Definition and function
The core function of the constant temperature and humidity test chamber is to simulate various temperature and humidity conditions that may be encountered in nature or during product use. By coupling the four subsystems of refrigeration, heating, humidification and dehumidification, the equipment outputs a stable and controllable temperature and humidity combination in the same space, thereby expanding the product reliability evaluation from a single temperature stress to the scope of temperature and humidity dual stress and even multi-stress combination. In the modern quality system, the constant temperature and humidity test chamber is not only a tool for product resistance verification, but also a scientific instrument for material aging model calibration, drug registration data generation, and electronic component failure rate prediction.
Principle
The working principle of the constant temperature and humidity test chamber is based on the basic laws of thermodynamics and fluid mechanics, and the precise regulation of the hot and humid environment in the chamber is realized through the closed-loop control system. Its working process involves the coordinated operation of four basic functions: heating, cooling, humidification and dehumidification.
The heating function is achieved by means of a nichrome finned heater, and the required power can be estimated according to the thermal equilibrium equation:
Q = cmΔT/t
where Q is the heating power and c is the specific heat capacity of air (about 1.0 kJ·kg⁻¹· K⁻¹), m is the effective air quality, ΔT is the target temperature difference, and t is the expected heating time. In practical engineering applications, a 30% safety margin is often added to compensate for wall heat loss. The controller outputs a pulse width modulation signal, which is triggered by the solid-state relay crossing zero to achieve stepless power regulation, and the steady-state deviation is converged to ±0.1°C.
The cooling function mainly relies on the vapor compression refrigeration cycle. When the target temperature is lower than -40°C, the exhaust temperature of the single-stage refrigeration cycle exceeds the limit due to the high compression ratio, and a laminated refrigeration system is required. The system uses medium-temperature refrigeration working fluid (such as R404A) as the high temperature stage and low temperature working fluid (such as R23) as the low temperature stage, which can extend the temperature in the chamber to -70°C through the coupling of the evaporative condenser. As a throttling element, the electronic expansion valve has a response time of less than 10 seconds, which saves more than 12% energy compared with traditional thermal expansion valves.
The humidification function generally adopts shallow trough steam humidification technology. The bottom of the equipment is equipped with a 316L stainless steel shallow tank, built-in immersion heating tube, the surface of the heating tube is slightly boiled to produce pure steam with a particle size of 0.1–0.3μm, and the circulation air system is evenly mixed with the main airflow to achieve "no water droplet" humidification to avoid secondary pollution of the test product. The humidification water requires deionized water or purified water with a conductivity ≤ 5μS·cm⁻¹, and is equipped with automatic water replenishment and drainage solenoid valves, and the water is forced to change every 24 hours to inhibit the growth of scale and microorganisms.
The dehumidification function is based on the principle of dew point condensation. When the air in the box flows through the surface of the evaporator, if the wall temperature is lower than the dew point temperature of the current air, the water vapor condenses into a liquid film, and the moisture content decreases. The dehumidification amount can be approximately expressed as:
W = ρ· V· (d₁-d₂)/t
where ρ is the air density, V is the circulating air volume, and d₁ and d₂ are the moisture content when entering and exiting the evaporator. The evaporator adopts a dual-zone design, with the front end as the cooling zone and the rear end as the reheating zone, and the air is returned to the set value by using the condenser waste heat or independent heating tubes to avoid temperature loss of control.
The whole system adopts the "feedforward + feedback" control strategy, which prioritizes ensuring temperature steady state when the temperature and humidity change at the same time, and then fine-tunes the humidification or dehumidification amount to reduce the energy coupling oscillation between heating and dehumidification, cooling and humidification.
Measurement method
The performance verification of the constant temperature and humidity test chamber needs to be measured and calculated according to the procedures stipulated by the national standards. Measurements are usually performed at full load to more realistically simulate real-world operating conditions.
Measurement and calculation of temperature uniformity: Place at least nine test points (including eight corner and center points) in the workspace. After the equipment is stabilized, the temperature and humidity values of all test points are recorded every 2 minutes, and a total of 15 data is collected within 30 minutes. Temperature uniformity is obtained by calculating the average of the difference between the highest and lowest temperatures in each test data.
Measurement and calculation of temperature fluctuation: Using the data of 15 tests at the center point within 30 minutes, half of the difference between the highest and lowest temperature is calculated, and it is marked with "±", which is the temperature fluctuation of the center point at this temperature.
Determination of temperature deviation: Using the temperature data measured in the constant stage, the difference between the maximum temperature, the minimum temperature and the nominal temperature is calculated respectively, which is the temperature deviation under the set conditions.
Humidity measurements are usually measured using precision dew point meters or calibrated hygrostor-capacitor sensors, and the measurement results must be within the tolerances specified by the standard. For the alternating test, the temperature and humidity data measured during the heating and cooling phases need to be plotted into a change curve to evaluate the equipment's ability to follow dynamic working conditions.
Key factors that affect performance
The actual performance of the constant temperature and humidity test chamber is restricted by a variety of factors, which are directly related to the accuracy and reproducibility of the test results.
The load characteristics of the sample are one of the important influencing factors. When the proportion of the volume of the test specimen to the effective volume is too high, the temperature and humidity uniformity in the box will be significantly deteriorated. The study showed that when the sample volume accounted for more than 30%, the temperature difference between the center and the corner of the box could be magnified from the nominal ± 0.5°C to ±2.1°C. If the windward area accounts for too much of the cross-section of the studio, the measured cooling rate may drop by 42%, which cannot meet the test profile requirements specified in the standard.
The airflow organization directly affects the heat and mass transfer efficiency. The air duct design based on computational fluid dynamics simulation can ensure that the wind speed in the working area is continuously adjustable at 0.5~2.0 meters per second, eliminating local eddy currents and ensuring temperature and humidity uniformity. When placing the sample, ensure that any surface is not less than 100 mm away from the air duct intake, humidifier, evaporator, and not less than 150 mm from the inner wall to maintain that the boundary layer is not damaged.
Environmental conditions cannot be ignored either. The ventilation conditions of the equipment installation environment, the fluctuation of ambient temperature, and the temperature change of cooling water will affect the condensation efficiency of the refrigeration system, which in turn affects the performance of the whole machine. The inlet air temperature of the condenser should be controlled below 35°C, and if the laboratory heat dissipation is poor, auxiliary cooling measures should be taken to prevent high-pressure jumping.
Water quality management directly impacts the stability and longevity of humidification systems. Excessive conductivity of humidification water will lead to scale accumulation, reduce humidification efficiency, and may breed microorganisms and contaminate test specimens.
Typical application areas
The application of constant temperature and humidity test chambers has expanded from simple resistance testing in the early days to product research and development, failure analysis, life prediction and other full-process fields.
In the electronics and communications industry, this device is used to evaluate the humid and thermal reliability of components, printed circuit boards, and complete machines. Typical tests include capacitance drift test of multilayer ceramic capacitors at 85°C/85% relative humidity for 1000 hours, insulation resistance test of printed circuit boards at 40°C/93% relative humidity plus 12 volts bias for 500 hours, and RF power stability verification of 5G base station equipment after 48 hours of live operation at 55°C/95% relative humidity.
It is used in the field of medicine and medical devices for drug stability research and medical device aging evaluation. According to ICH Q1A guidelines, chemicals need to be tested for 12 months at 25°C/60% relative humidity, and the data is used for shelf life extrapolation. Biological products need to simulate the cold chain interruption scenario at 2~8°C to evaluate the degree of protein aggregation. Medical consumables, such as disposable syringes, are stored at 40°C/75% relative humidity for 6 months to detect the change of piston sliding resistance.
Automotive and rail applications include in-vehicle displays checking for bubble delamination after 100 cycles between -30°C/50% relative humidity and 85°C/85% relative humidity, and compressive set rate testing of wiring harness rubber seals at 70°C/95% relative humidity for 1008 hours.
The new energy field involves the capacity recovery rate test of lithium-ion batteries after 300 hours of storage at 45°C/85% relative humidity, and the evaluation of the peel strength retention rate of photovoltaic backplane materials after high-voltage accelerated aging test.
Agricultural and biological breeding research uses this equipment to observe artificial accelerated aging of seeds and insect behavior, such as measuring the decline in germination rate after 72 hours of treatment at 20°C/95% relative humidity.
Selection decision elements
The selection of a constant temperature and humidity test chamber is an investment decision that needs to consider the current needs and future development, and once determined, it will lock in the user's testing capacity for a considerable period of time.
Volume determination is the first step in selection, and four rigid boundary conditions need to be followed. The first is the geometric boundary distance, and any surface of the sample must reserve enough space from the air duct inlet and inner wall. The second is the volume ratio, the non-heating sample should not exceed 20% of the effective volume, and the heating sample should not exceed 30%, of which the effective volume is about 0.85 of the nominal volume, and the space occupied by air ducts, evaporators and other spaces needs to be deducted. The third is the windward area ratio, the ratio of the maximum windward cross-sectional area of the sample to the total cross-sectional area of the workshop should not exceed 35%. The fourth is the heat-to-mass ratio, the heat capacity of the sample should not exceed 0.4 times the heat capacity of the air in the cavity. The empirical selection formula can be expressed as:
Veff = Vobj × k
Veff is the minimum effective volume, Vobj is the maximum shape volume of the sample, and k is the heating coefficient (5 for no heat, 3 for 50 watts for ≤ heat, and 2 for 50 watts > heat).
The determination of performance parameters is based on specific test standards. The temperature range should cover the most severe level required for the test, and focus on whether the bias zone can remain unexpanded at extreme coupling points such as 180°C/98% relative humidity. The humidity range should account for low humidity capability, with some standards requiring a stable output of 5% relative humidity at 10°C, which requires a dry gas seal or dual-stage compressor coupling. The temperature rise and fall rate should meet the requirements of the test profile, and for the 24-hour cycle profile of the vehicle specification level, it is necessary to confirm that the specified rate can still be reached under large load conditions.
Structural and material considerations involve liner materials, sealing systems, and heat exchanger design. The electronics industry should use 316L mirror stainless steel to resist sulfide corrosion, and the pharmaceutical industry needs 304L stainless steel with fillet full welding and surface roughness Ra≤0.4 microns, which is convenient for cleaning and sterilization. The door seal should use aviation-grade fluorosilicone rubber hollow O-ring, which has a low compression set rate and can ensure that the leakage rate is still controlled at a low level after long-term use.
Data and compliance requirements are increasingly becoming an important dimension of selection. The device should have data integrity features that comply with FDA 21 CFR Part 11, including electronic signatures, audit trails, tamper-proof hash storage, and more. Humidity sensors should have a calibration certificate that can be traced back to national or international metrology standards. The equipment itself should pass international certifications such as UL and CE to meet the requirements of mainstream standard groups such as GB/T 2423, IEC 60068, MIL-STD-810, etc., to ensure global mutual recognition of test reports.
When making a selection decision, users should first establish a list of key requirements based on their own test profiles, check third-party certificates and original data item by item, and consider the growth redundancy of 30% volume and 50% load-bearing to adapt to future product upgrades and standard updates. Only by taking indicator redundancy, data traceability, and risk warning as hard requirements can we ensure that the equipment avoids the loss of secondary procurement and re-verification throughout its life cycle.