Read the hydrometer and the thermometer in the same sample at the same time. Look up the pair in ASTM Table 53B, with the reading across and the temperature down; where they meet is the density at 15 °C. Between printed figures, read in a straight line.
What you need
- A clean, clear glass jar (the hydrometer jar).
- A calibrated hydrometer for the range of your fuel. Petrol and diesel need different ones.
- A calibrated thermometer.
- Your company’s copy of Table 53B, the Table 53B look-up or the calculator.
The MDG makes it mandatory for a dealer to have calibrated hydrometers, thermometers and the ASTM density conversion table at the outlet.
Step by step
- Take the sample. From the nozzle in the morning, after letting a little fuel through; from each compartment’s manifold when a tanker arrives.
- Fill the jar and let air bubbles rise. Keep it out of the wind and direct sun.
- Lower the hydrometer gently. Let it float free, away from the jar’s wall. Give it a slight spin to break any surface film.
- Read the hydrometer at the level of the fuel’s flat surface, with your eye level with it. This is the observed density.
- Read the thermometer in the same jar straight away.
- Look it up. Reading across, temperature down.
Example 1: petrol
Hydrometer 741.0 kg/m³, temperature 26.5 °C.
Column 741, row 26.5 °C: 751.2 kg/m³ at 15 °C.
Example 2: diesel, between rows
Hydrometer 815.0 kg/m³, temperature 27.3 °C. The table is printed every 0.5 °C, so 27.3 falls between two rows. Read in a straight line between them, 27.3 being 0.6 of the way from 27.0 to 27.5. The answer is 823.7 kg/m³. The calculator does this step for you.
What to do with the answer
- Morning: write it in the density register before sales start, by 10 AM at the latest.
- Tanker: compare it with the invoice density. The difference must stay within ±3.0 kg/m³. See the density variation limit.
- Check the range: BS-VI petrol is 720 to 775 kg/m³ and BS-VI diesel 810 to 845 kg/m³ at 15 °C. A figure outside is a reason to stop and check.
Figures here are for reference. Your oil company’s certified calibration chart, approved instruments and the Marketing Discipline Guidelines always prevail.
Questions
Can I just add or take off a fixed figure per degree?
Not safely. The change per degree depends on the density and is not quite constant. For petrol near 745 kg/m³ it is roughly 0.9 kg/m³ per degree; for diesel near 830 it is roughly 0.7. Use the table or the calculator.
How precise should the reading be?
Read the hydrometer to 0.5 kg/m³ and the thermometer to 0.5 °C. The table is printed to 0.1 kg/m³.