科学家们用“空洞”一词表示臭氧浓度低于220个多布森单位（多布森单位是用来度量大气中臭氧柱尺度的单位。
Scientists use "hole" as a metaphor for the area in which ozone concentrations drop below the historical threshold of 220 Dobson Units.
例如，可以使用pv度量大小可变的上传操作的进度。
For instance, you can use pv to measure the progress of a sizable upload.
带表外径千分尺用于测量大中型工件外尺寸的高精度量具。
The outside micrometers with dial indicators are a high precision tool used to make the outside measurement of large-sized and medium-sized workpieces.
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