Data loggers: To Bury, or Not To Bury (part II)
There are many reasons why you should never bury your data logger.* Most scientists who try it, fail (see part 1). However, there is one innovative team at Washington State University who has found a way to overcome many of the problems which plague buried data loggers. They now happily collect data from the road, sitting in the cab of a truck.
Collecting Data by Radio
Caley Gasch decided she wanted to bury data loggers in an actively managed field at the Cook Agricultural Farm, so they weren’t constantly taking down data loggers for cultivation, spraying, and harvest. She says, “We wanted a better system because after we took the data loggers down, they often did not get put back up for weeks, leaving giant gaps in our data. The idea of burying the data loggers and simply reading them by radio had crossed our minds, but we were stymied by four questions.”
How would we bury the dataloggers so that we could find them again?
To solve this problem, the team buried the data loggers with a radio power identifier ball, originally made by the 3M Corporation, for locating buried power lines. She says, “It’s a radio monitor that transmits a radio signal, and we have an instrument that we can then use to find them.” Caley buries a radio marker with the data logger so that if the flag that marks the logger location gets removed by farming equipment or the weather (which always seems to happen), she still has the ability find the buried data logger.
How would we avoid filling them with water, especially on a large scale?
Caley says she’s had success keeping water out of all but three of her forty-two data loggers. She says the shallow soil in those three locations gets easily saturated in the winter, so they are still trying to modify the system. However, the method they have developed works well for the other 39 data loggers.
Their method is to place the data loggers inside a pelican case, which is a plastic, water-tight box. She says, “We modify the boxes so the sensor cables leaving the data logger can exit the box through cable entry connectors which we tighten down with a plastic screw. We make a watertight seal where the cable can go in and out of the box, and we also add some heat shrink tubing on the cables themselves to tighten that connection. We put silica desiccant packs inside of the pelican box along with the data logger to keep the humidity low. This will collect any condensation that builds up or even soak up small amounts of water that leak in.” Caley says that any water leakage they have had is probably through the ports where they’ve modified the pelican box for cable entry, but in most locations, it’s not a problem.
How could we get radio signal to transmit out of the soil far enough?
Caley says, typically, she can connect with the radio signal up to 100 meters away from the loggers when they are buried. She adds, “We have successfully connected to loggers that are 0.5 km away, but it depends on the landscape, the amount of water in the soil, the season, the kind of crop that’s growing, and the terrain that’s between the scientist and the data logger. We have to get closer to most loggers. 100 meters is convenient enough for the farms that we are working on. The roads are within that distance to each of the loggers, so we never have to actually leave the vehicle to collect our data.”
How long will the batteries last?
Caley says they’ve gotten away with only changing the batteries once a year. She usually collects data twice each year and changes the batteries in the spring. She says, “By the time March comes around the batteries are pretty close to being dead, but we’ve been successful with just five alkaline AA batteries lasting about a year.”
In some cases, the loggers haven’t been buried deep enough, and farm equipment crushed them, or the seeder penetrated the boxes. Caley says, “We just have to make sure they are buried deep enough. We typically bury them at least 30 cm deep, and that seems to work pretty well with the current farm equipment.”
For the Future:
Caley has a new idea for modifying the locations that are prone to flooding. She will keep the loggers buried most of the year, and then dig them up during the winter. “After the harvest in the fall, when the grower gives us permission, we will go out and dig up the boxes and mount the dataloggers on a short post, so they can spend the winter above ground. Then, after the soil has dried a little in the spring, but prior to seeding to minimize disturbance, we will bury them again.” Caley says that even though digging them up in the winter is more work, it’s worth her time. She concludes, “It’s still worth it to bury the loggers during the growing season so we don’t continually have data gaps while growers are seeding, spraying, or making a pass over the field.”
*Note: METER’s (formerly Decagon) official position is that you should never bury your data logger. But we couldn’t resist sharing a few stories of scientists who have figured out some innovative ideas which may or may not be successful if tried at other sites.
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