There are at least few times you may need to add glycol to your chilled water system, but most commonly:
- during install
- topping off the tank after a fluid leak
- refilling the tank after a flush
If you're confident the glycol you have is mixed as indicated by the label this is a pretty simple math problem. However, if you're adding glycol to your chiller system with a new/different glycol mix it's advisable to use a refractometer to directly measure just how much glycol is in your system. This is necessary to be sure it's freeze protected to the extent you require. Keep in mind, you'll want to mix the glycol rich enough for protection down to your lowest winter temps, and/or to the refrigerant suction side temp. Suction temp can vary significantly, so you should consult your chiller manual for guidance. Our chillers generally run at suction temps around 30F below LWT for our Regular Temp chillers (~50F LWT / glycol mixed to approx. 20F pretection), and at suction temps of 10-15F below Low Temp (~25F LWT / glycol mixed to 10F protection).
There are plenty of refractometer tools out there, but one very affordable model which we frequently recommend can be found on Amazon for ~$20: