Measurement: tank dipping:
Factor affecting the accuracy of tank dips
- Sinking tank floors and or barrelling (stretching in the middle) which requires a new calibration table to be made.
- Sludge - at the bottom of the tank (may not be important for differential readings - see below)
- Water bottoms - where the total amount in the tank is what matters (such as when a quantity of oil is returned to duty suspension) - the presence and size of the water bottom is detected with a water paste smeared onto the tape, which needs to be left in place for up to minute for fuel oil but perhaps only 5 seconds for motor spirit.
- Floating roofs with snow - that adds weight causing the roof to sink lower resulting in the weight of snow being measured as oil (due to increased displacement ) - the trader has to remove the snow before any dipping.
- Listing ship - or pitching - ships are rarely level.
- Entry and exit valves - must be closed before dipping.
- Entry lines (pipes) - should be empty and drain into the tank - if they are not they need to be made to do so, or the measurement will be less than it should be.
- Exit lines (pipes) - should be full or the reading will be exaggerated.
These last two factors are especially important when a reading is taken at the start of receipt of oil into a tank, or the start of loading a delivery, and a second reading is taken later in order to see how much has been received or loaded. They are also important where the pipelines are long - such as to a ship and where the ship could be well below the level of the jetty.
Dip carried out by trader or his representative
The trader does the dip but you must witness it.
The trader, or his representative, who does the dipping must keep the tape or rod steady and it must not swing or the reading will be affected. If water paste is used the tape, which will have a weight attached to the bottom of it, must be held in place long enough to react - a minute in fuel oil. The weighted tape must of course be held steadily against the dipping point marked on the tank.
Requirements for dipping
These need to be available for every tank used for holding oil on which potential revenue is to be assessed whilst the oil is in the tank. They must use metric measures and show the volume in litres for each height in no less than 2mm intervals. If the calibration was done at greater intervals than 2mm, a supplementary table should be provided saying what the volume is calculated to be at the 2mm intervals between the readings.
Dip rods and tapes with selection of weights
Like the tank calibration table, the dip tape or rod should also be marked in no less than 2mm gradations. A dip tape must not vary over its length by 8mm (this is to be reduced to the new standard of 5mm).
Weights are attached to the bottom of dip tapes (due to viscosity); the heavier the oil to be measured then the heavier the weight to be used.
When applied to the bottom of the tape or rod, it changes colour. The tape or rod needs to be left in place for up to 2 minutes in order for the colour change to take place. The volume of water (water bottom) by reference to the tank calibration table needs to be deducted from the overall dip volume calculation in order to accurately determine the remaining volume of oil in the tank.
A fixed dipping point
This will normally be a fixed plate at the top of the tank (at the dipping point) which provides a reference height.
Health and safety requirements
If our safety standards are not met then the measurement should not take place until either this is put right or an alternative form of measurement is introduced that is acceptable.
Large cylindrical tanks should have a diagonally ascending railed in stairway around the side as opposed to a vertical ladder if it is to be used for revenue accounting purposes and thus available for inspection by our officers. Vertical ladders up the side of smaller and squared construction tanks should have caged-in ladders. There should be a suitable guard rail around the top of the tank.