
58-6 Chemical Variables
constant voltage conditions, or feedback circuitry can be used to maintain the lament resistance con-
stant,
bu
t
in ea
ch
ca
se,
ch
anges
in ga
s
co
mposition
le
ad
to an ou
t-of-balance
vo
ltage
in th
e
Wh
eatstone
br
idge
c
ircuit
.
e TC
D
is a un
iversal
de
tector;
ho
wever,
it is le
ss
se
nsitive
th
an
ot
her
de
tectors
su
ch
as th
e
FI
D
an
d
is pr
incipally
us
ed
fo
r
de
tection
an
d
me
asurement
of pe
rmanent
ga
ses
su
ch
as ox
ygen,
ar
gon,
ni
trogen,
ca
rbon
mo
noxide,
an
d
ca
rbon
di
oxide,
wh
ich
ei
ther
ca
nnot
be me
asured
by th
e
FI
D
or re
quire
sp
ecial
pr
etreatment
of th
e
e
uent
ga
s
(s
ee
su
cceeding
te
xt)
. It ca
n
be sh
own
th
at
th
e
se
nsitivity
is gr
eates ...