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Sounding from an ullage: correct it first, then read across

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Sounding from an ullage: corrected ullage and sounding share one reference heightA cargo tank in section. The reference height, from the reference point at the top of the gauging pipe down to the datum plate on the tank bottom, is 15.15 m. The ullage gauged with the portable unit is 6.52 m; the trim correction from the tank tables, 0.08 m, is added to it: corrected ullage 6.60 m, measured down to the cargo surface. The sounding is measured up from the datum plate to the same surface: 15.15 − 6.60 = 8.55 m.corrected ullage6.60 msounding8.55 mreference pointreference height15.15 mdatum plategauged 6.52 m+ trim correction0.08 m (tank tables)sounding = 15.15 − 6.60 = 8.55 mthe ullage is corrected first, then read acrossEven keel after the trim correction · gauged figures to scale

The rule

  • The ullage is the empty space above the cargo, measured down from the reference point at the top of the gauging pipe; the sounding is the depth of cargo, measured up from the datum plate on the tank bottom.
  • Both hang on one number from the tank tables, the reference height: the distance from the reference point down to the datum plate.
  • When she is not on an even keel, apply the trim correction from the tank tables to the ullage you gauged — before you turn it into anything else.
  • Sounding = reference height − corrected ullage. As the tank fills, the ullage falls and the sounding rises.

Formula

  • Corrected ullage = gauged ullage + trim correction (sign from the tank tables)
  • Sounding = reference height − corrected ullage

Worked example

Tank reference height, ullage datum to the datum plate15.15 m
Ullage gauged with the portable unit6.52 m
Trim correction from the tank tables, to be added0.08 m
  1. Corrected ullage = 6.52 + 0.08 = 6.60 m
  2. Sounding = 15.15 − 6.60 = 8.55 m

Answer8.55 m

Common mistake

Writing the corrected ullage, 6.60 m, into the sounding column.

Both are believable depths in metres, so nothing on the sheet looks wrong — but one is measured from the top and the other from the bottom: here the cargo stands 8.55 m deep, not 6.60 m, and the tank table will turn the wrong one into the wrong volume.