Page 343 - Instrumentation Reference Book 3E

326 Chemical analysis: spectroscopy

Figure 16.8 Barringer remote sensing correlation spectrometer.
designed to correlate sequentially in a positive and Most of these applications make use of the fact
negative sense with absorption bands of the target that different elements will form colored solu-
gas by rotation of the disc in the exit plane. The tions with reagents. The absorption of these
light modulations are detected by photomultiplier colored solutions is then proportional to the con-
tubes and processed in the electronics to produce a centration of that particular element.
voltage output which is proportional to the optical
depth (expressed in ppm meters) of the gas under 16.1.4 Measurements based on reflected
observation. The system automatically compen- radiation
sates for changes in average source light intensity
in each channel. The basic principle of this method Just as measurements of moisture, or other com-
rests on comparison of energy in selected propor- ponents, may be made by comparison at two
tions of the electromagnetic spectrum where wavelengths of transmitted infrared radiation, the
absorption by the target gas occurs in accordance method will work equally well by measuring the
with the Beer-Lambert law of absorption. attenuation when infrared is reflected or backscat-
Typically, this instrument covers the range tered. The principle is illustrated in Figure 16.9.
1-1000 ppm m or 100-10000ppm m, this unit For water measurement of paper or granulated
being the product of the length of the optical path material on a conveyor belt, the intensity of the
through the gas and the average concentration reflected beam at the moisture absorption wave-
(by volume) over that length. length of 1.93pm may be compared with the
The second instrument which covers absorp- intensity at a reference wavelength of 1.7 pm.
tion in the visible in liquids is the Brinkmann The beams are produced by interposing appro-
Probe Colorimeter. This instrument is basically priate filters contained in a rotating disc in front
a standard colorimeter consisting of a tungsten of a lamp producing appropriate radiation. The
light source, the output from which passes radiation is then focused onto the measured
through one of a series of interchangeable filters material, and the reflected beam focused onto a
covering the wavelength range 420-880 nm, then lead sulfide photoelectric cell. By measuring the
through a light pipe at the end of which is a probe ratio of the intensity of radiation at two wave-
cell. This cell has a reflecting mirror at one end lengths, the effects of source variation, detector
and so the optical path length is twice the length sensitivity, and drift in the electronic circuitry are
of the cell. The light then returns to the instru- minimized. Furthermore, calibration has shown
ment via a second light pipe to a photomultiplier, that for a number of materials the results are
the output of which is amplified and fed to a substantially independent of the packing density.
recorder in the usual way. This instrument is ideal However, if the measured material is a strong
for measuring turbidity in liquids and has the absorber of radiation, a powerful source of radia-
advantage that very small volumes of liquid tion such as a water-cooled quartz halogen lamp
(down to 0.5 ml) may be examined. Its other uses may be necessary.
include general quality control, chemical ana- With this type of instrument on-line measure-
lyses, pollution control, and food processing. ment of the moisture content of sands, clay, dust,