How to check fuel density at a petrol pump.

• A clean glass measuring jar (sample cylinder).
• A hydrometer for the right range — one for petrol, one for diesel.
• A thermometer.
• The density conversion chart, or a density at 15 °C calculator.
• The density register, and the tanker's challan when checking a delivery.

1. Rinse the jar with a little of the product, then fill it with a fresh sample. For a tanker, sample from the compartment you are about to decant.
2. Let the sample stand so air bubbles rise out.
3. Lower the hydrometer gently and let it float freely, without touching the sides.
4. Read the hydrometer at eye level, at the bottom of the curved surface of the liquid.
5. Put the thermometer in the same sample and read the temperature.
6. Convert the observed density to density at 15 °C using the chart or the calculator.
7. Compare it with the density at 15 °C on the challan, and write both in the register.

Fuel gets lighter as it gets warmer. A reading taken at 32 °C will be noticeably lower than the same product's density at 15 °C, which is the reference temperature printed on challans. Comparing the raw reading with the challan would make good fuel look wrong, and could hide bad fuel. Conversion uses the standard ASTM tables (Table 53B for petroleum products).

Oil companies allow a small tolerance between the challan density and the observed density corrected to 15 °C — commonly ±3 kg/m³; follow your own OMC's current guidelines. If the difference is outside the tolerance:

• Re-check with a fresh sample and a clean jar.
• If it is still outside the limit, do not decant that compartment.
• Record the reading and inform your oil company's sales officer.

Do the same check on each tank every day for the density register, so a change in a tank shows up the day it happens.