关于一个化学分析的The enclosure may be sealed and the mixing fanoperated for a period of up to 12 hours before the initial HC concentrationreading and the initialmethanol concentration reading is taken and the four-hour backgroundmeasurement
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关于一个化学分析的The enclosure may be sealed and the mixing fanoperated for a period of up to 12 hours before the initial HC concentrationreading and the initialmethanol concentration reading is taken and the four-hour backgroundmeasurement
关于一个化学分析的
The enclosure may be sealed and the mixing fanoperated for a period of up to 12 hours before the initial HC concentrationreading and the initialmethanol concentration reading is taken and the four-hour backgroundmeasurement period begins
有几个词连起来了，这里是修改过的
The enclosure may be sealed and the mixing fan operated for a period of up to 12 hours before the initial HC concentration reading and the initial methanol concentration reading is taken and the four-hour background measurement period begins.
enclosure 实验间 HC碳氢 methanol 甲醇，上下文我也没法连接，
关于一个化学分析的The enclosure may be sealed and the mixing fanoperated for a period of up to 12 hours before the initial HC concentrationreading and the initialmethanol concentration reading is taken and the four-hour backgroundmeasurement
外壳可密封和混合fanoperated一段长达12小时前的初始HC concentrationreading和initialmethanol浓度读数和四小时backgroundmeasurement期开始
密闭搅拌12小时，得到初始HC浓度及初始甲醇浓度，然后开始4小时的背景测量周期。。。。。。。。不知道对不对，具体还得联系上下文。你的原文中可能少了几个空格