BL635 Temperature Calibrator Salt Bath (300℃~670℃)

Why the salt bath is more stable than oil bath?

A high-temperature molten salt thermostatic bath uses molten inorganic salts (nitrate, nitrite, and other mixed salts) as the heat transfer medium. It is widely used in metal heat treatment and high-temperature metrological calibration. Compared with oil baths and air furnaces, it offers smaller temperature fluctuations and more uniform temperature distribution. Its stability comes from both the medium itself and the structural design.

A. Physical Properties of the Molten Salt Medium Itself (Fundamental)

• High specific heat capacity and extremely high thermal capacity: Molten salt has a large heat storage capacity. When heating power or heat dissipation fluctuates slightly, the medium temperature does not oscillate readily. In contrast, air has very low heat capacity—it heats up quickly with a little heat input and cools down just as fast when heat is lost. Oil baths tend to crack at high temperatures and cannot reach very high temperatures.

• Excellent thermal conductivity and strong natural convection: In the liquid state, molten salt exhibits vigorous natural convection, allowing heat to be rapidly distributed throughout the bath. Local hot or cold spots do not develop, and the temperature across the entire bath tends to be uniform. Note: Molten salt develops strong natural convection even without mechanical stirring; high-end models may incorporate auxiliary stirring to further improve uniformity.

• Fully liquid operating range with no phase-change interference: Within the melting-to-boiling range, the salt remains in the liquid state, with no solid–liquid latent heat disturbances. As long as the boiling point is not reached, the temperature does not drift dramatically. The bath temperature must not exceed the decomposition or boiling point of the salt mixture; once decomposition occurs, the system becomes unstable.

• Low vapor pressure (within the operating temperature range): At high temperatures, molten salt has very low volatilization, so significant heat is not carried away by evaporation, which would otherwise cause continuous temperature drift. In contrast, silicone oils volatilize at high temperatures, leading to gradual temperature drift.

B. Stability from Equipment Structure and Temperature Control Design

• Large-volume bath and large salt mass:  A large quantity of molten salt acts as a massive "thermal buffer." When the external environment is disturbed (e.g., opening the door or inserting workpieces/specimens), the thermal disturbance is absorbed by the medium, and the temperature recovers quickly.

• Heating layout: zoned heating to avoid localized overheating. Heating elements are immersed inside the salt bath rather than radiating heat through air. Heat is transferred directly to the molten salt, not to the air first, resulting in minimal thermal lag. Many models employ multi-zone segmented heating to reduce single-point overheating.

• Strong insulation structure: Multi-layer thermal insulation significantly reduces heat loss to the environment, stabilizing the heat dissipation rate and minimizing the on/off cycling of the temperature controller at high power.

• PID closed-loop temperature control + temperature sensor directly immersed in the molten salt. The temperature probe is directly immersed in the molten salt medium, reading the actual temperature of the medium rather than the air temperature. This minimizes measurement lag, allowing the controller to rapidly compensate for thermal changes.