Measurement: automatic Tank Level Gauges- other factors to consider
Factors which may affect measurement by Automatic Level Gauges
- Lack of use - mechanical ones can seize up.
- Low levels in the tank - some gauges can stop working below a certain level and do not take account of liquid being received into, or discharged from the tank, whilst measurement is in progress.
- Rust - unlikely in an oil environment - but this can happen to mechanisms where their parts or components are regularly exposed.
- Turbulence - although many ALGs are designed to dampen down turbulence and to find the true level even though the top surface could be rippling.
- Temperature - oil can form layers at different temperatures. Some meters compensate for this by having an averaging beam that takes and averages temperature measurements at a range of different depths across layers.
- Tank Dips - see HCOTEG177700, because an ALG is effectively an automatic dipping machine, the same provisions apply.
Automatic Level Gauges - other considerations
ALGs have to be fitted and adjusted to each tank, and their measurements must be converted by a calibration table which has been compiled and tested uniquely for each tank (this is most likely to be done electronically).
The fixings of the meter and all equipment that is used to adjust and maintain the meter and the read-out system also need to be controlled effectively by the trader in order to stop fraudulent interference.
Warehouse-keepers should have a control system in place that regularly (usually monthly) compares the ALG to a manual dip/ullage. Any differences outside of the agreed tolerance (currently +/- 8 mm) should be investigated and corrective action taken.
Please note that all new equipment installed after October 2006 should meet the new accuracy tolerance of +/- 5mm when compared to the results of manual dipping.