Section 8. Theory of Operation
precipitation, etc.) in the optical path. The values of these signals by themselves is
not very informative: all you are guaranteed of is that they will be somewhere
between 0 and 65535, but more typically between 25000 to 50000. To get a useful
diagnostic out of
A
co
and
A
wo
, one needs to know the expected value for a par-
ticular instrument when everything is clean and normal. If
A
cx
is the maximum
expected value for
A
co
, and
A
wx
the maximum expected for
A
wo
, then we could cal-
culate a signal strength
S
c
for CO
2
, for example, from
8-31
S
c
is on a 0 to 100 scale, where 100 is clean. This value is not ‘clipped’ at 0 or 100,
so may slightly exceed 100, especially after signal strength is reset. This simply
reflects slight differences from reality of the temperature compensation function
f
(T)
, as shown below in equation 8-34.
The problem with an implementation this simple is that the raw signals are a func-
tion of not only the optical throughput (obstructions, source and detector aging,
etc.), but also temperature. Unless one accounts for that, the
S
c
could actually
respond to diurnal temperature changes as much or more than contamination
changes. Thus, the proper
A
cx
or
A
wx
needs to be a function of temperature. For-
tunately, we characterize this response at the factory as part of the factory cal-
ibration of each individual unit, which allows us to use the following formulations
for signal strengths for CO
2
and H
2
O:
8-32
8-33
where
C
x
is the maximum value of
A
co
that we would expect from this unit.
W
x
is
the maximum expected value of
A
wo
. The function
f(T)
is the same for both CO
2
and H
2
O, and characterizes how the raw reference signal varies with temperature,
Tblock
, in degrees C.
8-34
Average values of
b
and
c
are both 0.05 for the LI-7200/RS, but the values for a par-
ticular unit are found on its calibration sheet.
on the facing page illus-
8-8
LI-7200RS Enclosed CO₂/H₂O Analyzer
Содержание LI7200RS
Страница 1: ...Enclosed CO2 H2O Gas Analyzer Instruction Manual...
Страница 2: ......
Страница 11: ...ix Appendix C Suppliers Appendix D Configuration Grammar Appendix E Warranty Appendix F Index...
Страница 12: ...x...
Страница 20: ...Section 1 General Information 1 8 LI 7200RS Enclosed CO H O Analyzer...
Страница 54: ...Section 3 Operation 3 12 LI 7200RS Enclosed CO H O Analyzer...
Страница 112: ...Section 4 User Calibration 4 12 LI 7200RS Enclosed CO H O Analyzer...
Страница 130: ...Section 5 Maintenance 5 18 LI 7200RS Enclosed CO H O Analyzer...
Страница 138: ...Section 6 Troubleshooting 6 8 LI 7200RS Enclosed CO H O Analyzer...
Страница 184: ...Section 8 Theory of Operation 8 12 LI 7200RS Enclosed CO H O Analyzer...
Страница 187: ...Gas Analyzer Dimensions Appendix A Specifications A 3 Appendix A Specifications...
Страница 188: ...Appendix A Specifications LI 7550 Dimensions A 4 LI 7200RS Enclosed CO H O Analyzer...
Страница 192: ...Appendix B Pin Assignments B 2 LI 7200RS Enclosed CO H O Analyzer...
Страница 196: ...Appendix C Suppliers C 4 LI 7200RS Enclosed CO H O Analyzer...
Страница 225: ......
