The History and Future of Water Potential

by Lauren Crawford on March 16, 2015

I often hear researchers complain about the accuracy of our TEROS 21 water potential sensors. We still have room to improve, but we’ve certainly come a long way! People have been attempting to make water potential measurement in the field for over 100 years. The following is a brief overview of the evolution and history of water potential measurements over that time.

Pre-MPS-1 prototype.

Livingston Discs

The Livingston disc, developed in 1908, was one of the first attempts at determining water potential in the field. The Livingston Disc was actually a primitive, manual version of the technology used in our MPS6 ceramic disc. Here is how it worked: first, you’d weigh the dry disk, then put it in the soil and let it equilibrate. After that, you would dig it up and weigh it again. Using the water retention curve of the disc, you could then determine the water potential.

Gypsum block

In the 1940s gypsum block sensors were invented as the first solid matrix equilibrium technique for water potential. This method tried to continuously sense water potential with a simple electrical conductivity measurement in a solid porous (and naturally occurring) gypsum matrix. However, because naturally occurring gypsum doesn’t have a consistent pore size distribution and it degrades over time, the instrument was not very accurate.

In the 1940’s gypsum block sensors were invented as the first solid matrix equilibrium technique for water potential. Image: www.soilmoisture.com

Tensiometers

In the 1960’s a liquid equilibration technique called tensiometry was discovered that allowed water potential measurement with good accuracy even in the presence of positive pore water pressures. Tensiometers work extremely well in wetter soils with water potentials between 0 and -80 kPa and should be the choice for all wet soil applications, especially above -9 kPa where the MPS6 will not work (the air entry value for its ceramic is -9 kPa). However, when the soil dries out the water under tension in the tensiometer eventually cavitates, causing the output to be useless until they are refilled. Thus solid equilibrium techniques like the TEROS 21 are the best choice across the dry range.

Tensiometers are the most accurate way to measure water potential in the field in the wet range, but are limited to the plant optimal range of about -100 kPa and above.

The Evolution of Ceramic Discs

We learned with the gypsum blocks that one of the challenges in solid matrix water potential measurement is finding a material that will create the same water retention curve every time. In quest of this goal, the ceramic discs in sensors like the TEROS 21 have taken years of development. Because of the limited range of the tensiometer, we wanted to develop a water potential sensor that could measure over a larger range. The hardest part about developing that ceramic was getting a variety of pore sizes so the instrument could read said wide range of water potentials. This started years ago in the lab of Dr. Gaylon Campbell at Washington State University where his technician, Kees Calisendorf, experimented over a long period of time to come up with the perfect recipe.

The MPS1 was our original matric potential sensor released in 2001. It allowed for long-term monitoring in the field because, unlike gypsum, the ceramic did not degrade over time.

Even after we found a consistent ceramic, there were still outliers. So creating a calibration method was essential to making an accurate sensor. The first challenge was to be able to store calibration points in the sensor, which required a microprocessor. The second, and more difficult task, was to establish a method to calibrate large numbers of sensors at once. We tried many different approaches like pressure plate, equilibration over salt solutions, and even centrifugal force, but nothing worked. Finally, in a discussion with our partner, UMS, we discovered the key. We now can accurately calibrate 50 sensors at a time in only 12 hours. Still, even with these advanced techniques, we only have a sensor with an accuracy of plus or minus 10%, but considering the history of how hard it’s been to develop consistent ceramic, this accuracy is exciting for the range that we can get.

The MPS 2 was our second matric potential sensor which offered two-point calibration and a temperature sensor, improving accuracy.

What’s Next?

Now that we’ve created a reliable calibration method, we can turn our attention toward further improving the sensor measurement range as well as its accuracy. Testing different ceramics, or other porous media, may hold the key to a solid equilibrium technique sensor reading all the way to 0 kPa, eventually replacing the need for tensiometers in the field.

The two key innovations in the MPS6 (now called TEROS 21), released in 2014, are the addition of a microprocessor to the sensor and fast, accurate equilibration at multiple points.

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