Read the tank history from the bottom layer upwards. The latest delivery note will not show an older heel, a previous cleaning agent or material changed by heat and storage time.
The bottom layer may pre-date the substance on the latest delivery note. Dates, rinses and unexplained gaps help the contractor plan for the material that could actually be present. Record that sequence on the tank contents sheet and compare it with chemical tank cleaning where compatibility is unresolved.
Follow the sequence of contents, not only the current label
Dates, concentrations and any unknown gaps help the contractor avoid assuming that one familiar product describes every layer. Tank contents history also informs compatibility, segregation and the route for recovered washings before cleaning is priced.
Leave gaps visible for the contractor to investigate
A missing record should stay missing rather than being replaced with a guess. Photographs from safe positions, sampling information supplied by the site and previous removal notes can narrow the uncertainty.
Note whether the vessel was drained, rinsed or left with a heel after each product. Those events explain why the visible layer may not match the latest delivery.
Keep the history with the vessel reference and pass it to the contractor offering the quotation. For a practical format, use the tank contents information sheet.