当差压变送器输出压力大于4mA时，除差压变送器正压室内留有一部分静压外，并假定差压变送器的安装位置与测量液位保持在同一水平，即H为0；所以当H取大值时，就又能得到ΔP=ρgH+ρgh的等式，此时变送器的输出压力已经超出了极限值20mA，由此说明，由ρgh产生的静压力属于多余部分，必须消除冗余部分，才能得到一种迁移类型，称为正迁移。1.2   负迁移如图2所示为负迁移的原理图，如果使用差压变送器的取压室，将密闭容器中的液体或气体注入取压室，测量管线会受到很大程度的损坏，甚至严重腐蚀，为了密闭容器内的液体或者气体流窜。因此，将隔离罐分别安装到了差压变送器的正负压室与取压点之间，并且注入了密度为ρ1的隔离液。2 差压变送器零点迁移故障探讨2.1   正迁移故障在判断正迁移状态下差压变送器的测量精度时，会遇到以下两个问题：一是仪表输出错误。如果安装了正常操作顺序，应先关闭差压变送器三阀组的正、负压测量，然后打开平衡阀，排空仪表塞，4mA的极限值应远于仪器的输出值。如果仪表输出值于极限值，说明操作装置被堵塞，可能的故障部位是三阀组和正压室引线；二是偏移或零位显示异常。

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