Interpretation of Results:
The mini burette has 20 numbered major divisions. Between each there are 5 unnumbered minor
divisions. The major divisions correspond to 0.2 mg/I. The minor divisions correspond to 0.04 mg/l.
For example, a reading of 18 major divisions and 2 minor divisions (18.2) would then correspond to:
18 times 0.2 = 3.60 mg/l
2 times 0.04 = 0.08 mg/l
3.68 mg/l of dissolved oxygen in the sample.
Sample handling is important for meaningful results. The amount of oxygen dissolved in water
changes with depth, temperature, and factors such as sludge, currents, and microbes in the
water. Test results can vary depending on how much the test sample was exposed to air, how
long it was kept before "fixed" and other factors. It is recommended to take several samples at
different depths and locations and perform the test several times for good results.
The amount of oxygen dissolved in water varies considerably with changes in temperature and pressure.
Table 2-2 shows, for combinations of temperature and pressure, the amount of oxygen in mg/l present in a
pure water sample that is saturated with oxygen. This table can be used as an approximate guide to
compare the measured oxygen content to the maximum possible oxygen content for a sample of water at
the temperature and pressure of the test sample. Differences between the maximum possible amount in
the table and the actual measured amount are due to many factors, such as biological and chemical
activity and physical factors such as turbulence.
In using the table you must first determine the pressure for the test sample. If the sample was taken near
the surface, the sample pressure is equal to the atmospheric pressure. If it was taken below the surface ,
the sample pressure is the atmospheric pressure plus the pressure of the water itself. The table lists
pressure in terms of inches and millimeters of mercury (Hg). One inch of water exerts a pressure equal to
that of 0.074 inches of mercury. Approximate the pressure for a sample taken below the surface as
follows. Multiply the sample depth in inches by 0.074. Then add that number to the atmospheric pressure
in inches of mercury at the time of measurement to get the sample pressure in inches of mercury.
For instance, if the sample was taken 12 inches below the surface of the water source:
Inches sample was submerged: 12 inches of water Atmospheric pressure at time of measurement:
27.56 in Hg The temperature of the water is 68°F.
Multiply times conversion factor: 12 x 0.074 = 0.888 Pressure of sample = 27.56 + 0.888 = 28.45
inches of Hg (rounded).
from the table the pressure value listed closest to 28.45 is 28.54. For a temperature of 68 °F, the
maximum amount of oxygen that can be dissolved in water is about 8.7 mg/l.