The determination of freezing point sounds simple—cool the liquid and observe when it begins to freeze. However, the SH/T 0090 method for measuring freezing point is far more complex. It does not measure the "temperature at which freezing begins," but rather a more physically meaningful parameter: the peak temperature rise caused by the heat released during crystallization. Understanding this principle explains why the measurement process requires continuous automatic monitoring of the temperature curve.

The latent heat effect of crystal precipitation

When the solution cools to a certain temperature, the solute (ethylene glycol or other components) begins to crystallize out as solid crystals. Crystallization is an exothermic process (releasing latent heat of crystallization), which temporarily prevents further temperature drop and may even cause a slight temperature rise.

The temperature variation curve of the test sample exhibits the following pattern: it continuously decreases → slows down significantly or even rises at a certain temperature → reaches a peak value before decreasing again. This peak value is defined as the freezing point in SH/T 0090.

Why not directly record the temperature at which crystallization begins

Under ideal conditions, the freezing point of a pure substance is the starting temperature of crystallization, making it easy to determine. However, coolant is a mixed solution, and the crystallization process is more complex: the activity coefficients of each component in the mixture vary with concentration, the actual crystallization temperature is influenced by the supercooling effect (the solution may begin to crystallize below the theoretical freezing point), and the component ratios of different batches of samples may slightly differ. Directly recording the onset of crystallization results in significant errors and poor repeatability.

The maximum temperature回升 (freezing point) is a stable parameter determined jointly by crystallization kinetics and solution thermodynamics, unaffected by undercooling effects, offering better repeatability and more accurately reflecting the actual low-temperature performance of the coolant.

The automatic capture logic of HZBD-1177

The instrument begins detection at 15°C above the expected freezing point, recording temperature every 15 seconds and plotting the temperature-time curve in real time. When the system detects a rebound after a continuous temperature decline and determines that the rebound has reached its peak before starting to decline again, it records this peak as the freezing point. The entire process requires no manual intervention—algorithms automatically identify the inflection points on the temperature curve, providing more accurate results than visual inspection and eliminating errors caused by reaction time.