Published in Soil Sci. Soc. Am. J. 68:709-710 (2004).
© 2004 Soil Science Society of America
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COMMENTS & LETTERS TO THE EDITOR
Comments on "Tensiometer modification for diminishing errors due to the fluctuating inner water column"
Joel M. Hubbell* and
J. Buck Sisson
Idaho National Engineering and Environmental Laboratory P.O. Box 1625 MS 2107 Idaho Falls, ID 83415
In the recent paper by Thalheimer (2003), the changing level of water in a tensiometer is identified as a significant source of error in tensiometer measurements, and a design change is proposed that reduces such error. This design change involves monitoring pressure at a point below the water level in the tensiometer, thereby removing the height of water in the tensiometer from the measurement. This results in considerable ease in analyzing the data since the drift in measurements is essentially eliminated. This self-compensating feature works until the water level in the tensiometer falls beneath the point of pressure measurement.
We propose simplifying the tensiometer design further and at the same time reducing the error resulting from temperature changes at land surface. Starting with Eq. [1] of Thalheimer (2003),
 | [1] |
where h is the measured pressure at the pressure sensor, he is the soil water matric potential, zT is the height of water above the point of measurement, and zB is the height of water on the capillary tube in Thalheimer's design (see Fig. 1a)
. Equation [1] simplifies to
 | [2] |
Thalheimer's design can be further simplified by placing the pressure sensor at the same depth as the point of measurement as shown in Fig. 1b (typically the centerline of the porous cup), resulting in zB = 0 or
 | [3] |
The tensiometer now provides estimates of matric potential.
The advent of small waterproof pressure sensors (e.g., Motorola, Phoenix, AZ, model MPX4250A; Electronic Engineering Innovations, Las Cruces, NM, model 15) allows sensors to be placed within the water column at the point of measurement (Fig. 1b, 2)
while using the technique presented by Thalheimer (2003) to remove the effects of changing water levels.
While the capillary tube and air expansion bulb diminish the effect of changing water level in the tensiometer from the pressure measurement, the upper section of the tensiometer remains full of air and exposed to temperature fluctuations that cause large unwanted variations in the data. Placing the entire water column connecting the pressure sensor to the tensiometer cup below the soil surface has been shown to reduce the effect of temperature fluctuations on measurements (Hubbell and Sisson, 1996, 1998, 2003). These temperature fluctuations tend to drive water into and out of the tensiometer, such that refilling is typically required once or twice a week. Reducing these temperature changes can increase the operational interval to months or years (Sisson et al., 2002).
The proposed system, when combined with the advanced (or portable) tensiometer design, self-compensates for changes in the hanging water column, becomes a matric potential sensor, can be installed at depths >10 m, isolates the sensor from temperature fluctuations, and increases the time interval between maintenance events.
NOTES
* jmh{at}inel.gov 
buck{at}reallife.mgrf.net
REFERENCES
- Hubbell, J.M., and J.B. Sisson. 1996. Portable tensiometer use in deep boreholes. Soil Sci. 161:376–382.
- Hubbell, J.M., and J.B. Sisson. 1998. Advanced tensiometer for shallow or deep soil water potential measurements. Soil Sci. 163:271–277.
- Hubbell, J.M., and J.B. Sisson. 2003. Self-compensating tensiometer and method. Number US 6,539,780. Date issued 1 Apr. 2003. U.S. Patent and Trademark Office, Washington, DC.
- Sisson, J.B., G.W. Gee, J.M. Hubbell, W.L. Bratton, A.L. Ward, and T.G. Caldwell. 2002. Advances in tensiometery for long-term monitoring of capillary pressures. Vadose Zone J. 1:310–315.[Abstract/Free Full Text]
- Thalheimer, M. 2003. Tensiometer modification for diminishing errors due to the fluctuating inner water column. Soil Sci. Soc. Am. J. 67:737–739.[Abstract/Free Full Text]
