Ethanol purity is important as even low levels of contaminants can have a significant effect on the ethanol's behaviour. Ethanol is available as several different grades, with the most pure being 99.9 to 100 per cent pure. Specific applications require specific levels of purity. If the ethanol is not the specified purity then problems can occur that produce incorrect answers
Fill up a large graduated cylinder with the ethanol solution. The volume of the hydrometer will depend on the size of the hydrometer. The cylinder needs to be taller than the hydrometer; normally a 500 millilitre cylinder is large enough.
Place the hydrometer in the ethanol solution in the cylinder and give it a twist to set the hydrometer spinning. When the hydrometer stops spinning make a record of what scale line the hydrometer is floating on. Make sure the hydrometer is not touching the sides or bottom of the cylinder, if it is use a taller cylinder. This is where the top of the ethanol solution touches the hydrometer. Be aware that the hydrometer scale is increases when going from the top of the hydrometer to the bottom. This type of scale is different from what is found on a thermometer.
Change hydrometers if the liquid level is off the hydrometer scale. There are different scales on hydrometers, and you want to use one that measures the scale of the expected ethanol content. Try to use a hydrometer with the expected measurement in the middle of the hydrometer scale range.
Use a thermometer to determine the temperature of the ethanol solution. Hydrometers are made to work at specific temperatures, normally 25 degrees Celsius or 20 degrees Celsius. If the temperature of the solution is different from the temperature specified on the hydrometer you need to factor in a temperature correction.
Look up the temperature correction needed on an ethanol hydrometer chart. Apply this correction to the hydrometer reading. Compare your specific gravity reading to the known specific gravity. If it is very different form the known specific gravity, then your ethanol is contaminated. Always take at least two measurements -- three is even better -- to get a true reading. Issues can occur that can make the reading be off. By taking multiple readings you reduce the possibility of random errors.