Installing the Gas Analyzer and Sonic Anemometer Combination Mount

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The open-path gas analyzer and sonic anemometer mount (part number 7900-320) is used to mount an LI-7500x gas analyzer in an eddy covariance system with a Gill WindMaster/Pro or R3-50 sonic anemometer. In this document we describe how to use the mount.

The mount provides a number of advantages over a standard Nu-rail and riser bar mount, including the following:

  • Maintains the ideal vertical and horizontal positioning relative to the anemometer
  • Maintains the ideal separation from the anemometer, regardless of orientation
  • Makes it easy to rotate the gas analyzer and anemometer together as a unit and independently of each other, so that the system can be oriented in the ideal direction.

Components

Description Quantity
Combination mount 1
Bolts (M6 x 35) 3
Flat washers (A6) 6
Lock washers (J6) 3
Nuts (M6 x 35) 3
Set pins 2

Assembling the mount

Note: We recommend assembling the components on the ground and installing the completed assembly on your tower or tripod as a unit. Once it is installed on the tower or tripod you can loosen the bolts to make adjustments. Determine the prevailing wind direction for your site before you install the Analyzer and Sonic mount. That information will guide the orientation of the anemometer and the position of the gas analyzer.

You need the following things to assemble the mount:

  • Analyzer and Sonic Mount (part number 7900-320)
  • Sonic anemometer mounting post (part number 900-11242)
  • Gill WindMaster, WindMaster Pro, or R3-50 sonic anemometer
  • LI-7500 or LI-7500A/RS/DS and the gas analyzer mounting post (part number 9975-010)
  • Standard Nurail® crossover fitting (part number 7900-342) that mounts to a tower or tripod cross-arm.

Keep some tape nearby to hold the bolts in place. If possible, have an assistant help position the components while you tighten the bolts.

  1. Insert the bolts into the anemometer bottom plate with a flat washer, as shown.
  2. It might be helpful to apply some tape to the top of the bolts to help hold them in place for the rest of the installation.
  4. Tighten the nuts to finger-tight.
  5. Install the gas analyzer in the mounting apparatus.
  6. Use a ~4 mm or 3/32" hex key to tighten the set pins. Tighten each set pin until it contacts the pipe or post, and for aluminum pipe, tighten about 1 full revolution more, or for a stainless steel post, tighten about ¼ revolution more.
  8. Install the assembly into a Nurail® crossover fitting on a tower or tripod cross-arm.
  9. Be sure that there is adequate clearance beside the Nurail cross-over fitting and cross-arm for the anemometer cable. If the bottom of the anemometer is above the cross-arm, there will be insufficient clearance for the cable. Rotate the cross-over fitting 180° on the cross-arm to gain adequate clearance.

  1. Orient the anemometer
  2. To begin, orient the north notch toward magnetic north. Then rotate the anemometer clockwise so that the north notch is opposite the prevailing wind direction at your site. Record the offset from magnetic north (positive degrees clock-wise).
  4. Orient the gas analyzer
  5. The analyzer and sonic mount can be rotated independently of the anemometer (loosen the three bolts first). Rotate the mount until the gas analyzer is oriented as desired. Ideally, the gas analyzer will be adjacent to one of the anemometer spars, downwind of the predominant wind direction. This can minimize airflow distortion from the predominant wind direction.
  6. Determine the eastward and northward separations.
  7. The distance (cm) of the gas analyzer from the sonic anemometer must be known along the north-south and east-west axes. The northward separation is positive if north and negative is south. The eastward separation is positive if east and negative if west.
Figure 2‑1. Measure the Northward and Eastward separations after installing the instruments and establishing the orientation.
  1. Tighten all the mounting hardware.
  2. Tighten the three anemometer nuts and bolts. Tighten the two set pins that secure the gas analyzer, and tighten the set pins in the cross-over fitting.
  3. Enter the North offset in the gas analyzer software site metadata.
  4. In the LI-7500A/RS or LI-7500DS software, the north offset is entered in Site Setup > Sonic Anemometer > North offset (°) as degrees positive from magnetic North.
  6. Optionally, in EddyPro, under Basic Settings > North Reference, select Use magnetic north (this is the default setting used by the SmartFlux System, so applying it should not be necessary unless you've changed the settings previously).
  7. Enter the Northward Separation, Eastward Separation, and Vertical Separations in the gas analyzer software site metadata.
  8. In the LI-7500A/RS or LI-7500DS software, the separations are set under Site Setup > CO2/H2O Analyzer.
Table 1. Northward and eastward offsets for the gas analyzer. Positions correspond with the 12 markings on the mounting apparatus, as indicated. The center of the anemometer represents 0, 0 on an x, y coordinate system. Offsets are based on the assumption that position 1 is oriented toward magnetic north. If position 1 is oriented in any other direction, recompute or remeasure the offsets.
Position Northward Offset (cm) Eastward Offset (cm) Vertical Offset (cm)
1 18.5 6.0 0
2 13.0 14.4 0
3 4.1 19.0 0
4 -6.0 18.5 0
5 -14.4 13.0 0
6 -19.0 4.1 0
7 -18.5 -6.0 0
8 -13.0 -14.4 0
9 -4.1 -19.0 0
10 6.0 -18.5 0
11 14.4 -13.0 0
12 19.0 -4.1 0