Sometimes a Girl Wants . . . An Infrared Thermometer

Long ago, I fell in love with the idea of a non-contact infrared (IR) thermometer. A week before Christmas, I dared mention this to my husband. And Christmas morning, I burst into giggles when I opened the package–it was just what I wanted!

Why? It gives me a new sense to see the world, something beyond color, smell, and feel.

Electricians and mechanics use IR thermometers to find hot spots. Versions that are safe for the eyes are used to check people’s temperatures. My OSU colleague has one that makes great thermal images–but it cost $10,000. (See thermal image below by my grad student.) The one I got was under $40 and is fine for playing around. The downside is that it measures large spots–at 8 feet, it measures a spot 1 foot in diameter. But the temperatures are repeatable, and they check out with other thermometers we have around.

Already I’ve opened the oven door to see how well the oven is calibrated. I’ve learned how much of our heat is hanging up at the ceiling. I can see that the new windows are better insulated than the ones we didn’t replace, and that the wound on my hand where the dog bit me, is it really hotter than the rest of my hand.  Our pick-up truck doesn’t have an outside thermometer, but I’ve opened the window to get a few readings on the bank and vegetation–and on the road surface.

That brings me to the measurements. The details are sticky about what anyone really measures when measuring temperature. We don’t usually worry about it. With this toy, I can see more about temperature than I’d ever thought about. Something’s temperature isn’t entirely inherent to the object itself. It depends on what the object is radiating at (open sky? a nearby roof?) and what’s radiating at it (a hot fire? a cold puddle?). Some objects store a lot of heat or cold (stones, barrels of water), but they also take longer to heat or cool than objects that can’t store as much (cushions, dry wood). And some objects are more conductive than others–which explains why it’s a bad idea to use metal nails in the seat of a wooden sauna. Even given all that, if I want to “know what temperature it is outside,” I aim at a few places and get a composite, and go with it.

Here’s what got me intrigued about IR thermometers in the first place. In grad school, my professor was questioned by the cops for aiming a gun at a tree next to a running trail–but it was his IR thermometer mounted on a gunstock. With his lilting Swedish accent, my prof explained that he wanted to see if he could use temperature to observe when plants transpire. He told them that transpiration is the process of opening the pores on the undersides of leaves, so CO2 can diffuse in for photosynthesis. But when the pores are open, he told the cops, a lot of water evaporates out–an unfortunate loss. Transpiration cools the leaves because evaporation (which is the pulling apart of water molecules, like the pulling apart of magnets) takes work (energy, heat)–and that should make a transpiring leaf cooler than one that isn’t transpiring. My professor was let off, but he was told to be less stupid in the future. Later that day, still cranky, he told me, “You must always grab plants in restaurants. If they’re cool Barbara, the plant is alive. If they’re room temperature, the plant is fake.”

I suppose I could shoot restaurant plants with my IR thermometer. More likely, I’ll play with outdoor plants to see if they’re doing what they’re supposed to do. In the summer, they should begin transpiration when the sun shines on them, close their pores mid-day when the air is so warm they’ll lose too much water per CO2 gained, then transpire again until dark.

A droughted Douglas-fir seedling (L), 2 °C warmer than a well-watered one (R) (Mosel 2017)

I’ll also see what they’re doing through the season. As summer goes on, a plant may not have enough water to cool its leaves through evaporation. If leaves get too hot, their proteins (the compounds that lets photosynthesis happen) denature (change shape, become non-functional, “fry”) and the leaves die. If too many leaves die, the branch or the entire plant dies. I should be able to measure if the leaves are getting too hot–and if so, which leaves, and where.

Huge-leaved plants require much water to shed heat.

Leaves facing different directions may shut off transpiration at different times. Same for leaves on trees in wet spots. Same for large vs. small leaves. Large leaves have more “boundary layer” (a thicker coat of unmoving air if there’s a breeze) and so they can’t count on breezes to blow off their extra heat. They need to rely on transpiration for cooling. That’s a major reason that a lot of chaparral plants, desert plants, sun plants, and conifers have evolved small leaves. It’s also why huge-leaved plants like skunk cabbage have to have a lot of water–otherwise, they’d fry.

Canopy dieback in Douglas-fir. The final cause is probably excess heat.

Most of us around here have seen Douglas-fir branches die back in dry years. Part of the story is thermal. That is, the actual cause of death is almost certainly from heat overload from not having enough water to cool themselves of. Here are a couple of ideas on how drought could cause that. A) Insufficient water reaches the foliage because trees have too much leaf area (where water evaporates) per root area (where water comes in). B) Insufficient water is transported through the stem because the wood in the stem has too much resistance (due to wood quality issues or because the stem is too long), or because there just isn’t enough transport area (due to too narrow sapwood, or insects or pathogens making some of the sapwood unable to transport water).

This summer, it’ll be interesting to check on the temperatures of some of the branches. Already I’m trying our houseplants to see which ones wake up first in the morning. I’m measuring whose fingers are colder, mine or my husband’s.  This afternoon, I shot the laundry and then something else in the room to learn if water was still coming off the laundry. What new will come up next week? How hot is a young plantation? How cool is shade? Who knows what I’ll ask–and that’s part of the fun, too.

And maybe, just maybe, someone close to you wants an IR thermometer and hasn’t dared ask.

Mosel J (2017) Physiological responses of loblolly pine and Douglas-fir seedlings from various provenances to timing and frequency of drought stress. M. S. thesis, Dept. of Forest Ecosystems & Society, Oregon State University.

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