The principle of the ethylene glycol concentration meter is based on the measurement of the refractive index. The refractive index is a unique physical property of a substance, and it varies among different substances. Ethylene glycol also has its own specific refractive index. Therefore, when ethylene glycol is added to a liquid, the refractive index of the liquid changes accordingly. Utilizing this principle, the ethylene glycol concentration meter can determine the concentration of ethylene glycol in the liquid by measuring its refractive index.