根据汉语完成句子。

1. We felt as if we____the whole thing ．

   (与过去事实相反)我们感到这一切都是我们亲眼所见似的.

2. He opened his mouth as if he____ something.

    (与将来事实相反)他张开嘴好像要说什么.

3. It looks as if ___.

    看起来要下雨的样子.(陈述事实)

某仪器测量工件尺寸的理论标准差为0.004㎜，如要求计量结果的总不确定度小于0.005㎜（置信概率99.73％），问至少须测几次？