OPEC Eddy Correlation System (Tripod Design)

The following is a description of the methods and materials used in construction of an OPEC System:

Reference to a company trade name is made for identification purposes only and does not imply approval or recommendation. Other equivalent sensors and data logger can be used to build an OPEC system but the available program was written for a Campbell data logger.


Directions for assembly:


The OPEC system consists of several main components. The first is the anemometer/thermocouple assembly. To construct it first cut the EMT pipe into one 4', four 18", and two 7" sections. Then cut the PVC pipe into thirteen 3 1/2" sections. These sections of PVC are to be used as spacers between the EMT pipe and the aluminum fittings . Place one fitting and spacer in the center of the 4' length of pipe. This will be where the mast attaches. Next place two more spacers and fittings on either end of the 4' pipe. Through each of these fittings slide one of the 18" sections up to their center. You should now have something that looks like an "I". Place the last four fittings on the ends of the 18" pipe sections, and orient them so they are perpendicular to the 18" pipe sections. Insert two of the remaining 18" sections (with spacers) on either side of the assembly. Next, place the final two short 7" sections in the remaining fittings. These two short sections will hold the anemometers and the two longer ones will hold the thermocouples.

To attach the anemometer sockets you will need to use the threaded pipe fittings, the increasing pipe fittings and the electrical conduit fittings. This is done by taking the anemometer socket base and screwing the threaded fitting into the end. Next screw the increasing fitting onto the end of the threaded fitting. The electrical fitting should screw on the end of the increasing fitting and the other end should should be able to slip on the end of the short pipe. It is best not to screw the anemometer mast and the propeller on to the socket base until just before installation, because it is fragile.

To install the thermocouples onto the other two pipes you must first prepare the rubber stoppers to hold the thermocouples . This is done by first drilling a small hole through the center of the stopper. After this is done, then the stopper needs to be slit from one edge to the center (intersecting the hole). The thermocouple should now be able to slide down the slit and fit snugly into the hole in the center. The stopper can be plugged into the end of the pipe, thus holding the thermocouple in place. The next step is to install the wires for the anemometers and thermocouples. The thermocouples will come with a wire that will be a direct plug in assembly. The anemometers will require you to solder the ends of the 30' multistranded shielded wire to the leads on the anemometer sockets with the one labeled A as positive and B as negative. The wire should have 4 strands. You will want to cut off the unused strands to keep them out of the way. Once the wiring is complete the wire should be run through the four vertical pipes. Use a couple cable ties to attach the wires to the 4' pipe after they come out of the bottoms of the vertical pipes. These ties will bear the weight of the wires and prevent the soldered connections and the plug connections from coming loose over time. The anemometer/thermocouple assembly is now complete and simply needs the propellers on the anemometer masts to be screwed in place. It would be advisable to remove the thermocouples as they are extremely fragile and put them back in their carrying case to await installation on the mast. Each TC has two fine-wire thermocouples in parallel so that the signal continues even if one of the junctions

The next piece of the assembly is to prepare the rest of the hardware for installation. First, cut the guy wires. They come in a 100' coil and should simply be divided into equal lengths of 33'. At this time it would be prudent to check and make sure all the materials are ready because the remaining installation must be done on site. You will also need an assortment of tools including: a level, a heavy hammer, assorted wrenches, assorted allan wrenches, pliers, wire cutters, various screwdrivers, a painting ladder, wire strippers, and a utility knife.

Once on site, the first thing to set up will be the tripod. Unfold it and place it in the desired location. Use the level to make fine adjustments until the tripod is level. Drive the stakes (included in the kit) through the feet and into the ground to secure the tripod. The tripod should come with a heavy pipe as part of the kit that screws into the tripod. This pipe will not be used. In its place the mast will now be inserted. You may need to screw the mast to get it to slide through the threaded part of the tripod and all the way to the ground. Place a piece of scrap wood under the mast so it does not rest on the ground and rust. Next, install the anemometer/thermocouple assembly by carefully carrying it up the ladder and inserting it on the mast using the aluminum fitting. It should be tightened in place about 4" from the top of the mast. Install the propellers and the thermocouples at this time. The thermocouples should be oriented so they point towards the propellers. Adjust all the pipes so that they are perfectly level or vertical. Make sure all the set screws are tight. On the very top of the mast install the lightening rod that comes with the tripod .

Now telescope the mast up just about three feet and hold it in place using the set screw on the mast . . At this point place the ring for the guy wires and hold it in place using the included clamp. Install the guy wires by inserting them through the three equally spaced holes in the ring. The guy wires should be run through the holes and then twist them around themselves several times.
Once the mast is fully extended secure it by inserting a bolt through the holes drilled in the lower mast section and just below the upper telescoped section . A pin is included for this purpose, but it is too short and there is no way to hold it in place. Once the bolt is in place allow the uppersection of the mast to slide back down on top the bolt. Twist the upper section of the mast until the notches line up with the bolt and it locks in place (this is explained in the instructions that come with the mast). Once this has been accomplished move on to securing the guy wires.
From each section of wire cut a length of about 5' off the end (you should have plenty of wire). Take one of the sections and secure one end to one of the stakes holding the tripod. To the other end secure one of the turnbuckles. Make sure the turnbuckle is not fully unscrewed or fully screwed in. They should have room to adjust in either direction. Take the nearest hanging guy wire and secure it to the other end of the turnbuckle. Try to get the wire as tight as possible. Repeat this process with the remaining wires. After this has been done take the level and check to see if the mast is perfectly vertical. If it is not, then use the turnbuckles to make fine adjustments until it is.
Now it is time to install the enclosure. It should be secured with U-bolts to the mast just above the tripod. Inside the enclosure install the datalogger and the modem . The modem will have a cable that plugs into the modem and into the serial port on the datalogger. The cell phone should also fit in the enclosure. Plug it via the included phone line into the modem. The phone will also have a separate antenna. The antenna can be mounted virtually anywhere. We mounted ours on the end of the net radiometer arm. The antenna cable should be run through one of the conduits on the side of the enclosure and plugged into the phone
At this time you can install the net radiometer. To do this first secure the mounting arm to the mast using two U-bolts . The arm should be oriented so that it is facing south, unless you live in the southern hemisphere, then it will face north. The net radiometer will screw on the end of the arm and has a little window with a bubble for leveling.
.You will also need to place a couple of cable ties on the arm to prevent birds from landing on the arm and pecking at the plastic domes of the net radiometer. This is explained in the directions that are shipped with the net radiometer.
The soil heat flux plate should now be installed. We installed ours by burying it about 1" below the ground about 6" away from one of the legs of the tripod
You are now ready to wire all the instruments into the datalogger

The instruments should be wired according to the following on a Campbell data logger:

Thermocouple 1 to channel 1. chromel (purple)goes to 1H, constantan (red)1L.

Propeller 1 to channel 2. Positive to 2H, negative to 2L.

Thermocouple 2 to channel 3. chromel (purple)goes to 3H, constantan (red)3L.

Propeller 2 to channel 4. Positive to 4H, negative to 4L.

Net radiometer to channel 5. Black to 5L and red to 5H.

Soil heat flux plate to channel 6. Black to 6H and white to 6L.

Once the instruments are wired in you can connect up the power. The battery can simply sit on the ground in its plastic box . The box will protect it from moisture. You will need two lengths of power cord. One will be for the phone and the other will be for the datalogger. Cut them to the appropriate lengths. The end of the cord that connects to the battery will be attached using two of the crimp on ring connectors. These work well with the wing nuts on the terminals. Connect both power cords to their respective positive and negative sides and then to the datalogger and cell phone.The solar charger should also be wired into the battery at this time to provide recharging power. The charger can be mounted anywhere there is room. Be sure, however, that it does not block the "view" of the net radiometer.
Finally, you will need to drive the copper grounding rod into the ground and connect the heavy copper cable to it (see picture) and to the tripod. The tripod has places to connect the cable. There is also a copper wire to connct to the datalogger to ground it as well. This should complete the installation.



Program for cr10x data logger


Materials List:

From Campbell Scientific (www.campbellsci.com)
CR10x Datalogger
COM200 Modem w/power adapter
SC12 Two Peripheral Connector Cable
HFT3-L Soil Heat Flux Plate w/20' lead
Q7.1 REBS Net Radiometer (Fritschen)- Alternate source Radiation and Energy Balance systems. (REBS) P. O. Box 15512, Seattle, WA 98115-0512 Phone: (206) 624-7221

Soil Heat flux sensor ( HFT -3.1)-Alternate source Radiation and Energy Balance Systems
(2) Thermocouple probes TC-BR-3 Bowen Ratio Thermocouples (type "E" chromel-constantan thermocouple, Bowen ratio, 0.003 inch wire, affixed to half a ch-con connector) order four, use two, have two spares.
(2) Chromel-constantan cables to connect probes to the data logger.Choose either FWC (plugs into connector on probe, with 20-ft length) , or FWC-L (matching connector to probe , plus specified length of chromel-constantan wire ). Order three, use two, have a spare. For thirty foot connection order FWC-L 30' Connector Cable for Thermocouples
One TC-BR/ENC (enclosure for TC-BR probes, holds 4)
CM10 Tripod and Grounding Kit
ENC 16/18 Fiberglass Enclosure with mount

From R.M. Young Company

2801 Aero-Park Drive
Traverse City, Michigan 49686
phone: (231) 946-3980; fax: (231

) 946-4772

1) (2) propeller anemometer #27106 with foam propeller, std. bearings,0-500 mV output,
2) replacement foam propeller #08274, .
3) extra bearings (unsealed, 2 req. per anemometer) #27122,
4) propeller torque disk, #18310, (useful for testing bearings).

From Industrial Metal Supply Company (www.imsmetals.com) or other sources

(7) NU-RAIL 1" No. 10 Cross Fittings (see picture)


Other Materials

(3) Turnbuckles
(2) 10' Lengths of 1" EMT Electrical Conduit
(2) 3/4" Threaded Pipe Fittings
(2) 3/4" to 1 1/8" Increasing Pipe Fittings
(2) 1 1/8" Electrical Conduit Fittings
5' of 1 1/8" Thinwalled PVC Pipe
(4) U-Bolts suitable for mounting the enclosure and Pyranometer arm
Cellular Bag Phone w/antenna
(2) #6 Rubber Stoppers to fit the 1" EMT snugly
Assorted plastic cable ties
Crimp-on style electrical connectors
Crimp-on ring connectors (large enough to fit on battery terminals)
Plastic battery box with lid (large enough to hold the marine battery)

10W Solar Charger
20' Telescoping TV Antenna Mast
100' Guy Wire
(2) 30' Multistranded Shielded Wire
Roll of two stranded red/black power cord



Equipment suppliers:

Campbell Scientific, Inc.(www.campbellsci.com)

R.M. Young Company
2801 Aero-Park Drive
Traverse City, Michigan 49686
phone: (616) 946-3980; fax: (616) 946-4772

Industrial Metal Supply Company www.imsmetals.com

Radiation and Energy Balance systems Inc. (REBS)
P. O. Box 15512, Seattle, WA 98115-0512
Phone: (206) 624-7221