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| MR. GEM (MOBILE RESEARCH GAS EXCHANGE MACHINE) |
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MR GEM is a research tool at the USDA-ARS Research Lab in Morris, MN. It is used to measure soil respiration (carbon dioxide released from the soil), evaporation (water released from the soil), photosynthesis (carbon dioxide absorbed by plants), and evapotranspiration (water released from plants).
The tractor is a four-wheel drive, all-terrain forklift with a large, clear Tuffak® box mounted on the forks. The chamber covers two crop rows 30 inches appart. There are four mixing fans inside the chamber that circulate the air 16 times per minute for the small chamber and 5.5 times per minute for the large chamber. A shuttle pump is mounted on the tractor to circulate a sub-sample of chamber air to the cab for analysis. A magnetic switch is mounted on the base of the chamber to determine when the chamber touches the ground. |
There are locations on the chamber to mount infrared thermometers for measuring plant canopy or soil surface temperatures. A 5000-watt generator is mounted on the outside of the tractor that provides AC power.
The tractor cab contains a computer that controls the data acquisition system, stores data, and executes MR GEM's software, an infrared gas analyzer for measuring carbon dioxide and water vapor, and a global positioning system (GPS) for recording measurement locations. Wet and dry bulb thermocouples are located inside and outside the chamber for air temperature measurements. Radiation sensors (PAR and RI) are mounted on top of the cab for measuring light intensity.
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| During a typical measurement, the chamber is lowered over the soil or plant canopy for sixty seconds. The following data is collected once a second: carbon dioxide, water vapor, inside and outside chamber air temperatures, RI and PAR light intensities, and the position of the magnetic switch. The GPS is queried at the beginning of the cycle for latitude, longitude, and elevation. At the end of the measurement, the chamber is lifted off the soil or plant canopy and the computer calculates the fluxes and stores the data. After correction for appropriate lag times, fluxes (soil respiration/photosynthesis and evaporation/evapotranspiration) are calculated from a 30-second window of the 60 seconds of data. Two methods are used for calculating fluxes. The first is a linear fit of carbon dioxide or water vapor concentrations versus time, and the second is to fit a quadratic equation to these data. Statistical tests are performed to determine which method gives the best fit. The flux is calculated from the slope of the linear or the quadratic at the initial time of the measurement.
When making measurements with MR GEM, the chamber is pointed into the prevailing wind to prevent contamination from the generator's or the forklift's exhaust, which contain carbon dioxide. Measurements are repeated three to four times at each plot location depending on statistical use of the data. Depending on plot locations, MR GEM can make roughly 300 measurements during a 9-hour day that would provide 10 measurement routines of 3 drops at 10 different plots. In addition, MR GEM can be hauled to remote locations on a fifth wheel trailer pulled by a 1 ton truck. |
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Specifications |
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Small chamber dimensions
(width x length x height)
Small chamber volume
Large chamber dimensions
(width x length x height)
Large chamber volume
Aproximate tractor and
chamber weight
Overall length of the tractor
and chamber
Tractor width
Chamber clearance
Wheelbase
Distance between axles
Ground Clearance
Travel speed |
5 ft. x 5.81 ft. x 4 ft.
(1.53 m x 1.77 m x 1.22 m)
112 ft.3 (3.30 m3)
5 ft. x 5.81 ft. x 11.5 ft.
(1.53 m x 1.77 m x 3.5 m)
321 ft.3 (9.48 m3)
6.5 tons (5.9 metric tons)
21.5 ft. (6.5 m)
6.5 ft. (1.98 m)
11.5 ft. (3.5 m)
5.6 ft. (1.7 m)
5.8 ft. (1.78 m)
0.7 ft. (0.2 m)
23.8 mph (38 kph) |
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Last Modified: 01/14/2009
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