We've all used thermometers to measure temperature " H9 u/ C$ G& a( N, q; r+ M
but sometimes we need to measure temperature in places you (1) __
+ |2 e7 }) |8 F/ @4 i: {can' t put a thermometer -- for example, inside a rocket engine. : r: m% p- W% A: A
One of the best methods for those really hard-to-get-to
, D) L# Z) L1 Q' Y6 rplace is based on laser spectroscopy. Light from a laser point- (2) __ 4 `+ ^. m1 j" Y; J) ^
ing at the region of interest is absorbed by molecules and some (3) __
' s/ F7 c- M3 \8 F- A7 Aof it are re-emitted back towards the source where it can be (4) __ 5 } t7 ~! w+ d' i @( w: ?
measured. The re-emitted fight is shifted in frequency (changed ( k: F. n! w! E$ O# c7 T6 ~0 r
in wavelength) by amounts that depend on the molecule and ) h! w- h" ^+ P( A+ t# Q+ c
also the temperature and pressure of the surrounding gas be- (5) __ ) S" T9 o0 Z" l# `
cause of collisions among the molecules. The single, sharp S' A, o% u3 N9 l# N
frequency from the laser is thus smeared out into a whole 2 G- ~" K1 O u0 J
spectrum of frequency whose shape can be used to determine (6) __
6 g6 ]+ o b& Hthe temperature provided so that the molecular transitions and (7) __
$ L% s/ n4 N8 J, n D; `& Q) Q. Xline shapes are known. Because it is the main component of , C& j& _) s4 D, d- G) S @' q% h2 I
the air, it is convenient to use nitrogen since its molecular (8) __ 1 c# D! k0 ]) O: z
properties are known from room temperature to temperature
& b0 l% A: K# eof about 1,200 degree Celsius from experimental measurements. (9)
* E( C8 L! s% [; ^% \Although this is a large range, it is far short of what it is needed (10) __ ( S, U! X9 O; O) H( W2 s2 Q: Y
for, some applications. |
发表于 2012-8-14 11:35:50
