Posts from the ‘WP4C dew point hygrometer’ Category

Dec 26

How to Measure Water Potential

In the conclusion of our 3-part water potential series (see part 1), we discuss how to measure water potential—different methods, their strengths, and their limitations.

Image of a mountain with a little snow on the top

Vapor pressure methods work in the dry range.

How to measure water potential

Essentially, there are only two primary measurement methods for water potential—tensiometers and vapor pressure methods. Tensiometers work in the wet range—special tensiometers that retard the boiling point of water (UMS) have a range from 0 to about -0.2 MPa. Vapor pressure methods work in the dry range—from about -0.1 MPa to -300 MPa (0.1 MPa is 99.93% RH; -300 MPa is 11%).

Historically, these ranges did not overlap, but recent advances in tensiometer and temperature sensing technology have changed that. Now, a skilled user with excellent methods and the best equipment can measure the full water potential range in the lab.

There are reasons to look at secondary measurement methods, though. Vapor pressure methods are not useful in situ, and the accuracy of the tensiometer must be paid for with constant, careful maintenance (although a self-filling version of the tensiometer is available).

Here, we briefly cover the strengths and limitations of each method.

Vapor Pressure Methods:

The WP4C Dew Point Hygrometer is one of the few commercially available instruments that currently uses this technique. Like traditional thermocouple psychrometers, the dew point hygrometer equilibrates a sample in a sealed chamber.

WP4C Dew Point Hygrometer

A small mirror in the chamber is chilled until dew just starts to form on it. At the dew point, the WP4C measures both mirror and sample temperatures with 0.001◦C accuracy to determine the relative humidity of the vapor above the sample.

Advantages

The most current version of this dew point hygrometer has an accuracy of ±1% from -5 to -300 MPa and is also relatively easy to use. Many sample types can be analyzed in five to ten minutes, although wet samples take longer.

Limitations

At high water potentials, the temperature differences between saturated vapor pressure and the vapor pressure inside the sample chamber become vanishingly small.

Limitations to the resolution of the temperature measurement mean that vapor pressure methods will probably never supplant tensiometers.

The dew point hygrometer has a range of -0.1 to -300 MPa, though readings can be made beyond -0.1 MPa using special techniques. Tensiometers remain the best option for readings in the 0 to-0.1 MPa range.

Secondary Methods

Water content tends to be easier to measure than water potential, and since the two values are related, it’s possible to use a water content measurement to find water potential.

A graph showing how water potential changes as water is adsorbed into and desorbed from a specific soil matrix is called a moisture characteristic or a moisture release curve.

Example of a moisture release curve.

Every matrix that can hold water has a unique moisture characteristic, as unique and distinctive as a fingerprint. In soils, even small differences in composition and texture have a significant effect on the moisture characteristic.

Some researchers develop a moisture characteristic for a specific soil type and use that characteristic to determine water potential from water content readings. Matric potential sensors take a simpler approach by taking advantage of the second law of thermodynamics.

Matric Potential Sensors

Matric potential sensors use a porous material with known moisture characteristic. Because all energy systems tend toward equilibrium, the porous material will come to water potential equilibrium with the soil around it.

Matric Potential Sensor

Using the moisture characteristic for the porous material, you can then measure the water content of the porous material and determine the water potential of both the porous material and the surrounding soil. Matric potential sensors use a variety of porous materials and several different methods for determining water content.

Accuracy Depends on Custom Calibration

At its best, matric potential sensors have good but not excellent accuracy. At its worst, the method can only tell you whether the soil is getting wetter or drier. A sensor’s accuracy depends on the quality of the moisture characteristic developed for the porous material and the uniformity of the material used. For good accuracy, the specific material used should be calibrated using a primary measurement method. The sensitivity of this method depends on how fast water content changes as water potential changes. Precision is determined by the quality of the moisture content measurement.

Accuracy can also be affected by temperature sensitivity. This method relies on isothermal conditions, which can be difficult to achieve. Differences in temperature between the sensor and the soil can cause significant errors.

Limited Range

All matric potential sensors are limited by hydraulic conductivity: as the soil gets drier, the porous material takes longer to equilibrate. The change in water content also becomes small and difficult to measure. On the wet end, the sensor’s range is limited by the air entry potential of the porous material being used.

METER Tensiometer

Tensiometers and Traditional Methods

Read about the strengths and limitations of tensiometers and other traditional methods such as gypsum blocks, pressure plates, and filter paper here.

Choose the right water potential sensor

Dr. Colin Campbell’s webinar “Water Potential 201: Choosing the Right Instrument” covers water potential instrument theory, including the challenges of measuring water potential and how to choose and use various water potential instruments.

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Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together. Plus, master the basics of soil hydraulic conductivity.

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Nov 14

Water Potential Instruments used to Determine Where Alkali Bee Larvae Get their Water

Alkali bee beds are maintained by farmers near Touchet, Washington to pollinate fields of alfalfa, grown there for seed. The beds are typically a few acres in size and provide a nesting place for the bees, which can increase seed production by as much as 70 percent. Alkali bees are better than honeybees for pollinating alfalfa, as they don’t mind the explosive pollen release of the alfalfa flower.

Alkali Bee

USDA-ARS entomologist, Dr. Jim Cane, is trying to understand optimal bee-soil-water relations to ensure the bees will happily reproduce next year’s pollinators. Dr. Gaylon S. Campbell recently worked with Dr. Cane to measure water relations in bee nesting beds. Here’s what they found out:

Why Water Relations Matter

Alkali bees nest underground. They prefer salty soil surfaces which retard evaporation and discourage plant growth. The soil has to be the right texture, density, and have the correct moisture levels for successful nesting. In addition, the water potential of the larval food provision mass has to be low so it does not mold. Growers apply high levels of sodium chloride to the bee bed surface, and the soil is sub-irrigated to keep the salt near the surface and the subsurface soil moist.

Bottom right: a white larvae on a gold colored provision mass inside one of the tunnels dug by the female.

The female digs a tunnel down to a favorable depth, typically 15-20 cm or more, hollows out a spheroidal shaped cell around 1 cm diameter, and carefully coats the inside of the cell with a special secretion that appears to form a hydraulic and vapor barrier between the soil and the nest contents. She then builds a provision mass from pollen and nectar, shaped like an oblate spheroid with major axis around 6 mm and minor axis 3-4 mm. One egg is laid on the provision mass (which provides food for the larva), and the mother bee then seals up the entrance to the cell and moves on to the next one.

The female coats the inside of the cell with a special secretion that appears to form a hydraulic and vapor barrier between the soil and the nest contents.

Specialized Instruments for Each Measurement

In order to understand moisture relations between the soil, the larva, and the food provision mass, Dr. Cane carefully excavated three soil blocks from one of the bee beds, dissected them to find nests, and Dr. Campbell helped measure water potentials of the eggs, larvae, and provision masses. They also measured matric and total water potentials of bee bed soils.

A researcher with a instrument called a sample chamber psychometer sitting in front of him

A sample chamber psychrometer

A Sample Chamber Psychrometer is the only water potential device with a small enough sample chamber to be able to measure individual eggs and early-stage larvae, which it did. The provision masses were too dry to measure with the psychrometer, so several provisions were combined (to provide sufficient sample size) and measured in a Dew Point Potentiameter, along with the soil samples. Dr. Campbell measured matric potential of the highly saline soils using a tensiometer.

Water Potential Seems Important to the Bees

Dr. Campbell thinks matric potential is important in determining physical condition of the soil (how easy it is for the bees to dig and paint the inside of the nest), but probably has little to do with bee or larva water relations. The water potentials of the eggs and larvae were low (dry), but within the range one sees in living organisms. There was a consistent pattern of larva water potential decreasing with larval growth.

Image of an Alkali Bee seeking shelter in a rain storm in a little tunnel in the dirt

This alkali bee seeks shelter during the rain in a previously dug tunnel.

The exciting part of this experiment was the provision mass water potentials, which were so low that it is more convenient to talk about them in terms of water activity (another measure of the energy state of water in a system, widely used by food scientists). The intact provision masses were drier than any of the soil water potentials and not in equilibrium with the soil. Dr. Campbell says, “It’s interesting that all the provision masses were at water activities that would make them immune to degradation by almost all microbes, both bacteria and fungi.”

Another Interesting Observation

Dr. Cane found one provision mass covered with mold. Soil and plants are full of inoculum, so it is unlikely that the other provision masses lacked spores, but this one was wet enough to be compromised, and the others apparently weren’t. Dr. Campbell says, “There are two possibilities. Either it was put up too wet, or it got wet in the nest. The really interesting question is why all of them don’t get that wet. I think the hydrophobic coating of the nest eliminates all hydraulic contact from the soil to the provision mass, thus eliminating any liquid water flow, which would almost immediately wet the pollen balls. I think it also drastically reduces the vapor conductance from the soil to the ball, making water uptake through the vapor phase slow enough that the provision mass can usually be consumed before its water activity gets high enough for mold to grow.”

Image of a large green tool used to punch holes in the soil for Alkali Bees to nest in laying on top of the soil

Tool the grower uses to punch holes in the nesting beds for the bees to tunnel into.

How Do Larvae Stay Hydrated?

The water activity of the larvae were around 0.99, much higher than either the soil or the provision mass, inspiring the scientists to wonder how they stay hydrated. Dr. Campbell speculates, “They have a water source from their metabolism, since water is a byproduct of respiration (Campbell and Norman, p. 205). It is also possible for biological systems to take up water against a potential gradient by expending energy. There are reports of a beetle which can take up water from a drop of saturated NaCl (water activity 0.75), so it is possible that the larva gets water from the environment that way. There appears to be no shortage of energy available. On the other hand, it would seem like the larval cuticle would need to be pretty impermeable to maintain water balance since the salty soil, and especially the provision mass, are so much drier than the larva.” Dr. Cane notes that, ”For a few exemplar bee species, mature larvae weigh 30-40% more than the provision they ate, with the possibility that the provision undergoes a controlled hydration by the soil atmosphere through the uncoated soil cap of the nest cell.”

In the future, Dr. Campbell is hoping to see more experiments that will answer some of the questions raised, such as measuring individual provision masses to determine why there is some variation in water potential. Dr. Cane will be undertaking experiments to measure moisture weight gain of new provisions exposed to the soil atmosphere of the Touchet nest bed soil.

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References

Campbell, G. S. 1985. Soil Physics with BASIC: Transport Models for Soil-Plant Systems. Elsevier, New York.

Campbell, G. S. and J. M. Norman. 1998. An Introduction to Environmental Biophysics. Springer Verlag, N. Y.

Rawlins, S. L. and G. S. Campbell. 1986. Water potential: thermocouple psychrometry. In Methods of Soil Analysis, Part 1. Physical and Mineralogical Methods – Agronomy Monograph 9, 2nd edition.

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Apr 29

Does Early Planting Increase Risk to Winter Canola?

Many dryland winter canola growers assume that if they plant earlier, they will establish a stronger plant, but Washington State University researcher Megan Reese recently found that this was not the case. She and her team discovered that planting earlier increases risk to the plant, as more water is used, and the reduced amount of water then left after the winter season limits spring regrowth. Megan’s findings could be valuable as water is the most yield-limiting factor in eastern Washington state’s wheat-dominated dryland systems, where winter canola has newly emerged as a rotational crop.

Bright yellow canola field in full bloom

Winter canola is cold hardy, but it’s not as resilient as wheat.

Early Planting:

Winter canola is cold hardy, but it’s not as resilient as wheat. It’s planted in August, much earlier than winter wheat, which is planted in the late fall. In order to survive, winter canola has to establish a hardy taproot system so that plants have reserves to survive the winter. Megan says, “Opinions vary, but anecdotally, a dinner plate sized plant can survive winter fairly well, so that’s why winter canola is planted in August . However, because establishment and germination can be an issue, we decided to try planting in June at Ritzville, Washington, thinking the soil would be more moist and have a cooler seedbed. However, the early planting date had a negative effect on winter survival. Not one of the early plants survived. We found the plants that started earlier used a lot more water, and consequently, the winter rains weren’t enough to refill the soil profile. Excessive growth and bolting also contributed to low survivorship.”

Methods and Moisture Release Curves:

Megan monitored soil water in the profile several different ways. At one location she used a neutron probe and hand-sampled gravimetric soil moisture in the top 30 cm of the profile, and in other locations, she was limited to hand samples. Then she combined those measurements with local weather stations to provide the crop water balance for the canola. Using these data, she was able to determine soil water use as indicated by the water content change through the growing season and calculate the depletion of soil water.

Anecdotally, a dinner plate sized plant can survive winter fairly well.

Megan also took soil samples into the lab from each depth increment at every site and used a chilled mirror hygrometer to construct a moisture release curve. This helped her to define the apparent permanent wilting point at -1.5 MPa. She says, “I was able to then see how efficient canola was at extracting available water, and I could look at available water instead of total water contents, which was more useful in terms of plant accessible moisture in the soil profile. It allowed me a consistent platform to compare actual water amounts across sites with differing soil types. At one site, 12.5% of the water was unavailable, but in the sandier soils at another site, it was 4%. So there were significant differences in permanent wilting point.”

Water and Physiological Challenges Affect Winter Survival:

Megan found that the June planted canola used every milliliter of available water in the soil profile by late October/early November, but August-planted canola still had some water above wilting left in the profile over the winter, which helped the plants in the spring. She says, “It was a milder winter, so we didn’t get the usual amount of snow and rain, which probably played a role, but we did not see the profile refilled in the June-planted canola. In addition, those June plants were purple and wilted by November, so water stress could have hurt the plants in terms of its defenses. However, I think a larger issue was that they grew so large (the crowns actually elongated and bolted so they weren’t close to the soil) they were more susceptible to the harsh temperatures, whereas the August planted canola were much smaller and their crowns stayed right on the soil surface.” These findings are based on only one year of data, and Megan notes that early plantings have worked well in the milder climate of Pendleton, OR.

What Does it Mean for Farmers?

Megan says, “We were able to surprise a lot of farmers by showing that canola roots access water down to 1.5 to 1.7 m in the fall; it was hard to believe that a winter crop would do that. Also, in my second year’s data, we followed water use all the way through harvest, so we were able to show how much yield we gained for every millimeter of water used, and farmers liked hearing that number as well. I think it’s useful information that incorporates biophysics principles and answers some questions that these new canola producers are interested in. I have three locations this season that we are currently following to give farmers a further idea of what the water use looks like, when canola uses that water, and from where in the soil profile. Hopefully, this research will help them manage their rotations and look at the possibility of adopting canola.”

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Apr 25

The Tensiometer: Micro-sized

A strand of a spider’s web is 5 micrometers in width. Microelectromechanical systems (MEMS) devices range in size from 20 micrometers to one millimeter. That’s the incredibly small size of the components used in the tensiometer being developed by PhD candidate, Michael Santiago, and his collaborators, professors Abraham Stroock and Alan Lakso at Cornell University.

Spider wed with dew water on the strands

MEMS devices can be as small in width as 4 strands of a spider’s web.

The engineer/research team is using MEMS technology to develop a miniature tensiometer (microtensiometer) that has a 100 times larger range than existing tensiometers, is stable for months, communicates digitally, and can be embedded into plant stems to directly measure plant water potential.

Existing Tensiometer Limitations:

Water potential is the best measure of a plant’s hydration relative to growth and product yield. Unfortunately, directly measuring water potential in plant tissue is only possible through labor-intensive, destructive methods such as the leaf pressure bomb and stem psychrometer. A common alternative is to use ‘set-and-forget’ soil tensiometers to measure soil water potential as a proxy for plant water potential, but this method is unreliable for plants with high hydraulic resistance (vines and woody species), where plant water potential is often much less than the water potential in soil. Although soil tensiometers are very accurate and simple to use, they can be large and bulky, and cavitate as soils dry.

A 25 cent coin next to a prototype microtensiometer

Prototype microtensiometer made with MEMS components.

Solution:

The Cornell University research team wants to improve the design of the tensiometer so it can be used in the field for applications such as continuously monitoring and controlling plant water potential in vineyards to consistently produce high-quality wine grapes with an exact flavor/aroma profile. Santiago says, “We’ve basically miniaturized a tensiometer using microchip technology to the point where it’s this tiny chip inside a wafer. Because of the way we fabricated it, we are hoping to make it an embeddable tensiometer that can go in anywhere and measure tension down to about -100 bars (-10 MPa).”

Developing and Calibrating

Santiago is using a chilled mirror hygrometer to produce solutions of specific water potential to test, calibrate, and characterize the microtensiometer. He comments, “We’ve been testing it in osmotic solutions. We use the water potential meter for calibrating a solution of PEG (polyethylene glycol), and then we measure it with the tensiometer.”

One hurdle the team has to overcome is finding a membrane that keeps small molecules and ions out of the tensiometer pores: these pollute the water inside the tensiometer and cause measurement errors. Santiago explains, “Our solution right now is to test in solutions of large molecules, such as PEG of 1400 molecular weight. The tensiometer pores are about 3-4 nanometers, extremely small, but small molecules, such as sugars and salts, can still get through. It’s not a problem for the short term because we are directly submerging into solutions of just water and large molecules, but our goal is to go into the environment and insert the tensiometer into soils and plant stems where small molecules are ubiquitous, so we’ll have to find a membrane that works and can handle field testing.”

The team has been experimenting with materials such as Gore-Tex and reverse osmosis membranes [M5] [M6] hoping to find a membrane that allows water through and keeps ions out, but does not slow the measurement.

Researchers want be able to insert the device directly into plant xylem.

What’s Next?

Santiago says the calibrations have worked well. Now the challenge will be putting the tensiometer into different environments such as soil, concrete, and plants. For example, they want be able to insert the device directly into plant xylem, which will require a seal so water is not exiting the system. And that’s not the only complication. Santiago explains, “We are getting ready to do some testing in soils. The challenge will be getting good data because soil can be really heterogeneous, and we have this sensor with a much larger range than the usual tensiometer. So what do we compare it with? That’s going to be a bit of a challenge.” Santiago says the next few months will be spent getting into some different materials and obtaining some initial publishable data.

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Feb 11

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Estimating Relative Humidity in Soil: How to Stop Doing it Wrong

Estimating the relative humidity in soil? Most people do it wrong…every time. Dr. Gaylon S. Campbell shares a lesson on how to correctly estimate soil relative humidity from his new book, Soil Physics with Python, which he recently coauthored with Dr. Marco Bittelli.

Radioactive waste buried in steel containers will corrode if the humidity is too high.

A number of years ago a former student told me of a meeting he had with some engineers establishing a low-level radioactive waste repository in a desert area. The waste was to be buried, and at least some of it was in steel containers which would corrode if the humidity was too high. The engineers assumed the humidity in the soil would be pretty low because it was a desert, but they didn’t know how low. So, what is the relative humidity in soil? That sounds like it would be a hard thing to find out without measuring it, but it isn’t. Let’s apply a little physics to see what we can find.

The energy required to create an infinitesimal volume of water vapor can be found using the first law of thermodynamics. For an adiabatic system

where dE is the energy required, p is the pressure, and dV is the volume change.

The Boyle-Charles law, which gives the pressure-volume relationship for a perfect gas, is

where n is the number of moles of gas, R is the universal gas constant, and T is the kelvin temperature. Rearranging terms and taking the derivative of both sides gives

This equation can be substituted for dV in the first equation, giving

The total energy required to go from a reference vapor pressure, po (the vapor pressure of pure water) to the vapor pressure of the water in the soil, p is

We can divide both sides by the mass of water. The left side then becomes the energy per unit mass of water in the soil, which we call the water potential. On the right side, the number of moles per unit mass is the reciprocal of the molecular mass of water, and the ratio of the vapor to the saturation vapor pressure is the relative humidity hr so the final equation is

We can rearrange this and take the exponential of both sides, giving

In the second version of the equation the molecular mass of water, the gas constant and the temperature (298K) have been substituted.

We can use this equation to find the range of humidities we would expect in soil. When soil is very wet, the water potential is near 0, so the humidity is exp(0) = 1. At the dry end, the soil is dried mainly by plant water uptake. Even desert soils support some vegetation. The soil near the surface will be dried by evaporation, but a few decimeters below the surface the lowest water potentials are those to which plants can dry them. The nominal permanent wilting point (lower limit of plant available water) is -1500 J/kg. Desert vegetation can extract water to lower potentials. If we say their lower limit is -2500 J/kg, then the humidity is

so the relative humidity in the soil is around 98%. Sagebrush can go lower than -2500 J/kg. We measured -7000 J/kg under it at the end of the growing season. Even that, though, is around 95% humidity.

The conclusion is that the humidity in the soil is always near saturation, except in a shallow evaporation layer near the surface. I don’t remember what the engineers were expecting. I think anything above 60 or 70% was going to be a disaster for corroding the steel containers. I don’t know whether they believed the calculations or just went on thinking that desert soil is dry.

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Nov 13

Will Sample Disturbance Lead to Lower Accuracy?

Sampling soil for laboratory analysis of water potential is done for two basic reasons. The simplest is to determine the current water potential of the soil. The other is to determine the moisture release curve of the soil. Regardless of the reason for measurement, the question of sample disturbance is important to ensure an accurate result. Dr. Colin Campbell explains why:

Soil is disturbed when it’s removed from its natural structure.

Water Potential and Pore Size:

In soil samples, the void spaces (pores) in between soil particles can be simplistically thought of as a system of capillary tubes, with a diameter determined by the size of the associated particles and their spatial association. The smaller the size of those tubes, the more tightly water is going to be held because of the surface association.

In a clay, water will be held more tightly than in a sand at the same water content because clay contains smaller pores and thus more surface area for the water to bind to. But, even sand can eventually dry to a point where there is only a thin film of water on its surfaces and water will be bound tightly. In principle, the closer water is to a surface, the tighter it will be bound.

Sample Disturbance

Sample disturbance (disturbing soil pores when you remove a sample from the ground) becomes an issue depending on the water potential of your sample. Typically, the less negative (wetter) the water potential, the larger impact sample disturbance will have on the measurement. We can do a calculation that shows there are specific pore sizes associated with specific water potentials (see table 1).

If you disturb a sample with low water potential, permanent wilting point (-1.5 MPa) for example, the pores that are still filled with water would be approximately 0.2 um in diameter, far too small to be broken apart by scooping up a sample. Thus, we could reasonably assume that your WP readings won’t be affected much. But if you disturb soil with higher water potential, say field capacity (-0.033 MPa), it’s much more likely that water will be disturbed, as it fills pores to approximately 9 um.

Hygrometers

Still, this is only an issue if you are attempting to measure in a high WP range. If your chilled-mirror hygrometer only measures up to -1000 kPa, sample disturbance will not be an issue because those pores that will have broken will likely be larger than the sub-micrometer that are holding water, which is beyond the accuracy of your instrument. However, some hygrometers can now measure to an upper limit of -100 kPa, which approaches the point where sample disturbance will make a difference.

Tensiometers

If you are sampling to measure with a tensiometer (measures 0 kPa to -80kPa), it’s extremely important to keep your samples intact because tensiometers cover the emptying range of the largest pores found in soil. A soil collar (sample ring) pounded into the ground will yield the most intact soil core. It’s the best method to use if you need make sure soil pores remain undisturbed to yield an accurate water potential measurement.

For a more in-depth examination of the magnitude of the effects of sample disturbance, read this chilled-mirror hygrometer App Note detailing the subject.

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May 30

This Idea Must Die: Using Filter Paper as a Primary Method for Water Potential

In a continuation of our popular series inspired by the book, This Idea Must Die: Scientific Problems that are Blocking Progress, Dr. Gaylon S. Campbell relates a story to illustrate the filter paper method, a scientific concept he thinks impedes progress:

Folded old paper sitting on a wooden table with a gold antique pocket watch and pen laying in top

There are times when our independent verification turns out to be like the clock and the whistle, and we end up inadvertently chasing our tail.

I remember listening to a story about a jeweler who displayed a big clock in the front window of his store. He noticed that every day a man would stop in front of the store window, pull out a pocket watch, set the watch to the time that was on the large clock, and then continue on. One day, the jeweler decided to meet the man in order to see why he did that. He went out to the front of the store, intercepted the man, and said, “I noticed you stop here every day to set your watch.”

The man replied, “Yes, I’m in charge of blowing the whistle at the factory, and I want to make sure that I get the time exactly right. I check my watch every day so I know I’m blowing the whistle precisely at noon.”

Taken aback, the jeweler replied, “Oh, that’s interesting. I set my clock by the factory whistle.”

The Wrong Idea:

In science, we like to have independent verification for the measurements we make in order to have confidence that they are made correctly, but there are times when our independent verification turns out to be like the clock and the whistle, and we end up inadvertently chasing our tail. I’ve seen this happen to people measuring water potential (soil suction). They measure using a fundamental method like dew point or thermocouple psychrometry, but then they verify the method using filter paper. Filter paper is a secondary method—it was originally calibrated against the psychometric method. It’s ridiculous to use a secondary method to verify an instrument based on fundamental thermodynamics.

Tunnel looking up from the bottom with square holes in the sides going up to the top

Geotechnical engineers use natural material such as soil and rock in combination with engineered material to design dams, tunnels, and foundations for all kinds of structures.

Where the Filter Paper Method Came From:

Before the development of modern vapor pressure measurements, field scientists needed an inexpensive, easy method to measure water potential. I.S. McQueen in the U.S. Geological Survey and some others worked out relationships between the water content of filter paper and water potential by equilibrating them over salt solutions. Later, other scientists standardized this method using thermocouple psychrometers so that there was a calibration. Filter paper was acceptable as a kind of a poor man’s method for measuring water potential because it was inexpensive, assuming you already had a drying oven and a balance. The thermocouple psychrometer and later the dew point sensor quickly supplanted filter paper in the field of soil physics. However, somewhere along the line, the filter paper technique was written into standards in the geotechnical area and the change to vapor methods never occurred. Consequently, a new generation of geotechnical engineers came to rely on the filter paper method. Humorously, when vapor pressure methods finally took hold, filter paper users became focused on verifying these new fundamental methods with the filter paper technique to see whether they were accurate enough to be used for water potential measurement of samples.

What Do We Do Now?

Certainly, there’s no need to get rid of the filter paper method. If I didn’t have anything else, I would use it. It will give you a rough idea of what the water potential or soil suction is. But the idea that I think has to die is that you would ever check your fundamental methods (dewpoint or psychrometer) against the filter paper method to see if they were accurate. Of course they’re accurate. They are based on first principles. The dew point or psychrometer methods are a check to see if your filter paper technique is working, which it quite often isn’t (watch this video to learn why).

Which scientific ideas do you think need to be revised?

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Jan 19

Modeling Available Soil Moisture

Both the amount and the availability of water in soil is important to plant roots and soil-dwelling organisms. To describe the amount of water in the soil we use the term water content. To describe the availability we talk of water potential. In thermodynamics, the water content would be referred to as the extensive variable and the water potential as the intensive variable. Both are needed to correctly describe the state of water in soil and plants.

Measuring soil moisture with the WP4C

In addition to describing the state of water in the soil, it may also be necessary to know how fast water will move in the soil. For this, we need to know the hydraulic conductivity. Other important soil parameters are the total pore space, the drained upper limit for soil water, and the lower limit of available water in a soil. Since these properties vary widely among soils, it would be helpful to establish correlations between these very useful parameters and easily measured properties such as soil texture and bulk density. This paper will present the information needed for simple models of soil water processes.

Click here to download the paper.

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Dec 22

Measuring Osmotic Sap Water Potential

Sometimes networking with new scientists at conferences and workshops can pay dividends in terms of new ideas. Steve Garrity and I recently attended and taught practicum sessions at the PEPg (Plant Environmental Physiology group) Ecophysiology Workshop. The mission of this workshop was twofold: to invite the world experts on plant physiology measurements to come and lecture, and to invite the manufacturers to teach about instrumentation and provide hands-on training.

Workshop participants check the water potential of soil with a UMS T5 mini-tensiometer.

With three sessions per day using METER instrumentation and only two of us, neither Steve nor I could teach about leaf water potential using the WP4C chilled mirror dew point instrument. So, we asked another scientist who is an expert in plant water relations to teach it for us. Not only did he do a great job of teaching about measuring leaf water potential using a hygrometer, but he also inspired us to take another look at how to make this measurement as we learned about its importance to his research (to learn more about how to do this, watch our virtual seminar).

He’s developed a procedure where you can freeze the leaf and break all of the cells so you are left with the cell water (the symplastic water).

Later in the conference, this same scientist gave a talk about the importance of osmotic potential. He’s developed a procedure where you can freeze the leaf and break all of the cells so you are left with the cell water (the symplastic water). He was able to squeeze that sap out and test it in a thermocouple psychrometer, where he established a relationship between how tolerant plants are for drought and what their osmotic sap water potential (turgor loss point) was. We have made many of those sap measurements but had not used them in this manner. That’s really interesting to us at METER because we were unaware of this relationship, and we have now found another use for osmotic potential measurements in leaves.

We would never have realized this new idea without the help of our colleague. Meeting with other scientists at conferences and talking over ideas can be really important. Have you ever struck gold in terms of coming up with new ideas for research, funding, or inventing new research tools at a conference you’ve attended?

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Measuring and Modeling the Environment

Posts from the ‘WP4C dew point hygrometer’ Category

How to measure water potential

Vapor Pressure Methods:

Secondary Methods

Matric Potential Sensors

Tensiometers and Traditional Methods

Choose the right water potential sensor

Take our Soil Moisture Master Class

Why Water Relations Matter

Specialized Instruments for Each Measurement

Water Potential Seems Important to the Bees

Another Interesting Observation

How Do Larvae Stay Hydrated?

Early Planting:

Methods and Moisture Release Curves:

Water and Physiological Challenges Affect Winter Survival:

What Does it Mean for Farmers?

Existing Tensiometer Limitations:

Solution:

Developing and Calibrating

What’s Next?

Take our Soil Moisture Master Class

Take our Soil Moisture Master Class

Water Potential and Pore Size:

Sample Disturbance

Hygrometers

Tensiometers

The Wrong Idea:

Where the Filter Paper Method Came From:

What Do We Do Now?