<Li> Test strips--specialized, disposable indicators made for this purpose . </Li> <P> Although ethylene glycol hydrometers are widely available and mass - marketed for antifreeze testing, they give false readings at high temperatures because specific gravity changes with temperature . Propylene glycol solutions cannot be tested using specific gravity because of ambiguous results (40% and 100% solutions have the same specific gravity). </P> <P> Most commercial antifreeze formulations include corrosion inhibiting compounds, and a colored dye (commonly a fluorescent green, red, orange, yellow, or blue) to aid in identification . A 1: 1 dilution with water is usually used, resulting in a freezing point of about − 34 ° F (− 37 ° C), depending on the formulation . In warmer or colder areas, weaker or stronger dilutions are used, respectively, but a range of 40% / 60% to 60% / 40% is frequently specified to ensure corrosion protection, and 70% / 30% for maximum freeze prevention down to − 84 ° F (− 64 ° C). </P> <P> In the absence of leaks, antifreeze chemicals such as ethylene glycol or propylene glycol may retain their basic properties indefinitely . By contrast, corrosion inhibitors are gradually used up, and must be replenished from time to time . Larger systems (such as HVAC systems) are often monitored by specialist firms which take responsibility for adding corrosion inhibitors and regulating coolant composition . For simplicity, most automotive manufacturers recommend periodic complete replacement of engine coolant, to simultaneously renew corrosion inhibitors and remove accumulated contaminants . </P>

When should antifreeze be used in the cooling system