this window]
[in a new window]
|
Fig. 3. Comparison of average temperatures with time in the light-bulb and traditional ovens. The red bars indicate the upper and lower ends of the range of acceptable oven temperatures (100–110°C).
|
|
Once it was demonstrated that the oven could achieve the desired temperature range, a subsequent experiment was designed to compare the abilities of the light-bulb oven and a conventional soil drying oven to dry samples of different soils. Four local soil types were used to examine a range of soil textures (Fig. 4): Gila sandy loam, Sonora sandy clay loam, Pima silty clay loam, and an organic-rich composted sandy loam (referred to here as compost). Each soil was wetted uniformly in the laboratory. For each soil, water was added to saturate the sample to test the ability of the light-bulb oven to adequately dry a variety of very wet soils. Twelve samples of each wetted soil were placed in the light-bulb oven, and 12 samples of each wetted soil were placed in the traditional laboratory oven. The soils were dried for 24 to 48 h, following standard methods (Topp and Ferré, 2002). The gravimetric water contents determined in the light-bulb oven were very similar to those determined using a traditional laboratory oven. The RMSE of the measurements using the two ovens was 0.0009 kg/kg. Table 2 gives the average soil moisture and standard deviation for each of the four soils.
View this table:
[in this window]
[in a new window]
|
Table 2. Average values of gravimetric soil moisture (SM) for four soil types. The relation is 1:1 between the data from the traditional and light-bulb ovens. The RMSE is 0.0009.
|
|
During the tests shown in Fig. 2, the ambient temperature of the laboratory only ranged from 25.0 to 27.6°C; the average temperature was 26.6°C. We expect that if the light-bulb oven is operated within the temperature range of 20 to 28°C (e.g., the average temperature of most classrooms or laboratories), there will be negligible effects on its performance, especially since the light-bulb oven's temperature is readily controlled by adjusting the size of the air gap. When the light-bulb oven is opened to replace dried samples with new, moist samples, it is expected to heat up at least as fast as the initial heating. In addition to the laboratory tests shown here, the light-bulb oven has been used in sheltered, outdoor, humid conditions, and the drying times for moist soil samples were comparable to those found in this laboratory study.
|
CONCLUSIONS
|
|---|
Despite the rapid development of indirect, in situ methods for water content measurement, the gravimetric method remains the standard. This method has many advantages, especially for use in educational settings and for use by community groups with limited resources for instrumentation. We have developed a simple light-bulb oven that shows promise as a low-budget, low-technology method to dry soils for gravimetric soil water content analysis. The oven can be built based on a simple set of design schematics, construction instructions, and a list of readily available parts. The oven has been shown to achieve a constant temperature within the desired range of 100 to 110°C. Although the light-bulb oven required a longer period of time to achieve a constant temperature than a convection oven (12 h, compared with the traditional oven reaching the desired temperature within 1 h), the lower technology oven did not show the temperature oscillations seen in the standard oven used in our study. While 12 h may be an inconvenient length of time for an oven to preheat in a standard laboratory setting, we do not anticipate that this would be a hardship for K–12 students and their teachers. However, some professional soil scientists may find that the temperature stability of the light-bulb oven is an attractive feature for a laboratory oven, despite the 12-h heating time. Both the light-bulb oven and low-end convection oven can hold approximately 75 8-cm-diam. soil sample cans, allowing time-efficient analysis. Finally, gravimetric water contents determined with the light-bulb oven showed excellent agreement with those measured in the traditional oven for the four soil types tested. The cost of the light-bulb oven is approximately one-third that of a low-end traditional convection oven, which could make soil water content measurement affordable for interested educational and community groups.
|
ACKNOWLEDGMENTS
|
|---|
We gratefully acknowledge the generous funding from the National Science Foundation through the GLOBE Project. In addition, thanks are extended to Doug Tietema and Lars Peterson for their laboratory contributions to the project.
|
REFERENCES
|
|---|
- Jackson, T.J., and D. Lettenmair. 2004. [abstract] Soil Moisture Experiments 2004 (SMEX 04). Eos 85(17):ja181.
- Robock, A., K.Y. Vinnikov, G. Srinivasan, J.K. Entin, S.E. Hollinger, N.A. Speranskaya, S. Liu, and A. Namkhai. 2000. The global soil moisture data bank. Bull. Am. Meteorol. Soc. 81:1281–1299.[CrossRef]
- Topp, G.C., and P.A. Ferré. 2002. Water content. p. 417–545. In J.H. Dane and G.C. Topp (ed.) Methods of soil analysis. Part 4. Physical methods. 3rd ed. SSSA Book Ser. no. 5. SSSA, Madison, WI.
This article has been cited by other articles:
|
|
|
|
 
D. A. Robinson, C. S. Campbell, J. W. Hopmans, B. K. Hornbuckle, S. B. Jones, R. Knight, F. Ogden, J. Selker, and O. Wendroth
Soil Moisture Measurement for Ecological and Hydrological Watershed-Scale Observatories: A Review
Vadose Zone J.,
February 25, 2008;
7(1):
358 - 389.
[Abstract]
[Full Text]
[PDF]
|
|
|
TOP
ABSTRACT
INTRODUCTION
