Track a fuel-ethanol ferment with a hydrometer and a thermometer: note the starting density (original gravity, OG) and the reading when it stops falling for two days (final gravity, FG), and the alcohol is about (OG - FG) × 131.25 per cent by volume, so 1.075 falling to 0.995 gives 10.5%. A ferment that will not start, stalls early or smells of vinegar is nearly always too cold, short of nutrients or contaminated, and each has a check and a fix.
Part of Ethanol Fuel Production
Monitoring a fermentation means measuring density and temperature through the run; troubleshooting means reading those and the smell to find a stalled or spoiling batch.
Density Falls As Sugar Becomes Alcohol
Sugar raises a liquid’s density above water’s 1.000; alcohol lowers it below. A hydrometer (a weighted glass float with a scale) floats higher in dense liquid. Standard brewing tables put a 15% sugar wash near 1.061 and a 20% wash near 1.083 (recalled). As the ferment runs, the reading falls, and at the end it sits a little below 1.000 because alcohol is lighter than water. The common brewer’s rule converts the fall to alcohol: ABV ≈ (OG − FG) × 131.25.
If you have no hydrometer, the corpus describes making one: a sealed glass tube weighted with lead shot and calibrated against sugar solutions of known strength (Mash Preparation covers readings in a mash).
What You Need
- Hydrometer and a tall jar for a sample, or an improvised one
- Thermometer (or the hand-test)
- A log: date, temperature, reading, smell, bubble rate
Method
Step 1: Take OG before pitching. Draw a clear sample, cool it to the hydrometer’s calibration temperature, read at the liquid surface. Never return the sample to the fermenter (contamination).
Step 2: Check daily. Temperature 25-30°C; airlock bubbles; smell (bready, fruity, sharp).
Step 3: Read FG when bubbling has stopped; read again two days later. If it has not moved, it is finished.
Step 4: Convert. ABV = (OG − FG) × 131.25.
Worked Example
OG 1.075, FG 0.995: (1.075 − 0.995) × 131.25 = 0.080 × 131.25 = 10.5%. Cross-check against sugar: 1.075 is about 18% sugar by weight (recalled brewing table), so a litre weighing 1.075 kg holds about 195 g of sugar. At 0.511 × 0.95 = 0.486 kg of ethanol per kg of sugar that is 95 g, and 95 ÷ 0.789 = 120 mL per litre, 12%. The density rule gives 10.5%, the chemistry 12%: the two agree within about a point and a half, which is as good as these approximate rules allow. Use the hydrometer rule for tracking and the chemistry for planning.
Diagnose A Bad Ferment
| Symptom | Likely cause | Check | Fix |
|---|---|---|---|
| No bubbling after 24 hours | Wash too cold (below 20°C) or yeast dead | Thermometer; test the yeast in a cup of warm sweet water | Warm to 25-30°C; add fresh yeast |
| Bubbling stops early, gravity still high | Stuck ferment: nutrient shortage, or alcohol above the yeast’s tolerance | Gravity vs expected FG; yeast type | Add crushed grain or yeast nutrient (parent: ammonium sulfate); restart with a stronger yeast |
| Vinegar smell | Acetobacter turning alcohol to acetic acid; air got in | Airlock water level, lid seal | Batch is lost for fuel but can become vinegar; clean and re-seal next time |
| Fruity or solvent smell, normal bubbling | Warm ferment (above 30°C) | Thermometer | Cool the vessel |
| Foam out of the airlock | Over-full vessel | Fill level | Wider tube, empty more space |
| Mould on the surface | Air contact | Lid, cloth covers | Skim, fit airlock; discard if heavily covered |
The parent states the first three causes; the others are consequences of the same mechanisms.
Common Mistakes
- Judging “finished” by bubbles. A slow airlock can still hide falling gravity; read the hydrometer.
- Reading the hydrometer while foam clings to it, or without correcting for sample temperature.
- Distilling a wash that is still fermenting. Residual CO₂ and sugar lower the yield and foam over in the still.
Carbon dioxide during checks
Opening a fermenter in a closed cellar or pit exposes you to the gas it has collected; check outdoors or with the space ventilated (Fermentation works out how much a 100-litre wash makes).
What’s Next
- Learn next: Distillation, once the gravity has stopped falling
- Combine with: Yeast Selection & Propagation, for restarting a stalled batch