How to Read a Petrol Pump Dip Chart and Work Out Tank Stock in Litres
A dip rod gives you centimetres. Your business needs litres. The dip chart is the bridge between the two — here is how to use it correctly.
The fuel in your underground tank cannot be seen or weighed. The only way to know how much is there is to measure its height with a dip rod and convert that height into litres. The table used for this conversion is the petrol pump dip chart, also called the calibration chart of the tank.
What is a dip chart?
A dip chart is a table made for one particular tank. For every height of fuel it gives the volume in litres. Because tanks are cylindrical and lie on their side, the volume does not rise evenly: one centimetre near the middle of the tank holds far more fuel than one centimetre near the bottom or the top. That is why the chart is needed and why a rough calculation does not work.
Two points are important:
- Every tank has its own chart. The chart of one tank must never be used for another, even if the capacity is the same.
- The chart is only as good as the reading you look up in it.
How to take a dip reading
- Stop dispensing from the tank for the time of the reading, so the level is steady.
- Use a clean, straight dip rod with clear markings. Apply fuel-finding paste if you use it.
- Lower the rod gently through the dip pipe until it touches the bottom. Do not drop it — a splash gives a false reading.
- Pull it out and read the level at the wet mark straight away, before the fuel evaporates from the rod.
- Repeat once and check that both readings agree.
- Write the reading immediately with the tank, date and time.
Many pumps also check for water at the bottom of the tank with water-finding paste at the same time.
How to convert dip to litres
Find the centimetre reading in the chart and read the litres against it. If your reading falls between two lines of the chart — for example between two whole centimetres — take the proportionate quantity between them. Your chart may give a separate column for millimetres to make this easier.
The result is the physical stock of the tank at that moment.
Dip before and after a tanker delivery
A delivery is the most important time to take a dip, because it is the only proof of how much fuel actually entered your tank.
- Take the dip before decantation and note the litres.
- Decant the tanker.
- Allow the fuel to settle, then take the dip after decantation.
- The difference between the two is the quantity received. Compare it with the invoice quantity.
Keep sales from that tank stopped between the two dips, or account for them, otherwise the received quantity will look lower than it was.
Common mistakes
- Using the wrong chart for the tank.
- Reading the rod at an angle or after the wet mark has faded.
- Taking the dip while the nozzle of that tank is running.
- Taking the after-delivery dip too early.
- Writing the reading later from memory.
From dip reading to daily control
A dip reading is useful only when it is compared with something. Expected stock is what your records say should be in the tank: opening stock plus deliveries minus sales. Physical stock is what the dip says. The difference is the stock variance.
In Sanchay, daily dip readings are entered tank-wise and converted with the dip chart of that tank. Expected stock, physical stock and variance are shown together, and the before-delivery and after-delivery dip are part of every tanker entry. See tank and stock management and tanker and fuel delivery.
This guide gives general information on good practice. Follow the instructions of your oil company and the applicable rules for measurement at your outlet.
Do this in software, not in a register
Sanchay is complete petrol pump management software for sales, shifts, stock, staff, credit and loyalty. Book a demo to see it working.