unless如何理解在整句？ 个人觉得又是逗号惹得祸。楼主的逗号是这样放的： in general therefore valves tend to be nickel or copper based materials and unless the pressure differential is less than 1 bar g , a pipe section 10x diameter is also required in a nickel or copper based material both upstream and downstream of the valve position. 因此，阀门倾向于用镍或铜基材料，除非压差小于1bar 。 而且阀门位置上下游10倍管径处也要求是镍或铜基材料。 3楼的逗号是这样放的： in general therefore valves tend to be nickel or copper based materials , and unless the pressure differential is less than 1 bar g a pipe section 10x diameter is also required in a nickel or copper based material both upstream and downstream of the valve position. 因此，阀门倾向于用镍或铜基材料 。 除非压差小于1bar，阀门位置上下游10倍管径处也要求是镍或铜基材料。 觉得3楼的比较准确，但如果不是英语国家的人写的，那就得根据上下文好好推敲了。 还有，压差是2个压力的差值，应该直接用bar表示，而不用加g。查看更多
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