I want to measure sensible heat flux in the optic path of the LI7500 above a forest canopy with eddy covariance, but the datalogger was just on the ground surface for an old version of colsed path system (LI7000), so I have to use long-cable to collect the10-hz data. But I found the sensible heat flux calculated from the long-cable thermocouples was 30% lower than that from the CSAT3. Direct comparison between two thermocouples (a 1 m long and 100 feet long cable) also get similar result.
I want to know why long-cable attenuated significantly. Thanks!
Hi,
any cable acts as a resistance element and the cable length increases that resistance. Any voltage entering the cable will have a certain attenuation at the end of the cable. In the simplest case the attenuation is just a function of the cable length (direct current only) and the attenuation in terms of voltages can be calculated by
L(u) = 20 LOG u1/u2
when measured voltages u1 and u2. The actual attenuation is a function of cable length, material and also how the cable is constructed (shielded cable) etc.
If the signal is changing with a certain frequency there is another factor depending on that frequency added up to the simple case attenuation and you get:
L(f) = k1 + k2 * f + k3 * Sqrt(f)
with k1 = L(u) the attenuation of the direct current (resistance), k2 the attenuation parameter of the dielectricum (alternating current) and k3 the attenuation parameter according to the "skin-effect" of a cylindrical cable. Info on these parameters should be given by the cable specifications.
hope that's helps you
