An obsolete unit used in measuring the clarity of water: the lower the value, the clearer the water. Symbol, JTU or Jtu. Also called a Jackson Candle unit. Water containing 100 parts per million silica had a turbidity of 21.5 JTU. The scale was, in fact, built around suspensions of silica, in the form of diatomaceous earth.
The test relied on the human eye. The flame of a standard candle was viewed through a vertical tube. The tube was gradually filled with the water to be measured until the flame was no longer distinct. The depth of water in the tube, which had a scale marked in JTU, indicated the turbidity.
Both the unit and the measuring technique are obsolete; however, measurements in the units that replaced it (NTU and FTU) are numerically close to JTU measurements, but with a much lower margin of error.
ASTM D 1129, D-19; D 1889, D-19.
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Last revised: 1 June 2007.