Leaf Temperature Thermocouple Height

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Instructions for positioning the leaf temperature thermocouple.

Leaf temperature is an important input to many parameters calculated on the LI-6800, including stomatal conductance (gsw) and leaf intercellular CO2 concentration (Ci). The LI-6800 leaf temperature probe is a Type E thermocouple mounted on flexible material (Figure 1‑1). The thermocouple junction is a ~0.008” diameter bead. The bead is too small to be seen without magnification (Figure 1‑2).

Figure 1‑1. The LI-6800 leaf thermocouple assembly (LI-COR part # 9968-245).

Accurate measurement of leaf temperature requires that the thermocouple bead be high enough to ensure it is making contact with the leaf surface. If the thermocouple is not pushed into the chamber high enough, the thermocouple will instead measure air temperature in the chamber. If the thermocouple is pushed too far into the chamber, leaf tissue can put pressure on the flexible mounting board of the material. Two possible undesirable results can come from this. First, it is possible to tear or puncture delicate leaves. Second, too much bending can put undue stress on the thermocouple wires, which may eventually lead to failure.

One solution for ensuring consistent, correct thermocouple height is to place a few o-rings on the outside of the thermocouple mounting body (Figure 1‑3). Here we demonstrate that in this particular thermocouple, (3) 10 x 1.5 mm o-rings (LI-COR part # 192-14435, available in the LI-6800 spare parts kit), allow for appropriate thermocouple leaf contact with the standard gaskets. Due to tolerances in the manufacture of the thermocouples, the correct number of o-rings may vary and should be verified for each probe. In addition to preventing the problems discussed above, this will prevent the thermocouple from being pushed further into the chamber if the bottom of the thermocouple should be bumped or when setting down the sensor head.

Figure 1‑2. A) Front face and B) side view of the leaf thermocouple assembly. In both figures, the red arrow points to the thermocouple bead that must contact the leaf surface.
Figure 1‑3. A) Thermocouple installed on the 6800-01 fluorometer chamber using 3 o-rings (part number 192-14435) to ensure correct height. B) A close-up of the thermocouple bead, showing that the thermocouple bead is above the top of the lower gasket. With a leaf in the chamber, the probe will flex and ensure that the thermocouple bead contacts the surface of the leaf.