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Jun 26

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Understanding Avalanches: Thermal Conductivity of Snow (Part 2)

In a continuation of last week’s article “Understanding Avalanches,” we find out what conclusions Dr. Ed Adams and his colleagues in Montana State University’s avalanche studies program were able to make about measuring the thermal conductivity of snow.

Picture of a snow-capped mountain peak

In order to study the thermal properties of snow samples, the research team wanted a way to measure thermal conductivity in three directions.

In order to study the thermal properties of snow samples, the research team wanted a way to measure thermal conductivity in three directions. That ruled out flux plates. Thermal probes were an obvious alternative, but they brought a different set of challenges. Snow has a very low thermal conductivity, and as Shertzer explains, “if you add a lot of thermal energy to snow, since it’s very insulative, you’ll tend to raise the temperature. Not only do we want to avoid melting the snow in the neighborhood of the probe, but we want to prevent the probe from artificially inducing the same thermal processes we’re measuring—the ones that cause the crystals to change size, and shape, and orientation.”

Shertzer read an article about measuring thermal conductivity in liquids, where if you add too much heat, you induce convection. “Our situation is similar to that,” he explains. “Heating the needle induces local phase change.” The article gave him some ideas about delivering low levels of heat for a relatively long period of time, and he contacted Decagon to see if that option was a possibility.

Snow barriers in the Alps

Snow barriers in the Alps

Unbeknownst to him, Decagon’s research scientists had just completed a year-long project focused on reducing the contact resistance errors that occur when using the large TR1 needle to measure thermal conductivity in large-grained samples.  This made the TR1 needle a good candidate for measuring thermal conductivity in snow. The scientists were excited about modifying TEMPOS firmware to produce a low-power version that would work in snow. The resulting modification has given Shertzer some good data.

“I can definitely say that the anisotropy is there [in the snow samples]. It’s measurable and it’s significant. As the crystals reorient in these depth hoar like chains, the ice network is more conductive than the air in between. The orientation of the chains follows a direction of increased conductivity, and the directions that are perpendicular to the chains tend to decrease in conductivity. Qualitatively, it’s always made sense, and we were just looking for a way to actually relate it to properties like conductivity. Using needles to measure in three different directions simultaneously has given us the ability to measure those properties like conductivity. We expect that this orientation also affects other properties like strength and stiffness.”

Researchers stand at a sign of an avalanche

Signs of an avalanche

Thermal conductivity studies may ultimately lead to a better understanding of the conditions that cause the snowpack to fracture and trigger an avalanche—and information that may help save lives among the growing number of people who ski and snowboard the backcountry.

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Jun 22

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Understanding Avalanches: Thermal Conductivity of Snow (Part 1)

Reading through our archives the other day, I came across this article about thermal conductivity and snow. It’s a unique application for a thermal properties analyzer, an interesting story, and something that may ultimately even save the lives of backcountry skiers and snowboarders.

Researcher climbing a snowy mountain while snow blows off a mountain in the distance

Rich Shertzer, who finished a PhD in the program at Montana State, thinks snow may be unique among natural materials because “the thermal environment it’s exposed to every day can cause pretty remarkable changes in its microstructure.”

When Wired Magazine wrote up Dr. Ed Adams and his colleagues in February 2011, they didn’t refer to them as a team of civil engineers studying granular mechanics. Instead, they named them one of seven teams of “Mad Scientists” and called them “Snow Bombers.”

It’s not hard to find articles about Montana State University’s avalanche studies program. Just describing a typical field study makes for a good story: to investigate real-world avalanche conditions, MSU researchers sit in an outhouse-sized shack bolted to the side of a mountain while colleagues trigger an avalanche up-slope.

But this isn’t just a story about explosions and extreme sports. At its heart, it’s a story about the microstructure of a very fascinating and difficult material. Rich Shertzer, who finished a PhD in the program at Montana State, thinks snow may be unique among natural materials because “the thermal environment it’s exposed to every day can cause pretty remarkable changes in its microstructure.”  A cold, sunny day in the mountains can cause significant changes in snow crystals. It can change their size and shape, but more significantly it can cause a directional orientation in snow layers.

Snow laying on a mountain with tracks running down it

Signs of a recent avalanche.

It’s long been empirically understood that avalanches tend to form above “weak layers” of snow. Shertzer and his colleagues are studying how the orientation of snow crystals correlates with weak layers. Most models of granular mechanics assume that the material’s microstructure is randomly arranged. However, snow layers seem to show a regular arrangement.

As Shertzer explains, “Qualitatively, people have known for a while that when you look at certain snow layers, chains of these ice grains seem to be forming. What I was trying to mathematically model is how that might affect the material properties [of snow], including thermal properties.”

Avalanche running down Mt. Everest

Avalanche on Mt. Everest.

In order to study the thermal properties of snow samples, the research team wanted a way to measure thermal conductivity in three directions. That ruled out flux plates. Thermal probes were an obvious alternative, but they brought a different set of challenges. Snow has a very low thermal conductivity, and as Shertzer explains, “if you add a lot of thermal energy to snow, since it’s very insulative, you’ll tend to raise the temperature. Not only do we want to avoid melting the snow in the neighborhood of the probe, but we want to prevent the probe from artificially inducing the same thermal processes we’re measuring—the ones that cause the crystals to change size, and shape, and orientation.”

Read about how the team addressed these problems next week in part 2 of “Understanding Avalanches.”

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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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May 23

Gore-Tex, House Wrap, and Stomatal Conductance

Want to develop an appreciation for Gore-Tex?  All you need is five minutes in a rubber raincoat.  But how do you know whether the North Face knock-off you’ve just purchased in China for a ridiculously low price is Gore-Tex or rubber?  If you’re a METER researcher, you dash back to your hotel room and clamp a porometer onto the fabric.

The leaf porometer was designed to measure stomatal conductance in leaves.  It’s typically used by canopy researchers to relate stomatal resistance to canopy attributes like water use, water balance, and uptake rates of herbicides, ozone, and pollutants.  Yet, from the beginning, Dr. Gaylon Campbell, the porometer’s designer, saw the possibilities:  “Give this to someone with only a passing interest in research, a ten-year-old kid for example, and they’ll go around the garden and come back with some really interesting observations,” he said.  “There are lots of questions about what loses water and what doesn’t that you can answer with this instrument.”

Researcher Clamping a LEAF POROMETER onto a Leaf in a Forest

SC1 Leaf Porometer

Dr. Campbell was probably thinking the questions would be about organic material—but it hasn’t always turned out that way.  By putting a wet paper towel on one side of an inorganic material and clamping the towel and the material into the porometer head, you can measure how well water vapor diffuses through the material.  Using this strategy, the researcher in China discovered that his raincoat was pretty much impermeable (unlike real Gore-Tex, which is a good vapor conductor).  Spotting the fake North Face coat is now a favorite part of METER’s canopy seminar.  And the coat is not the only leafless item that has been tested. “People will clamp the porometer on just about anything,” Doug Cobos, a METER research scientist, admitted. He himself grabbed it when a local contractor brought in a sample of some supposedly unique house wrap.

Siding is supposed to protect a house from the elements, but most building codes now require that houses be wrapped under the siding.  House wraps provide a secondary defense against liquid water and increase energy efficiency by preventing drafts.  As with raincoats, high-performance house wrap needs to repel water and stop wind while remaining permeable to water vapor.

A House Under Construction with Trees Around it

House under construction with protective wrap under the siding.

The practice of applying a sheathing of tar paper under siding is a hundred years old, but in the last fifteen years, high tech house wraps made from polypropylene in combination with a push towards energy efficiency have made the house wrap market big and competitive.  Upstart wraps try to gain market share through innovation and the one brought in by the local contractor came along with an outlandish claim.  According to the manufacturer’s rep, this plastic wrap would allow water vapor to diffuse out while preventing any from diffusing in.  Some builders might have scratched their heads and moved on.  Our local man decided to check it out.  He brought a sample of the mystical wrap to Decagon.  Out came the porometer and a quick scientific study of house wrap was born.

Dr. Cobos tested industry standard Tyvek house wrap along with the great one-way pretender.  The results?  “The vapor conductance of the new material was basically the same, regardless of which side of the material faced wet filter paper,” Dr. Cobos said.  “And, in fact, the material didn’t diffuse well at all.  Its conductance was similar to cheap perforated plastic.  It didn’t come close to the performance of Tyvek.” Ultimately, the newfangled wrap was retested by the manufacturer and taken off the market.

Probably the porometer’s best and highest use is still in canopy research, but it still gets pulled out to measure whatever seems interesting, organic and inorganic alike.  That dovetails with Dr. Campbell’s vision of it as a tool for routine use in canopy studies—and everywhere else.  Can you use it on yourself?  “Oh sure,” says Dr. Campbell.  “People clamp the porometer on their fingers all the time.  That’s a quick way to see if it’s working.”  He grins.  “Maybe you could use it as a lie detector on your kids.”

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May 15

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What is the Future of Sensor Technology?

Dr. John Selker, hydrologist at Oregon State University and one of the scientists behind the Trans African Hydro and Meteorological Observatory (TAHMO) project, gives his perspective on the future of sensor technology.

Researcher Pointing to Something while Walking through a Forest

Dr. John Selker (Image: andrewsforest.oregonstateuniversity.edu)

What sparked your interest in science?

I was kind of an accidental scientist in a sense. I went into water resources having experienced the 1985 drought in Kenya. I saw that water was transformative in the lives of people there. I thought there were lots of things we could do to make a difference, so I wanted to become a water resource engineer. It was during my graduate degree process that I got excited about science.

What was the first sensor you developed?

I’ve been developing sensors for a long time.  I worked at some national labs on teams developing sensors for physics experiments. The first one I developed myself was as an undergraduate student in physics. I was the lab instructor for the class, and I wanted to do something on my own while the students were busy. I made a non-contact bicycle speedometer which was much like an anemometer. I took an ultrasonic emitter, trained it on the tire, and I could get the beat frequency between emitted sound and the backscatter to get the bicycle speed.

What’s the future of sensor technology?

Communication

Right now one of the very exciting advances in technology is communication. Having sensors that can communicate back to the scientists immediately makes a huge difference in terms of knowing how things are going, making decisions on the fly, and getting good quality data.  Oftentimes in the past, a sensor would fail and you wouldn’t know about it for months.  Cell phone technology and the ability to run a station on a few AA batteries for years has been the most transformative aspect of technological development.  The sensors themselves also continue to improve: getting smaller and using less energy, and that’s excellent progress as well.

A Picture of a Orange Maple Leaf in the middle of Fall

What often happens is that you install a solar sensor, and then a leaf or a dust grain falls on it, and you lose your accuracy.

Redundancy

I think the next big thing in sensing technology is how to use what we might call “semi-redundant” sensing.  What often happens is that you install a solar sensor, and then a leaf or a dust grain falls on it, and you lose your accuracy.  However, if you had a solar panel and a solar sensor, you could then do comparisons.  Or if you were using a wind sensor and an accelerometer you could also compare data. We now have the computing capability to look at these things synergistically.

Accuracy

What I would say in science is that if we can get a few more zeros: a hundred times more accurate, or ten times more frequent measurements, then it would change our total vision of the world.  So, what I think we’re going to have in the next few years, is another zero in accuracy.  I think we’re going to go from being plus or minus five percent to plus or minus 0.5 percent, and we are going to do that through much more sophisticated intercomparisons of sensors.  As sensors get cheaper, we can afford to have more and more related sensors to make those comparisons.  I think we’re going to see this whole field of data assimilation become a critical part of the proliferation of sensors.

What are your thoughts on the future of sensor technology?

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May 9

Founders of Environmental Biophysics Series: Sterling Taylor

Gaylon Campbell’s first experience with environmental measurement came in the lab of Dr. Sterling Taylor at Utah State University, where he was asked to make water potential measurements in order to understand plant water status. What he learned with Dr. Taylor became the start of four scientific companies and gave Dr. Campbell the tools and the confidence to become one of the world’s foremost authorities on physical measurements in the soil-plant-atmosphere continuum.  Here’s what Dr. Campbell had to say about his association with Dr. Taylor:

Sterling Taylor 1918-1967 Image: dlscience societies.org

Sterling Taylor 1918-1967 Image: dlscience societies.org

Who was Sterling Taylor and why is he considered one of the Founders of Environmental Biophysics?

Sterling Taylor was professor of Soil Physics at Utah State University.  He did his undergraduate work at what was Utah State Agricultural College, and earned his PhD at Cornell University. He worked on both theoretical and practical problems in soil physics.  His practical work focused on research in the area of plant-water relations and irrigation management.   Dr. Taylor worked out water potential limits for both maximum and reduced growth rates of crops. The irrigation limits tables that he put together are still used in today’s handbooks.  His theoretical contributions were on linked transport and applications of non-equilibrium thermodynamics to soil physics, which he was working on at the time of his death.   Dr. W. H. Gardner, a soil physicist of the time, called the amount of work Dr. Taylor and his students did “unparalleled” and noted that attendees at regional conferences often had to carry Taylor’s “weighty reports” home as overweight baggage.

Corner of a bound note book

Attendees at regional conferences often had to carry Taylor’s “weighty reports” home as overweight baggage.

What was your association with him, and how did he influence your life and your science?

Sterling was a kind of second father to me and to many other young scientists.   He loved to help boys and teach them what their potential was.  At that age, I didn’t have any idea that I could do anything in science. The first assignment he gave me was to set up an experiment to measure the simultaneous movement of salt and water in soil.  I had no idea what I was doing, and it was a challenging project.  It would be challenging for me to do it right now!  But he’d give me ideas about how to do the next thing, I’d try to do it, and eventually I got some data that he thought was useful.  He did some analysis of it, and that’s how I learned to measure electrical conductivity and salt concentration in water and soil.  Sterling’s lab is also where my brother Eric and I learned how to make thermocouple psychrometers and other instruments for environmental measurements.  Those insights led directly to the start of Wescor and Decagon.  Campbell Scientific, Juniper systems and others eventually came from those beginnings.

Dr. Taylor was also a very patient man. He made a precision constant temperature bath out of an old washing machine.  It had an agitator in the middle to stir the water while cooling it with coils around the outside of the tub.  It was a wonderful setup, and he took a lot of pride in how well it worked.  He came into the lab one day while I was making some modifications to it.  I was drilling a hole through the outer jacket around the Freon(™) coils where the refrigerant ran.  He said, “Now be careful if you’re drilling holes through that thing so you don’t hit the coils”.  And I said, “Yes, I’m being careful.”  But I wasn’t.  The coils were a couple of inches apart, and I thought, There’s no way I’m going to hit one.  I didn’t even get a ruler.  I just eyeballed it, drilled a hole, and hit the tube dead on.  I couldn’t have hit it more perfectly if I’d measured as carefully as I could. All the refrigerant came hissing out, and I thought he would hear it over in his office.   He probably did hear it, but he didn’t come out to see what was going on.  One of the hardest things I ever did in my life was to go in and tell him I’d drilled a hole in his refrigerant tube.  He just said, “Well…I guess we’ll have to get some new refrigerant.”  He was just patient, and knew how to work with young people.

Student Examining a Textbook Reading the Pages at a Desk in a Classroom

I made a career choice to be a teacher and have students.

But that wasn’t the only way he influenced me.  As it came time for graduation he gave me some advice that had an enormous impact.  Once when I was trying to choose between soil physics and medical biophysics he said “do you want to be a little duck in a big puddle or a big duck in a little puddle?”  I decided on the little puddle.  On another occasion, I was wondering what kind of soil physics position would be best.  One of his former students had gotten a job at an experiment station near Kimberly, Idaho, and I thought that would be ideal.  He observed, “Those can be fun jobs, but if you go to a position like that you just don’t have any offspring.”  That resonated with me, and I thought, “I would like to have offspring.”  So I made a career choice to be a teacher and have students.  It was wonderful to have had that kind of advice at that critical time.

What do you think we missed because he died so early?

It’s interesting to think about scientific contributions and other types of contributions people make.  One of my sons gave me a book of science cartoons, and one of those cartoons shows a couple of scientists talking together. One of the scientists says to the other, “Isn’t it sad to think that everything we come up with now will be disproved in 20 years?”

It just shows you what a transient thing our work is. We think it’s so important, but the important contributions that Sterling made were the numbers of people that he influenced so profoundly.  I’m not the only one he was a second father to.  Sterling Taylor had a huge family of students.  Many went on to prestigious institutions like CalTech (California Institute of Technology), making important contributions over their careers.  And they trace it back to Sterling’s influence on them.

How can scientists today emulate the great man that he was?

I think it would be to not take science so seriously but to take interactions with their fellow travelers seriously. There is a quote by Clayton Christensen from an article in Harvard Business Review on how to emulate what Sterling Taylor was. Christensen says, “I’ve concluded that the metric by which God will assess my life isn’t dollars but the individual people whose lives I’ve touched.  I think that’s the way it will work for us all. Don’t worry about the level of individual prominence you have achieved; worry about the individuals you have helped become better people. This is my final recommendation: Think about the metric by which your life will be judged, and make a resolution to live every day so that in the end, your life will be judged a success.”

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May 2

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Small Company, Big Mission: The Phoenix Mars Lander & TECP Sensor

On May 25, 2008 NASA’s Phoenix Lander successfully landed on the surface of Mars and used a robotic scoop arm to deliver regolith samples to the suite of instruments on the deck of the Lander—with one exception. The Thermal and Electrical Conductivity Probe (TECP), designed by a team of Decagon (now METER) research scientists, was mounted on the knuckle of the robotic arm and made direct contact with the regolith. It measured thermal conductivity, thermal diffusivity, electrical conductivity, and dielectric permittivity of the regolith, as well as vapor pressure of the air.

But, that’s starting at the end of the story.  The fact is that TECP almost didn’t get started.  After seeing a thermal properties needle at the American Geophysical Union meeting in San Francisco, Mike Hecht (project leader on the Mars Environmental Compatibility Assessment (MECA) instrument suite) encouraged his colleague Martin Buehler to call Decagon (now METER) to see if we’d be willing to participate in the Phoenix Lander project. When Martin called one Friday afternoon, announcing that he was from JPL and wondering if we would be willing to fly our sensor on the Phoenix Lander, I was instantly intimidated. I knew JPL was associated with NASA, and I couldn’t imagine why they would be calling Decagon.  I always thought there was a fundamental relationship between NASA and Lockheed Martin, Northrop Grumman, and other major companies that did NASA work.  I told him that Decagon, which was much smaller in those days, didn’t have the capacity to develop instrumentation for space flight. He suggested they come up for a visit and at least consult with us on what they would need to do to obtain this measurement.  The following Monday, we were talking Martian science and inexorably hooked on the idea of joining the team.

The NASA Logo in Front of the NASA Building

I knew JPL was associated with NASA, and I couldn’t imagine why they would be calling Decagon.

Deciding to put one of our sensors on Mars did nothing to lessen the intimidation factor. But, working with Mike and his team at JPL/NASA taught us that doing amazing science can be an inspiring and collaborative effort. I’d always imagined NASA as a group of uber-scientists and engineers sitting in glass offices dreaming up and executing great projects that would be impossible for mere mortals.  The reality is that sending something to Mars and having it do real science requires the combined effort of thousands of smart, dedicated people who are not that much different from the rest of us.

This idea was really brought home when we finally visited JPL. Although the things they were doing were amazing and on a much grander scale, they weren’t that much different from the things we do at Decagon.  They had testing facilities, development facilities, production facilities, and support personnel all working together on projects, just like us.  However, the projects were pretty amazing. We watched the robot arm being tested in a lab for the ability to dig martian soil analogs. We observed an ice probe working in a 55-gallon drum trying to prove it could melt its way down through the thick Martian polar ice caps. We were mesmerized by prototypes of Mars rovers being programmed and executing maneuvers on Martian surface analogs.

It was fun to discover who the Jet Propulsion Lab is and how enjoyable it is to collaborate with people that are thinking about new applications of technology.  This collaboration also benefitted METER’s thermal properties instrument because the mathematical models we developed for Mars made this sensor much more accurate and effective. The Mars project expanded both the depth of our understanding and the breadth of our perspective. Even so, it was fun to find out that scientists who work at JPL have to put their pants on one leg at a time, just like all of us.

Watch this virtual seminar where Dr. Mike Hecht talks Mars, poetry, and Decagon’s (now METER’s) involvement in the Mars Phoenix Lander Mission.

 

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Apr 22

The Scientific Instrumentation Museum of Horrors

Chris Chambers is the primary technical support scientist at METER.  Deep within the recesses of his office, there is a collection of scientific instrumentation we like to call the “Museum of Horrors”.  It showcases the many instruments that have been mangled and destroyed over the years by insects, animals, or the environment.

Melted Serial Cable sitting on a stone

This serial cable melted when it got too close to a sample heating oven.

We get a few instruments back every year that are burned up in a fire, chewed up by rodents, and occasionally we get one that’s been exploded by lightning. We interviewed Chris to find out how to prevent scientific instrumentation from being damaged or destroyed by these types of natural disasters.

Soil Moisture Sensor that got Eaten by Ants

Beware of ant hills. This soil moisture sensor got eaten by ants.

Animals and insects:

The single most important thing you can do to prevent damage from animals is to protect your cables. You can protect your cables with cable armor, electrical conduit, or PVC pipe. Even better is to place cables in some type of conduit and then bury it.  Keeping things tidy around the data logger and avoiding exposed cables as much as possible will go a long way toward preventing animals and insects from ruining your experiment.

An ECH2010 Laying in Dirt and Chipped by a Shovel

A retired ECH2O10 that was hit by a shovel.

Lightning:

Lightning is not as big of a danger on METER loggers as it is with third party loggers (read about logger grounding here). Where we typically see people run into problems with lightning is when they have long lengths of cable between the data logger and sensor. Long cable runs act like lightning harvesting antennae.  The best thing to do is to keep the cables shorter and do not spread them out in lots of different directions.

TEROS12 with a Bent Needle from Being Pushed into a Rock

This soil moisture sensor was pushed into a rock.

Wildfire:

We have a few instruments every year that get burned up in fires, but there is not much you can do about this hazard except for watching for reports of encroaching fires that may be in your surrounding area and evacuating important instrumentation.

Data Logger that was Struck by Lightning Laying in Bark

A data logger that was struck by lighting.

Flooding:

The worst killer of data loggers is flooding.  We have a lot of customers that try and bury their loggers, and that’s generally a terrible idea.  Unless you can guarantee the logger will be waterproofed and put some desiccant inside the box, it will probably end badly.  There are a few scientists out there that have done a really good job of waterproofing, but they generally spend almost as much effort and money waterproofing as they do purchasing the actual logger.

There’s always going to be some risk to your scientific instrumentation because you’re installing it outside, but hopefully, these tips will help you avoid disaster and keep your system out of the museum of horrors.

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Apr 16

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Do the Standards for Field Capacity and Permanent Wilting Point Need to Be Reexamined?

We were inspired by this Freakonomics podcast, which highlights the book, This Idea Must Die: Scientific Problems that are Blocking Progress, to come up with our own answers to the question:  Which scientific ideas are ready for retirement?  We asked METER scientist, Dr. Gaylon S. Campbell, which scientific idea he thinks impedes progress.  Here’s what he had to say about the standards for field capacity and permanent wilting point:

Canola Field right next to an eroded soil cliff

A layered soil, a soil that has a fine-textured horizon on top of a coarse-textured soil, will hold twice as much water as you’ll predict from the -⅓ bar value.

Idea:

The phrase, “this idea must die,” is probably too strong a phrase, but certainly some scientific ideas need to be reexamined, for instance the standard of -⅓ bar (-33 kPa) water potential for field capacity and -15 bars (-1500 kPa or -1.5 MPa) for permanent wilting point.

Where it came from:

In the early days of soil physics, a lot of work was done in order to establish the upper and lower limit for plant available water.  The earliest publication on the lower limit experiments was by Briggs and Shantz in 1913. They planted sunflowers in small pots under greenhouse conditions, letting the plants use the water until they couldn’t recover overnight, after which they carefully measured the water content (WC).  The ability to measure water potential came along quite a bit later in the 1930s using pressure plates.  As those measurements started to become available, a correlation was found between the 15 bar pressure plate WCs and the WCs that were determined by Briggs and Shantz’s earlier work.  Thus -15 bars (-1.5 MPa) was established as the lower limit of plant available water.  The source of the field capacity WC data that established a fixed water potential for the upper limit is less clear, but the process, apparently, was similar to that for the lower limit, and -⅓ bar was established as the drained upper limit water potential in soil.

Sunflowers against a blue sky

Briggs and Shantz planted sunflowers in small pots under greenhouse conditions, letting the plants use the water until they couldn’t recover overnight, after which they carefully measured the water content (WC).

Damage it does:  

In practice, using -15 bars to calculate permanent wilting point probably isn’t a bad starting point, but in principle, it’s horrible. Over the years we have set up experiments like Briggs and Shantz did and measured water potential. We have also measured field soils after plants have extracted all the water they can.  Permanent wilting point never once came out at -15 bars or -1.5 MPa.  For things like potatoes, it was approximately -10 bars (-1 MPa), and for wheat it was approximately -30 bars (-3 MPa).  We found that the permanent wilting point varies with the species and even with soil texture to some extent.

Of course, in the end it doesn’t matter much as the moisture release curve is pretty steep on the dry end, and whether you predict it to be 10 or 12% WC, it doesn’t make a huge difference in the size of the soil water reservoir that you compute.

However, on the field capacity end of the scale, it matters a lot.  If you went out and made measurements of the water potentials in soils a few days after a rain, it would be an absolute accident if any of them were ever -⅓ bar (-33 kPa).  I’ve never seen it.  A layered soil, a soil that has a fine-textured horizon on top of a coarse-textured soil, will hold twice as much water as you’ll predict from the -⅓ bar value.  On the other hand, if you’re getting pretty frequent rains or irrigation, that field capacity number becomes irrelevant. Thus, -⅓ bar may be a useful starting point for determining field capacity, but it’s only a starting point.

Why it’s wrong:

Field capacity and permanent wilting point are dynamic properties.  They depend on the rate at which the water is being extracted or the rate at which it’s being applied.  They also depend on the time you wait to sample after irrigation. Think of the soil as a leaky bucket.  If you were trying to carry water in a leaky bucket and you walked slowly, the bucket would be empty by the time you get the water where you want it. However, if you run fast, there will still be some water left in the bucket.  Similarly, if a plant can use water up rapidly, most of it will be intercepted, but if a plant is using water slowly, the water will move down past the root zone and out the bottom of the soil profile before the plant can use it.  These are dynamic phenomena that you are trying to describe with static variables.  And that’s where part of the problem comes.  We need a number to do our calculations with, but it’s important to understand the factors that affect that number.

Rye Field

Rye field

What do we do now:

What I hope we can do is better educate people. We should teach that we need a value we call field capacity or permanent wilting point, but it’s going to be a dynamic property.  We can start out by saying: our best guess is that it will be -⅓ bar for finer-textured soils and -1/10 bar (-10 kPa) for coarser-textured soils. But when we dig a hole and find out there is layering in the profile or textural discontinuities, we’d better adjust our number.  If we’re dealing with irrigated farmland, the adjustment will always be up, and if we’re dealing with dryland or rain-fed agriculture where the time between water additions is longer, we’ll use a lower number.

Some Ideas Never Die:

One of the contributors to the book, This Idea Must Die, Dr. Steve Levitt, had this to say about outdated scientific ideas, and we agree:  “I love the idea of killing off bad ideas because if there’s one thing that I know in my own life, it’s that ideas that I’ve been told a long time ago stick with me,  and you often forget whether they have good sources or whether they’re real. You just live by them. They make sense. The worst kind of old ideas are the ones that are intuitive. The ones that fit with your worldview, and so, unless you have something really strong to challenge them, you hang on to them forever.”

Harness the power of soil moisture

Researchers measure evapotranspiration and precipitation to understand the fate of water—how much moisture is deposited, used, and leaving the system. But if you only measure withdrawals and deposits, you’re missing out on water that is (or is not) available in the soil moisture savings account. Soil moisture is a powerful tool you can use to predict how much water is available to plants, if water will move, and where it’s going to go.

In this 20-minute webinar, discover:

  • Why soil moisture is more than just an amount
  • Water content: what it is, how it’s measured, and why you need it
  • Water potential: what it is, how it’s different from water content, and why you need it
  • Whether you should measure water content, water potential, or both
  • Which sensors measure each type of parameter

Take our Soil Moisture Master Class

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.

Watch it now—>

Download the “Researcher’s complete guide to soil moisture”—>

Download the “Researcher’s complete guide to water potential”—>

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Apr 12

Thoughts on Soil Sensor Installation from a German Precisionist

Many researchers carefully choose the right instrumentation for their projects, but when it comes to installing the soil sensor into the soil, they are less than careful about the process. Researchers need to know how to install sensors in a way that will allow them to get the most accurate data the instruments are capable of.

Georg Von Unold

Georg von Unold

Georg von Unold has almost two decades of experience installing all types of soil sensors and a German eye for precision that is unmatched in our experience. As the president and founder of UMS (now METER Ag), a German company that develops and manufactures precision soils instrumentation, and a close friend, we thought there would be no one better to share a couple of ideas on careful installation.  Here’s what he had to say:

Pick the Right Place to Install your Sensors

When we develop research instrumentation we look at the accuracy and the resolution of our instruments from a technical point of view.  However, the heterogeneity of research sites can be so vast that we have to take care to select a research site that is representative from a scientific point of view of the results we would like to publish.  We do this first by analyzing the biosphere above the soil that is visible to us, and then perhaps doing some auguring into the soil at various sites to investigate what might be going on in different areas of the field.  If you are researching on a farm, it is important to ask the grower where he’s had good and bad harvest results, where he’s needed to irrigate, and where he’s had problems with erosion.  Always interview people who know the history and specifics of the sites first, because if the sites are flooded or at risk for landslides, it will be a bad choice for long-term monitoring.  Investigating the right place for your sensors before you install will save you time and help you obtain the most applicable and accurate data for your research.

Flat Gravel

We knew that gravel would have bad capillary contact because the stones would have holes between them.

Be Careful with the Way you Install Sensors

One of our research projects used tensiometers to try and determine how water flowed through gravel.  We knew that gravel would have bad capillary contact because the stones would have holes between them. So we decided to make a slurry of fine material from this gravel soil and put it in the installation hole so that the tensiometer would have better capillary contact.  It was a good idea, but it led to misleading results.  What we ended up with was a kind of water reservoir with fine material around the tensiometer which had nothing to do with the true moisture situation in the gravel.  The tensiometer gave us wonderful readings: very constant but with no dynamics that would have been typical for a gravel soil.  When we took it into the lab to investigate, we realized we’d built an artificial soil around our tensiometer.  We weren’t measuring the gravel but were measuring our artificial error which we had created so carefully.  The other thing we found is that over the course of time our slurry would move away from the tensiometer, and within a few years, the tensiometer would be simply hanging in a big gap.  This project also contained fine, heavy soils. Eventually, we realized that we needed an auguring tool that would not push the soil away or compact the soil where we placed the tensiometer because compaction would mean different hydraulic behavior.  So we asked our friends at a Dutch company to make us an auger that was shaped in a form that wouldn’t change the natural soil density that we wanted to measure.

It is important to be careful when you install sensors. For example, if you have a clay soil and you auger a bigger hole than your tensiometer, you will have a water tube around your sensor.  If your soil flooded, the water would flow down your shaft to where your tensiometer is placed, and then what are you measuring?  Thus it is necessary to seal the shaft or to prevent access of surface water to a deeper horizon.

Researcher squatting letting sand fall through his fingers

You need to remember that if you want to measure temperature at a depth of one meter below the surface, the thermal conductivity is strongly dependent on the kind of soil and the moisture of the soil.

Beware of Simple Mistakes

You can also make simple mistakes with other types of soil sensors, such as temperature probes.  You need to remember that if you want to measure temperature at a depth of one meter below the surface, the thermal conductivity is strongly dependent on the kind of soil and the moisture of the soil.  If, for example, you put a temperature probe wired with copper wires in a dry sand or gravel, you will get an average value of the temperature of the sunlight exposed hot cable. The reason is that the copper is leading the temperature down to where you measure and has a much higher conductivity compared to dry, coarse soil.  Thus it is important to think through your installation processes because it is likely you will have a different installation method in a clay soil versus a gravel soil.

Download the “Researcher’s complete guide to soil moisture”—>

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