TM 10-6630-245-13&P
1.10.4.1. Colorimetric Blank.
Visible color in any liquid is due to light being absorbed by the liquid. The WQAS-PM Spectrophotometer is an optical
and electronic test instrument that is used to measure the amount of light absorbed by a treated water sample. An
untreated sample of water is placed in a special container called a reference cell. The reference cell is then placed in the
spectrophotometer. Light is shined through the reference cell. The intensity of the light passing through moves a meter
needle. The spectrophotometer is electronically adjusted so that the reading on the meter scale corresponds to a zero
concentration of the substance being tested for. This is called adjusting to the blank.
1.10.4.2. Sample Measurement.
Another container identical to the reference cell is called the sample cell. It is filled with the water sample after treating it
to produce the color change. The sample cell is placed in the spectrophotometer, and light is shined through the cell.
The intensity of light passing through the cell is electronically measured. Electronics move the meter needle an amount
depending on the intensity of the light. The spectrophotometer must be adjusted so that light absorbed by the blank cell
produced a "zero" reading on the meter scale. Therefore the reading on the meter scale with the sample cell is due only
to the light absorbed by the colored substance in the sample cell and not by the sample cell itself or the pure water.
1.10.4.3. Meter Readings.
The spectrophotometer has several cards that can be placed behind the meter needle. On each are printed scale
markings for a particular test. These markings have been developed so that the meter needle position will correspond to
the concentration of the substance being tested for. In addition one card is marked in units of pH. These are units for
measuring the acidity of water. Another card is marked in units for turbidity, and yet another in units of the percent of the
light that is transmitted. The correct meter card is specified for each test procedure.
1.10.4.4. Wavelength.
Different colors seen by the eye are due to different wavelengths of light. The spectrophotometer filters out wavelengths
of light other than the one needed for the color being measured in a particular test. This is done to prevent absorption at
other wavelengths (colors) from interfering with the test. Each test procedure specifies the correct wavelength for that
test, which is then set before performing the test.
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