3. Water & Environmental

Q. How do we identify which type of glycol we have in our HVAC system?

A. Yes it is possible to differentiate between ethylene glycol and propylene glycol using  both refractive index and  specific gravity.

You will need:

Digital refractometer for glycolhydrometer 1.000-1.500 with subdivisons 0.005, hydrometer jar 300x50mm









The key to differentiate the two is to use the physical differences with regard to specific gravity and refractive index.

Specific Gravity

Perform the specific gravity test first. Fill the cylinder with 250-300ml of the unknown sample. Record the numerical value of the nearest graduation mark, if the value is 1.05 or above the solution is ethylene glycol based. It is impossible for propylene glycol solutions to have a specific gravity at or above 1.045 at 70 degrees fahrenheit.


Refractive Index

If the Specific Gravity is between 1.0 and 1.05 (0.05 margin of error added), then you must do the Refractive Index to determine which glycol is present. We recommend using a hand held refractometer such as the one shown above. To perform this test all you have to do is place a few drops of your unknown sample liquid on the measurement prism at the end of the refractometer. Close the cover so the liquid spreads evenly across the entire surface of the prism without any air bubbles or dry spots. Allow this to remain on the prism for approximately 30 seconds.

It is best to use the instructions that come with the refractometer. Using a hypothetical Specific Gravity of 1.03 from the test above. A Specific Gravity of 1.03 means that you either have an Ethylene Glycol solution between 25% and 30% by weight, or you have a Propylene Glycol solution between 35% and 40% by weight. Which is it? In the refractometer field of view below you can see an area of grey scale and an area of white. The junction of these two determines the percent strength of a known Glycol. Here it shows that the solution could be 30% Propylene Glycol. Or perhaps it could be approximately 30% Ethylene Glycol.

Use the refractometer to identify an unknown glycol which we know to have two totally different percentages of strength. In our hypothetical example, if the grey and white shaded areas meet near the red line (red line added for the purpose of demonstration) our unknown is a Propylene Glycol solution. If, however, the grey and white shaded areas meet near the blue line (blue line added for the purpose of demonstration) our unknown is an Ethylene Glycol solution.

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