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为什么说多流速水表优于单流束水表?

答案

参考答案:

单流束水表比多流束水表体积小重量轻,便于携带,且其结构简单。多流束水表比单流束水表的灵敏性能好,在极小流量时也能计量,单流束水表的来水方向只有单一方向,不如多流束水表的来水是叶轮盒的周围均匀来水。所以叶轮轴尖及其轴承易使部分磨损也易产生计量不准的缺点。单流束水表没有产生压力损失最大的叶轮盒。但单流束水表的体积小,过滤网也小,也会产生一定的压力损失。但不比多流速大。单流束水表,其水流来自一方向,故普通型单流束水表的叶轮,叶轮轴尖及其轴承易使部分磨损。如果水中混有泥沙铅屑,则较多流束水表很少引起故障。多流束水表的水流来自叶轮盒的周围,给予叶轮的冲击力较均匀,且喷水速度也大,可以做微量的调整。单流束水来自一方,很难做到微量调整,容易发生计量不准确的现象。总之多流速水表优于单流束水表。

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Burkina Faso student teacher Hema Cecile has a lot more time to crack the books thanks to a recent initiative from the World Bank and the International Finance Corporation (IFC). The launch of the Lighting Africa program by the two organizations this year has made it possible for Cecile to swap kerosene lamps for a solar-powered LED lantern.

Lighting Africa is a $12 million project which intends to bring light to the poorest regions across sub-Saharan Africa. The program works with the lighting industry to develop clean, affordable lighting and energy solutions for millions without access to electric grids. Its aim is to accelerate the market and to develop education programs that inform off-grid populations currently dependent on costly, inefficient and hazardous fuel-based lighting about modern alternatives.

Cecile used to spend $3-4 a month on kerosene for her lamp. That is a large proportion of her earnings—like 70 percent of the population she lives on less than $2 a day. In the weeks since buying her lantern she has managed to read four books including Madame Bovary. by Gustave Flaubert and Emile Zola’s Germinal. She is among the most learned in a society which has the world’s lowest literacy rate, according to a 2007 UN Human Development Report. When she graduates next year she will teach in a local junior school She makes ends meet by holiday jobs as a cleaner and an IT trainer. To earn her daily ration of cornmeal she does shifts from May to September in a corn field.

The lanterns are designed to look like the kerosene ones they are replacing in order to increase adoption among the population. Each has a small solar panel on the top and costs an average $30, although some cost $100, depending on the size of the battery and the number of LED lights it contains.

Because of the large number of sunlight hours in Burkina Faso, the lamps can be relied on to work whenever needed. The battery life is 2-4 years, and can be replaced once they lose their storage capacity. The LED lights last 5-10 years.

Although it is barely out of its trial period the project, Chabanne said there are signs the project is a boon for the population in areas other than household savings and education. "There are fewer people reporting eye problems to the local hospital.

The word "swap" ( Line 4, Para. 1 ) most probably means ()

A. to exchange

B.to destroy

C.to invent

D. to copy