Fill a clean glass jar with the sample, let the bubbles clear, and float the hydrometer free of the sides. Read the scale where the flat surface of the fuel meets it, with your eye level with the surface. Read the thermometer in the same jar, then convert with Table 53B.
What you need
- A glass jar, clear and clean, wide enough that the hydrometer floats without touching the sides.
- A hydrometer for the fuel’s range, with a valid calibration.
- A thermometer, also calibrated.
- A level, shaded spot out of the wind.
The MDG makes calibrated hydrometers and thermometers and the ASTM density table mandatory at every outlet, ready for an inspector.
Doing the test
- Rinse the jar with a little of the same fuel, then pour it away.
- Fill the jar slowly down its side so few bubbles form.
- Lower the hydrometer gently. Let go only when it is near where it will float. Give it a light spin.
- Wait until it is still and clear of the wall.
- Read the scale at the fuel’s flat surface, with your eye level with it.
- Read the thermometer straight away, with its bulb in the middle of the sample.
- Convert with Table 53B.
Mistakes that give a wrong figure
| Mistake | What happens |
|---|---|
| Hydrometer touching the jar | It sticks and reads wrong |
| Reading from above | You read the curve that climbs the stem, and the figure comes out wrong |
| Thermometer read later | The sample has warmed or cooled; the correction is wrong |
| Dirty or wet jar | Water or old fuel changes the density |
| Wrong hydrometer range | The reading falls off the scale or is coarse |
| Sample in the sun | Warms while you read; temperature and reading no longer match |
Figures here are for reference. Your oil company’s certified calibration chart, approved instruments and the Marketing Discipline Guidelines always prevail.
Questions
Do I read the top or the bottom of the curve on the stem?
For a clear liquid like petrol or diesel, read where the flat surface of the liquid, extended, would cut the scale: the bottom of the small curve that climbs the stem. Look from slightly below the surface and raise your eye until the surface becomes a line.
How long should the sample stand?
Long enough for air bubbles to rise and for the thermometer to settle, usually a minute or two. Do not leave it so long that it warms or cools in the sun.
Can I use one hydrometer for petrol and diesel?
Usually not. Hydrometers have narrow ranges for precision. Keep one for each product's range and check each against its calibration certificate.