We've all used thermometers to measure temperature ; z: q1 r0 `& Z! }
but sometimes we need to measure temperature in places you (1) __ {+ A/ L% u% P9 Z
can' t put a thermometer -- for example, inside a rocket engine.
1 i: L" E- X7 a0 C c) UOne of the best methods for those really hard-to-get-to ; D9 |7 ]# F1 K, c) o6 x" ~
place is based on laser spectroscopy. Light from a laser point- (2) __
' P8 ~" ?( S6 G7 l/ sing at the region of interest is absorbed by molecules and some (3) __ * X2 j E& u) o) _
of it are re-emitted back towards the source where it can be (4) __ + H" ]. o( d {/ D
measured. The re-emitted fight is shifted in frequency (changed
) s- J+ d- F0 `' v3 d; Bin wavelength) by amounts that depend on the molecule and
. [3 A+ n2 I. ?- nalso the temperature and pressure of the surrounding gas be- (5) __
6 Q: N* X. Q& `+ I* m& Pcause of collisions among the molecules. The single, sharp $ p2 K. S' S# P9 F- y' Q7 b
frequency from the laser is thus smeared out into a whole
, I5 d6 |: M3 X! A9 }& g4 V+ d' u! Ospectrum of frequency whose shape can be used to determine (6) __ : h# U$ C' T/ s8 O4 o n+ m
the temperature provided so that the molecular transitions and (7) __
/ {* N s8 Q& t T# [line shapes are known. Because it is the main component of
7 p9 k: n7 h0 f9 i$ tthe air, it is convenient to use nitrogen since its molecular (8) __
* k* f, d8 r. Y1 P# o0 D; tproperties are known from room temperature to temperature & }8 \) ?" u6 ?4 r7 D+ v5 b: q$ p* m, d
of about 1,200 degree Celsius from experimental measurements. (9) 7 }2 v7 N) l+ b7 Z
Although this is a large range, it is far short of what it is needed (10) __ , y1 M! j: ^8 K# j
for, some applications. |
发表于 2012-8-14 11:35:50
