You advise to clamp the sample leaf on the left side when facing the stem in order for the compass measurement to be accurate. I have measured small 10 cm long cereal leaves, which at first grow in very upright position in the fields. I'm right-handed and if I clamp the leaf on the left, I usually stand in front of it and shade it, so I’d prefer clamping on the right side. Because the leaves are very small in the beginning, it'd be difficult to perform the measurements with the left hand. Has the wrong measurement practice any other effects besides the compass values? Should I change the measuring direction after the plants grow and leaves turn to grow horizontally or keep the same 'wrong' position through the season?
Measuring from the right side instead of the left side of the leaf will only affect the 'leaf direction' parameter from the compass. If this parameter is not important to your project, then you can ignore clamping from the left side and clamp the leaf however it is most convenient for you. You do not need to keep the same 'wrong' position throughout the year, you can switch from the right side to the left side at any time. Also, currently the compass calibration is not working properly, so the compass data will not be accurate. Hopefully, we will update both the firmware and the mobile app soon to enable accurate compass calibrations.
It is much more critical to avoid shading the leaf during the measurement, which will have a significant impact on the chlorophyll fluorescence parameters (Phi2, PhiNPQ, PhiNO).