Gore-Tex, House wrap and Stomates
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 Decagon researcher, you dash back to your hotel room and clamp a porometer onto the fabric.
Decagon's 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. It's small, accurate, and perfectly easy to use-you just grab, clamp, and get the reading.
The sensor head for the porometer is a clamp holding two relative humidity sensors mounted along a fixed diffusion path. By measuring the vapor concentration at two different points along this path, it's possible to compute stomatal conductance in leaves (see History of the Porometer below)-and measure water loss in a whole range of other materials.
From the very beginning, Dr. 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."
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 Decagon'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 Decagon 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.
The practice of applying a sheathing of tar paper under siding is a hundred years old, but in the last fifteen years, hightech 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 its affordability, its portability, and its sheer practicality make it an every day kind of instrument. It gets pulled out to measure whatever seems interesting, organic and inorganic alike. That dovetails with Dr. Campbell's vision of it as an affordable 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."
History of the Porometer
The first work on measuring stomatal openings started in the late 1930's with scientists like M.C. Desai, F.G. Gregory, and H.L. Pearse. These pioneers would strip the leaf epidermis, make impressions with collodion, look at infiltration with liquids and observe stomata with direct microscopic investigation.
The 1960's opened new possibilities to make measurements with the advent of the Dunmore electronic humidity sensor. This sensor was not very good in retrospect, but it helped launch the concept of the hand-held porometer with Ellis Wallihan's research and design.
With the advent of the Vaisala fast response capacitance humidity sensor in the 1970's another leap forward was made. At this point the commercialization of porometers by Licor and Delta-T put the measurement into the hands of researchers that could not build their own equipment. Licor's design was based on work by C. van Bavel , C. Tanner, and E. Kanemasu. Across the Atlantic, Delta-T used designs based on work by J. Monteith.
As in the past, technology progresses, and now new and improved humidity measurement technology is available, allowing Decagon to provide a new, low cost porometer.
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