As the core functional unit of salt spray test chambers, temperature control directly determines the reliability of salt spray environment simulation and the reproducibility of test results. Temperature control devices achieve automatic control of heating and cooling actuators by sensing temperature variations within the test space, triggering physical effects within switching elements to initiate closing or opening actions. Based on different temperature-sensing principles and control methods, these devices can be classified into five technical categories, each with distinct structural characteristics and application scenarios.

I. Pressure-Type Thermostat: A Classical Control Solution Based on Phase-Change Transmission
The pressure-type thermostat was widely applied in early designs of salt spray test chambers, with its technical principle based on the phase-change pressure characteristics of the working fluid within a sealed temperature-sensing system. This device comprises a temperature sensing bulb, capillary tube, elastic diaphragm chamber, and microswitch to form a closed pressure transmission network. The bulb is filled with low-boiling-point liquids (such as chloromethane or R134a refrigerant) or saturated vapor. When the temperature inside the test chamber rises, the sensing bulb absorbs heat, causing internal working fluid vaporization and expansion. The pressure increment is transmitted through the capillary tube to the diaphragm chamber, driving the membrane to displace. When the temperature reaches the preset threshold, the mechanical force of the diaphragm chamber overcomes the spring preload, actuating the snap-action mechanism to rapidly open or close contacts, thereby achieving on-off control of the heating circuit.
The advantages of this thermostat lie in its simple structure, low cost, and no requirement for external power. However, its control precision is limited by the hysteresis of mechanical components, typically exhibiting a temperature deviation range of ±3~5°C, which cannot meet high-precision test requirements. Furthermore, the capillary tube length and bending radius significantly affect pressure attenuation, requiring strict adherence to technical specifications during installation to avoid mechanical damage that could degrade temperature-sensing sensitivity.
II. Snap-Action Thermostat: Thermo-Mechanical Response Mechanism of Bimetallic Strips
The snap-action thermostat employs a bimetallic strip as its core temperature-sensing element, with its technological essence lying in the mechanical stress effect formed by composite lamination of two metals with significantly different coefficients of thermal expansion (such as brass and Invar alloy). When the working chamber temperature of the salt spray test chamber rises, the active layer (high expansion coefficient) of the bimetallic strip elongates more than the passive layer, generating bending deformation toward the passive layer direction. When accumulated stress reaches the critical threshold, the strip completes a snap-action flip within milliseconds, driving the contact mechanism to rapidly open or close via an insulated pushrod. This process features a definite differential between action and reset temperatures (typically 5~15°C), effectively preventing contact chattering.
Such devices offer rapid response, reliable operation, and long service life (exceeding 100,000 cycles), making them particularly suitable for safety interlock circuits such as heating tube overheat protection and fan failure alarms. However, their temperature setpoint is determined by the bimetallic strip’s forming process, making post-manufacture adjustment difficult and lacking flexibility. Consequently, they are not appropriate for precision temperature control scenarios requiring continuous, smooth regulation.
III. Liquid-Expansion Thermostat: Linear Sensing Technology Based on Fluid Thermal Expansion
The liquid-expansion thermostat is commonly found in auxiliary temperature control circuits of mid-to-low-end salt spray test chambers, with its operating principle based on the volume effect of liquid thermal expansion and contraction. The temperature-sensing component consists of a stainless steel bellows, sensing tube, and capillary tube filled with high-expansion-coefficient liquid (such as ethanol, kerosene, or silicone oil). Temperature changes cause linear volume expansion of the liquid within the sensing tube, pushing the bellows to displace axially. This displacement is amplified by a lever mechanism and actuates the microswitch. This type of thermostat achieves temperature control accuracy of ±2°C with moderate cost and mature application in household appliances and light industry temperature control systems.
When applied in salt spray environments, special attention must be paid to the temperature-sensing medium’s resistance to chloride ion corrosion. The bellows material should be 316L stainless steel or surface nickel-plated. Additionally, capillary tube length is generally limited to within 3 meters; excessive length will increase thermal transmission delay, causing temperature control overshoot. Its mechanical structure determines relatively slow response speed, making it unsuitable for complex programmed temperature control requiring rapid switching.
IV. Digital Thermostat: Intelligent Integration of Sensors and Microprocessors
The digital thermostat represents the mainstream technological direction for modern salt spray test chambers, converting temperature variables into continuous electrical signals through NTC thermistors or Type K thermocouples. The sensing element is positioned within the airflow circulation path of the working chamber to acquire real-time temperature data. After conditioning by high-precision operational amplifiers, the signal is input to the A/D conversion port of a microcontroller (MCU), completing the conversion from analog to digital quantity. The controller incorporates PID algorithms or fuzzy logic control programs, performing proportional-integral-derivative calculations on deviations between measured and set values, and outputs PWM signals to drive solid-state relays (SSR) or thyristors, achieving continuous, fine adjustment of heating power.
The technical advantages of such devices are remarkable: temperature control accuracy can reach ±0.1~0.5°C, supporting multi-stage program setting and data storage, with RS485 or Ethernet communication functions for easy integration into Laboratory Information Management Systems (LIMS). Their high sensitivity derives from 16-bit or higher resolution A/D converters and digital filtering technology, effectively suppressing electromagnetic interference and sensor drift in salt spray environments. However, digital systems place higher demands on power quality, requiring independent filtering devices, and cost 3~5 times more than mechanical thermostats.
V. Electronic Thermostat: Continuous Regulation Scheme Based on Analog Circuits
The electronic thermostat is essentially an analog circuit control system, employing tungsten filaments, copper thermal resistors, or positive temperature coefficient (PTC) ceramics as temperature-sensing elements. The resistance value of such sensors changes approximately linearly with temperature and is connected to a Wheatstone bridge circuit composed of precision resistor networks, operational amplifiers, and comparators. When temperature deviates from the setpoint, the bridge’s unbalanced voltage is amplified to drive transistors or power MOSFETs, continuously regulating heating current and achieving contactless, stepless power adjustment.
This solution features a compact structure and small size, with drive power reaching several kilowatts, widely applied in household air conditioners and small constant-temperature equipment. Its advantage lies in the absence of mechanical contacts and high reliability. However, temperature control accuracy is limited by temperature drift and long-term stability of analog components, typically ±1~2°C. Additionally, analog circuits exhibit weak anti-interference capability. In the strong corrosive and high-humidity environment of salt spray test chambers, PCB boards require conformal coating treatment, increasing manufacturing costs. Compared with digital controllers, its functionality is limited, making it difficult to implement complex program control and data logging.
Technical Selection and Comprehensive Application Recommendations
Various temperature control devices can form complementary configurations in salt spray test chambers: The main control system should adopt a digital thermostat to ensure high precision and programmability of the test process; Critical safety circuits (such as over-temperature protection) should be equipped with snap-action thermostats for redundant protection; The start-stop control of air-cooled condensers can be managed by pressure-type thermostats; Auxiliary heating or defrosting functions may select liquid-expansion or electronic thermostats. During selection, it is necessary to comprehensively evaluate factors including test standard precision requirements, equipment budget, maintenance capabilities, and laboratory digitalization level to construct an optimal temperature control strategy, thereby ensuring long-term stable operation and data credibility of salt spray corrosion testing.