Articles -
May 2020
Which one – Refractometer or Hydrometer – to use for Glycol Concentration Testing?
The concentration of glycol in your system is what ensures that you have adequate freeze and burst protection. Glycol concentration can be measured in several different ways. This article describes two methods of testing that can easily be done at a job site, using inexpensive tools: refractometers and hydrometers.
Measuring with a Refractometer
A refractometer is a tool to measure refractive index, which describes how fast light travels through a material. The angle at which light appears to bend can be measured and converted to concentration.
A handheld refractometer is very easy to use. Just add a few drops of the sample on the prism, hold it up to a light source, and read the concentration directly on the scale inside the meter.
Measuring with a Hydrometer
A hydrometer is a tool used to measure specific gravity, which is the ratio of the mass of a liquid to the mass of an equal volume of distilled water. Specific gravity is correlated directly with concentration.
To use a hydrometer, fill the cylinder with the test sample. Put the hydrometer in with the bulb end down so that it bobs up and down in the sample. Note: Ensure that the hydrometer is not in contact with the sides of the cylinder when you take the reading. Be careful to take the reading at the meniscus—the flat plane of the water—not where the water clings up the neck of the hydrometer.
Hydrometers vs. Refractometers: Which is Better?
Hydrometers are good options for determining glycol concentration, and they are often chosen because they are inexpensive. However, hydrometers have some distinct disadvantages when compared to refractometers:
- They are made of glass, which is easy to break.
- They require significantly larger sample volumes than refractometers.
- The density of propylene glycol decreases at high concentrations, meaning two different concentrations may confusingly have the same specific gravity.
- Manual adjustments must be made if the hydrometer is measuring specific gravity at a different temperature than the reference temperature.
Overall, refractometers are highly advantageous because they are quick, accurate, compensate for temperature differences automatically, and result in less user error than hydrometers. They also have the added benefit of meeting the ASTM standard D3321.