问题 问答题 简答题

什么是仪表的精确度等级?什么是仪表的回程误差(变差)?

答案

参考答案:

仪表的精确度是用来反映测量结果与被测量真值的偏离程度。它的数值等于引用误差去掉百分号以后的绝对值,并将此值作为精确度等级指标。一般工业仪表的精确度等级有0.1、0.2、0.5、1.0

1.5、2.5、4.0等。

仪表的回程误差(变差)是指在规定的条件下,用同一仪表对被测量进行正、反行程的测量,对某一测量点所得到的正、反行程两次示值之差称为该测量点上的示值变差。在整个仪表量程范围内,各校验点的最大示值变差称为该仪表的变差。合格的仪表,其变差不得超过仪表的允许误差。仪表的变差一般以引用相对误差的形式来表示:

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In the early 1960s Wilt Chamberlain was one of the only three players in the National Basketball Association (NBA) listed at over seven feet. If he had played last season, however, he would have been one of 42. The bodies playing major professional sports have changed dramatically over the years, and managers have been more than willing to adjust team uniforms to fit the growing numbers of bigger, longer frames.

The trend in sports, though, may be obscuring an unrecognized reality: Americans have generally stopped growing. Though typically about two inches taller now than 140 years ago, today’ s people—especially those born to families who have lived in the U. S. for many generations--apparently reached their limit in the early 1960s. And they aren’t likely to get any taller. "In the general population to- day, at this genetic, environmental level, we’ve pretty much gone as far as we can go," says anthropologist William Cameron Chumlea of Wright State University. In the case of NBA players, their in- crease in height appears to result from the increasingly common practice of recruiting players from all over the world.

Growth, which rarely continues beyond the age of 20, demands calories and nutrients—notably, protein—to feed expanding tissues. At the start of the 20th century, under-nutrition and childhood infections got in the way. But as diet and health improved, children and adolescents have, on average, increased in height by about an inch and a half every 20 years, a pattern known as the secular trend in height. Yet according to the Centers for Disease Control and Prevention, average height—5’9" for men, 5’4" for women—hasn’t really changed since 1960.

Genetically speaking, there are advantages to avoiding substantial height. During childbirth, larger babies have more difficulty passing through the birth canal. Moreover, even though humans have been upright for millions of years, our feet and back continue to struggle with bipedal posture and can- not easily withstand repeated strain imposed by oversize limbs. "There are some real constraints that are set by the genetic architecture of the individual organism," says anthropologist William Leonard of Northwestern University.

Genetic maximums can change, but don’t expect this to happen soon. Claire C. Gordon, senior anthropologist at the Army Research Center in Natick, Mass. , ensures that 90 percent of the uniforms and workstations fit recruits without alteration. She says that, unlike those for basketball, the length of military uniforms has not changed for some time. And if you need to predict human height in the near future to design a piece of equipment, Gordon says that by and large, "you could use today’s data and feel fairly confident.\

The text intends to tell us that()

A. the change of human height follows a cyclic pattern

B. human height is becoming even more predictable

C. Americans have reached their genetic growth limit

D. the genetic pattern of Americans has altered