Coolant concentration should be checked when any additional coolant was added to system or after a coolant drain, flush and refill. The coolant mixture offers optimum engine cooling and protection against corrosion when mixed to a freeze point of -37 degrees C (-34 degrees F) to -46 degrees C (-50 degrees F). The a hydrometer or a refractometer can be used to test coolant concentration.
A hydrometer will test the amount of glycol in a mixture by measuring the specific gravity of the mixture. The higher the concentration of ethylene-glycol, the larger the number of balls that will float, and higher the freeze protection (up to a maximum of 60% by volume glycol).
A refractometer, Tool 8286 (Refer to 7 - COOLING - SPECIAL TOOLS) will test the amount of glycol in a coolant mixture by measuring the amount a beam of light bends as it passes through the fluid.
Some coolant manufactures use other types of glycols into their coolant formulations. Propylene-glycol is the most common new coolant. However, propylene-glycol based coolants do not provide the same freezing protection and corrosion protection and is not recommended.
CAUTION: Do not mix types of coolant - corrosion protection will be severely reduced.