在涡度协方差系统中，如何确保测量准确?

测量仪器本身是否会对测量结果造成偏差？
                                                 ——在涡度协方差系统中，如何确保测量准确

        三维超声风速仪是涡度协方差测量系统中的核心测量组件。有研究表明，在对风速进行测量时，哪怕超声风速仪传感器的体积很小，也会对风速测量结果产生偏差^{【1，2，3，4，5，6】}。另外，如果采用合体式设计思路，即把三维超声风速仪和气体分析仪合二为一。由于气体分析仪位于三维超声风速仪采样空间内部或与其非常接近^【7，8】，风速的测量误差就会很大（图1）。

图1 若物体距离三维超声风速仪太近，如气体分析仪，就会导致其风速测量不可靠。

 
        理论上，涡度协方差系统最好测量同一涡旋的风速和其对应的气体密度。但在实际测量时，却不能这样。合体式设计思路，由于其测量组件本身就会对涡旋造成扰动，这种扰动所导致的测量误差很难被量化，且不可进行后续订正【6，7，8，9】。
 
那怎么办呢？研究表明，一个简单的解决方案就是采用分体式思路：三维超声风速仪和气体分析仪以一定间距（10-20cm）分开测量。这种分体式测量，只需对原始数据做一个简单的数据订正就可以得到准确结果【10，11，12】。
 
li-cor的涡度协方差测量系统以严谨的科研成果为依据，采用分体式设计思路（图2），确保了涡度通量数据的准确、可靠。

图2 li-cor分体式涡度协方差测量系统设计思路

 
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