Page 336 - Instrumentation Reference Book 3E

Absorption and reflection techaiques 319

The detector consists of two sealed absorption The instrument is usually run from a.c. mains
chambers separated by a thin metal diaphragm. through a constant voltage transformer. Where
This diaphragm, with an adjacent perforated utmost stability is required an ax. voltage stabi-
metal plate, forms an electrical capacitor. The lizer may be used, as the constant voltage trans-
two chambers are filled with the gas to be former converts frequency variations to voltage
detected so that the energy characteristic of the changes. Generally, the instrument is insensitive
gas to be measured is selectively absorbed. to temperature changes, although the gas sensi-
The reference cell is filled with a non-absorbing tivity depends on the temperature and pressure of
gas. If the analysis cell is also filled with a non- the sample gas in the absorption tube. since it is
absorbing gas, equal energy enters both sides of the number of absorbing molecules in the optical
the detector. When the sample is passed through path which determines the meter deflection. For
the analysis cell, the component to be measured instruments sensitive to water vapor the detecting
absorbs some of the energy to which the detector condenser has a temperature coefficient of sensi-
is sensitized, resulting in an imbalance of energy, tivity of 3 percent per Kelvin, and it is therefwe
causing the detector diaphragm to be deflected necessary to maintain the detector at a constant
and thus changing the capacitance. This change temperature.
is measured electrically and a corresponding
reading is obtained on the meter. Table 16.1 Sensitivity of non-dispersive infrared analyzer
Any other gas also present in the sample will
not affect the result unless it has absorption Gas Mininmrn Gas itfinirnum
bands which overlap those of the gas being deter- concentration concentration
mined. I13 this event; filter tubes containing the for full-scale for full-scale
interfering gas or gases can be included in one or deflection, deflection,
(Vol. Yo)
(~QL?. yo.)
both optical paths, so that the radiation emer-
ging from these tubes will contain wavelengths co 0.05 NO2 0.1
which can be absorbed by the gas to be detected con 0.01 so2 0.02
but will 'contain very little radiation capable of H20 0.1 HCN 0.1
being absorbed by the interfering gases in the CH4 0.05 Acetone 0.25
sample, since such radiation have already been C2H4 0.1 Benzene 0.25
removed. N20 0.01
The length of absorption tube to be used
depends upon the gas being estimated and the The approximate maximum sensitivity to cer-
concentration range to be covered. The energy tain gases is given in Table 16.1.
absorbed by a column of gas 1 cm long and con- Errors due to zero changes may be avoided
taining a concentration c of absorbing compon- by the use of a null method of measurement
ent is alpproximately Elkc, where E is the illustrated in Figure 16.l(b). The out-cf-balance
incident energy and k is an absorption constant, signal from the detector is amplified, rectified
provided that kcl is small compared with unity.
Thus at low concentrations it is advantageous to by a phase-sensitive rectifier, and applied to a
servo system which moves a shutter to cut off
use long absorption paths, provided kcl remains as much energy from the radiation on the
small and the relationship between energy reference side as has been absorbed from the
absorbed and the measured concentration analysis side. and so restore balance. The shut-
remains reasonably linear. At higher concentra- ter is linked to the pen arm, which indicates
tions the energy absorbed is E[l - exp(-kcl)], the gas concentration.
and the relationship between energy absorbed
and concentration departs greatly from linearity
when absorption exceeds 25 percent. When the On-line infrared absorption meter using two wave-
absorption reaches this value it is: therefore, lengths In order to overcome the limitations of
necessary to reduce the length of the absorption other infrared analyzers and provide a rugged
cell, and the product c x 1 should be kept reliable drift-free analyzer for continuous opera-
approximately constant. tion on a chemical plant, IC1 Mond Division
The most convenient method of calibrating the developed an analyzer based on the comparison
instrument is to pass mixtures of the pure gas of of the radiation absorbed at an absorption band
known composition through the measuring cell and with that at a nearby wavelength. By use of this
note the output for each concentration of measured comparison method many of the sources of error
gas. For day-to-day checking a simple internal cali- such as the effect of variation in the source inten-
brating device is fitted, and it is only necessary to sity, change in the detector sensitivity, or fouling
adjust the sensitivity control until a standard deflec- of the measurement cell windows are greatly
tion is obtained. reduced.