Troubleshooting and Resolution of 12 Real-World Challenges in Constant Temperature and Humidity Test Chambers

Field Case Study: Troubleshooting and Resolution of 12 Real-World Challenges in Constant Temperature and Humidity Test Chambers

Constant temperature and humidity test chambers are among the most commonly used environmental simulation devices in the product reliability verification chain. However, in actual laboratory operating environments, a multitude of operational challenges exist between "the equipment being operational" and "the reliability of test data" — such as cooling performance degradation, humidity control failures, condensation interference, sensor drift, program interruptions, and condensation/droplet formation. Some of these issues stem from deviations in environmental conditions, some arise from inadequate equipment maintenance, and others result from systemic flaws in the operational procedures themselves.

This paper focuses on the MY-80L constant temperature and humidity test chamber (temperature range: –60 °C to 150 °C; humidity range: 20%–98% RH; equipped with a French Taikang compressor and a Korean TEMI880 controller) and compiles 12 real-world case studies of high-frequency or challenging field issues. Each case study reconstructs the troubleshooting process using a three-step framework: "fault symptoms → root cause analysis → corrective solutions." The conclusion provides quick-reference parameters for the equipment and practical experience summaries.

■ Equipment Background Overview

The MY-80L has an effective volume of 80 L, featuring an inner chamber constructed from SUS#304 stainless steel and an insulation layer composed of rigid polyurethane foam combined with glass wool. The refrigeration system employs a fully enclosed compressor from France's Taikang, equipped with the environmentally friendly refrigerant R404A; the expansion system utilizes a capacity-controlled design, incorporating a Danfoss expansion valve, an ALCO oil separator, and a filter. The control core consists of a 7-inch color touchscreen panel from Korea's TEMI880, capable of automatically calculating six sets of P.I.D parameters, with a program capacity of 100 sets comprising 5,000 segments, and features power-off memory for continuous operation. The following parameters were calibrated under no-load conditions at a room temperature of 20 °C:

▼ Below are 12 representative challenging case studies from the field

Case 01 | Severe degradation of cooling rate; failure to reach-60 °C over an extended period

[Fault Symptoms]During a low-temperature test set to-60 °C, the equipment began cooling from room temperature. When the temperature dropped to approximately-35 °C, the cooling rate suddenly decreased from the normal 1 °C/min to less than 0.3 °C/min; despite exceeding 90 minutes of operation, the target temperature of-60 °C was not reached. No alarm was displayed on the TEMI880 panel, and the compressor operated normally.

[Root Cause Analysis]The root cause was identified through a three-step investigation: ① Environmental factors — During summer, the laboratory air conditioning system exhibited insufficient cooling capacity, resulting in an indoor temperature of 33 °C, which exceeded the equipment's performance guarantee range (5–35 °C); consequently, the heat dissipation efficiency of the condenser significantly decreased. ② Load factors — The test specimen was a metal casting with a single-piece weight of 12 kg; the total thermal mass of the three specimens far exceeded the recommended load capacity for the 80 L chamber (which should not exceed one-third of the chamber's internal volume). ③ Maintenance factors — The condenser fins suffered from severe dust accumulation, and the inter-finned gaps were obstructed by dust, leading to a reduction in the cooling airflow rate. The combined effect of these three factors resulted in insufficient output power of the refrigeration system during the low-temperature operating range.

[Solution]Step 1: Set the laboratory air conditioning temperature to 22 °C to ensure that the room temperature remains stable within the range of 20–25 °C; Step 2: Reduce the number of test specimens to one, placing it centrally on the shelf to ensure unobstructed airflow circulation; Step 3: Use compressed air to alternately purge the front and rear sides of the condenser fins to remove accumulated dust (recommended to be performed quarterly). After these steps, restart the cooling procedure; the cooling process from 25 °C to –60 °C took 52 minutes, and the cooling rate then returned to normal.

Case 02 | Humidity fails to reach the set value over an extended period and remains stuck at 75% RH

[Fault Observation]When a constant humid-thermal test was set at 85 °C/85 % RH, after the temperature normally reached 85 °C, the humidity began to increase from the ambient humidity level, rose to 75 % RH, and then plateaued, remaining unchanged for 30 minutes—significantly lower than the set value of 85 % RH.

[Root Cause Analysis]Priority checklist for investigating insufficient humidity: ① Wet-bulb tank – inspection reveals normal water level, but the wet-bulb gauze has been in use for over 3 months; the gauze has become severely hardened and yellowed, resulting in loss of capillary water-absorption capability. This leads to inaccurate calculation of the temperature difference between the dry and wet bulbs, causing the controller to incorrectly assess the current humidity as higher than the actual value and consequently cease humidification; ② Humidification tube – the surface of the 1kW custom humidification tube exhibits significant scale buildup, reducing thermal efficiency; ③ Sealing – the door sealing strip at the right handle hinge shows slight lifting, allowing moisture to leak through the gap.

[Solution]Replace the wet-bulb gauze (use the non-woven fabric provided in the standard configuration; cut to appropriate dimensions, then moisten with pure water before installation); remove scale from the surface of the humidification tube (soak in a citric acid solution for 30 minutes, then clean with a soft brush); adjust the door hinges and replace the sealing strips. After these steps, restart the system; within 15 minutes, the humidity will rise from 75%RH to 85%RH and stabilize. Key tip: The wet-bulb gauze should be replaced monthly; do not substitute it with ordinary cotton fabric.

Case 03 | Sudden jump in humidity display value; data cannot be trusted

[Fault Symptoms]During constant temperature and humidity operation (60 °C / 90 % RH), the humidity display on the TEMI880 panel exhibits irregular fluctuations ranging from 82 % RH to 96 % RH, occurring every 5–8 seconds; the temperature display remains normal and stable. The test data cannot pass the evaluation criteria due to humidity fluctuations exceeding the allowable tolerance.

[Root Cause Analysis]The temperature remained stable while the humidity readings fluctuated; this observation, after ruling out drastic variations in the actual humidity within the enclosure, pointed to issues with the sensor or its signal circuitry. Upon inspecting the wet-bulb Pt100 sensor, it was found that the gauze was improperly positioned—failing to fully cover the sensing element of the sensor, and the lower end of the gauze not being submerged below the water surface in the bath. Consequently, the "wet-bulb temperature" required for the wet-dry bulb calculation effectively became a "semi-dry-bulb temperature," leading to unstable wet-bulb temperature signals. Further inspection revealed slight oxidation at the wiring terminals of the Pt100 sensor, and variations in the contact resistance were causing signal fluctuations.

[Solution]Reinstall the wet-bulb gauze: Ensure that the gauze completely encloses the temperature-sensing bulb of the sensor, with the lower end of the gauze extending at least 20 mm below the water surface of the tank, and that there are no air bubbles between the gauze and the sensor. Clean the Pt100 terminals (spray with an electronic contact cleaner, then wipe with a lint-free cloth). Activate the automatic dry-and wet-bulb calibration function (TEMI880 menu → Calibration → Automatic Dry-and Wet-Bulb Calibration); once calibration is complete, the humidity display will stabilize at 90±1% RH.

Case 04 | Failure of test result determination due to surface condensation on the test specimen

[Failure Observation]A temperature-and humidity accelerated life test was conducted on the PCBA circuit board, ranging from 25 °C to 85 °C at 85% RH. During the temperature rise phase, distinct water droplets (condensation) appeared on the surface of the circuit board; furthermore, optical inspection and insulation test results showed anomalies (insulation resistance dropped abruptly from the normal value of 500 MΩ to 5 MΩ). The customer therefore deemed the test invalid.

[Root Cause Analysis]The physical mechanism of condensation: When the surface temperature of the test specimen is below the dew point temperature of the air within the chamber, water vapor condenses on the specimen surface. During the heating phase, if the air temperature rises faster than the specimen temperature (particularly for metal substrates and heavy-duty specimens), a temperature differential window emerges—characterized by "warmer air and cooler specimen." Within this window, the dew point temperature exceeds the specimen surface temperature, inevitably leading to condensation. The equipment's default heating rate is set to the maximum value of 9.9; excessively rapid heating further exacerbates this temperature differential window.

[Solution]By leveraging the temperature slope function of TEMI880, the heating rate was reduced from 9.9 to 2.0, while a "preheating phase" was incorporated into the program: first, the temperature was increased to 85 °C under low-humidity conditions (25 °C / 40 % RH) over approximately 15 min; then, the temperature was held at 85 °C for 10 min to allow the test specimen to fully equilibrate; finally, the humidity was incrementally increased from 40 % RH to 85 % RH at a rate of 3 % RH/min. This "heat first, then humidify" strategy ensured that the test specimen reached the target temperature prior to the humidity increase, thereby eliminating the condensation window. After this process, no condensation was observed on the surface of the PCBA, and the insulation resistance remained consistently above 500 MΩ throughout the test.

Case Study 05 | Exceeding temperature and humidity accuracy drift limits after 30 consecutive days of operation

[Failure Symptoms]After 30 consecutive days of accelerated aging testing, the temperature deviation of the equipment expanded from an initial ±0.5 °C to ±1.8 °C (exceeding the ≤2 °C specification and approaching the critical threshold), while the humidity deviation expanded from ±1 %RH to ±4 %RH. No abnormal alarms were detected on the TEMI880 panel; however, a systematic deviation was observed between the values displayed by the external data logger and those displayed by the panel.

[Root Cause Analysis]After prolonged operation, the Pt100 sensor exhibits zero-point drift, which is a typical aging phenomenon observed in environmental test equipment. Detailed analysis: when the Pt100 temperature sensor operates continuously within a wide temperature range of-60 °C to 150 °C, the platinum wire resistance of its sensing element undergoes slight deviations in its temperature characteristic; for the wet-bulb Pt100 sensor, prolonged immersion and scale deposition result in reduced response speed. The P.I.D parameters of the controller were initially tuned based on the original sensor characteristics; however, after sensor drift occurs, these parameters no longer yield optimal performance, leading to degraded control accuracy.

[Solution]Step 1: Perform automatic dry-and wet-bulb calibration (TEMI880 menu → Calibration → Automatic Dry-and Wet-Bulb Calibration); after calibration, the temperature deviation is restored to ±0.6 °C, and the humidity deviation is restored to ±1.5 %RH. Step 2: Execute the P.I.D automatic recalibration: under no-load conditions, set a step response from 25 °C to 85 °C, allowing the controller to automatically test the system's response characteristics and recalculate the P/I/D parameters. Upon completion of this process, the temperature deviation is further reduced to ±0.3 °C, and the humidity deviation is reduced to ±0.8 %RH. Establish maintenance procedures: perform dry-and wet-bulb calibration quarterly, and execute the P.I.D automatic recalibration every six months or following a change in load type.

Case 06 | Power failure during the experiment: Is the program lost? Are the data valid?

[Fault Observation]During the 1,000-hour temperature and humidity cyclic test, at the 320th hour, the laboratory power supply tripped and the power was cut off. Upon power restoration, the equipment automatically restarted and resumed its test program; however, during the power outage period, the temperature inside the test chamber naturally decreased from the set value of 85 °C to the room temperature of 26 °C, and the humidity decreased from 85 % RH to the ambient humidity of 55 % RH, lasting approximately 25 minutes. During this interval, the test specimen underwent an uncontrolled temperature drop.

[Root Cause Analysis]The equipment's power-off memory function operates normally, and after power restoration, the program segments continue to execute without error. However, the issue lies in the fact that the temperature profile during the power-off recovery period (approximately 25 minutes) is not included in the valid test duration; furthermore, this power-down event may induce unexpected thermal stress on the test specimen, potentially compromising the validity of the test results. The equipment itself exhibits no functional failures; the concerns pertain to the reliability of the external power supply and the assessment of the test data's validity.

[Solution]Hardware Level: Deploy UPS (Uninterruptible Power Supply) systems in regions with unstable power supply to ensure that equipment can operate for at least 15 minutes following a power outage, thereby enabling the smooth completion of the current program segment. Operational Level: Record the power outage timestamp, the power restoration timestamp, and the temperature and humidity trend curves during the power outage in the test report; the test engineer shall then determine whether such interruption compromises the validity of the test—should the determination be "valid, but requiring compensatory runtime," an equivalent amount of compensatory runtime shall be appended at the end of the program. Standardization Level: Explicitly define in the test protocol the criterion that "tests involving a single power outage exceeding 10 minutes, or cumulative power outages exceeding 30 minutes, shall be repeated."

Case Study 07 | External condensation and dripping on the cabinet housing compromise laboratory safety

[Fault Symptoms]During the-40 °C low-temperature test, a large number of condensation droplets appeared on the side panels and the top of the equipment's outer housing; these droplets fell onto the laboratory floor, creating a slippery surface and posing a safety hazard. Distinct water streaks were visible on the surface of the stainless steel plates of the outer housing.

[Root Cause Analysis]The insulation layer of the equipment consists of a composite structure of rigid polyurethane foam and glass wool. During the low-temperature test, the inner chamber temperature was set at-40 °C. If the laboratory environment exhibited high humidity (measured at 75 % RH) and the ambient temperature was 25 °C, the surface temperature of the outer chamber could fall below the ambient dew point temperature (approximately 20.5 °C), inevitably leading to condensation. Inspection revealed slight deformation at the lower-left corner of the chamber door sealing strip; inadequate sealing in this area allowed cold air to leak out, resulting in locally lower surface temperatures on the outer chamber and exacerbated condensation.

[Solution]First, reduce the laboratory ambient humidity to below 60% RH (by activating the dehumidifier) so that the ambient dew point temperature remains below the surface temperature of the outer cabinet. Second, replace any deformed door sealing strips to ensure uniform tightness and sealing around the cabinet door perimeter (after closing the door, use a 0.05 mm feeler gauge to measure the gap at the sealing surface; no material should be able to pass through). Third, inspect the condition of the sealing strips on the three layers of vacuum observation windows (260×340×40 mm); replace any aged strips. Fourth, position the equipment away from direct airflow from air conditioning vents to prevent localized temperature fluctuations that could exacerbate condensation. After implementing these measures, no condensation shall be observed on the outer cabinet during the-40 °C test.

Case Study 08 | Frost formation on the low-temperature zone evaporator; aggravated temperature fluctuations

[Fault Symptoms]During the-50 °C low-temperature hold test, the temperature begins to exhibit periodic fluctuations of ±2.5 °C from its steady-state value (normal fluctuation range: ≤0.5 °C), with a fluctuation period of approximately 15 minutes. The TEMI880 indicates that the compressor continues to operate; however, after the temperature drops to-53 °C, it gradually rises back to-47 °C, repeating this cycle.

[Root Cause Analysis]This is a typical manifestation of frosting on the evaporator. During operation at low temperatures, residual water vapor within the evaporator chamber undergoes deposition on the evaporator fins, forming a frost layer. This frost layer increases the thermal resistance at the evaporator surface, thereby reducing heat transfer efficiency and causing the temperature to gradually rise; when the surface temperature of the frost layer exceeds 0 °C, part of the frost melts, temporarily restoring heat transfer efficiency, which in turn leads to a further decrease in temperature—thus creating a cyclic process: "frosting → efficiency drop → temperature rise → defrosting → efficiency increase → temperature decrease → renewed frosting." In equipment lacking an automated defrosting function, prolonged operation at low temperatures will inevitably result in this phenomenon.

[Solution]Emergency Procedure: suspend the test, execute the defrost cycle (increase the set temperature to 20 °C and maintain it for 30 min; then open the chamber door and wipe the surface of the evaporator fins with a dry, soft cloth to remove residual moisture), and only after confirming that the fins are completely dry, resume the low-temperature test. Preventive Measures: ① Before each low-temperature test, verify that the chamber interior is dry (optionally perform a "drying phase" at 30 °C/30% RH to eliminate residual moisture); ② Avoid frequent opening of the chamber door during the low-temperature phase (each opening may introduce a large amount of water vapor); ③ For prolonged low-temperature operation, insert a defrost maintenance phase every 48 hours. After corrective actions, maintain the temperature at −50 °C with temperature fluctuations within ±0.4 °C.

Case 09 | False trigger of overtemperature protection; test interrupted midway

[Fault Phenomenon]During the 125 °C high-temperature test, the equipment suddenly shut down after 3 hours of operation; the TEMI880 displayed a "Overtemperature Alarm," and the indicator light of the RKC overtemperature protection instrument lit up. The actual measured temperature inside the test chamber was 125.2 °C, which did not exceed the set value; however, since the set threshold of the protection instrument was configured at 125 °C, the upper limit of normal temperature fluctuation triggered the protection action.

[Root Cause Analysis]The overtemperature protection device serves as a secondary safety safeguard independent of the main controller. The issue stems from improper setting of the protection threshold: the operator set the overtemperature protection value to be equal to the test temperature of 125 °C, whereas the normal temperature fluctuation ranges from ±0.5 °C to ±1 °C; thus, when the peak temperature reaches 125.5 °C, the 125 °C protection threshold is triggered. The overtemperature protection threshold should be set at least 5–10 °C higher than the test temperature setpoint to account for temperature fluctuations.

[Solution]Adjust the temperature limit of the RKC overtemperature protection instrument from 125 °C to 135 °C (test temperature + 10 °C). Determination criterion: Overtemperature protection threshold = test temperature setpoint + 10 °C (minimum margin); for high-temperature tests (>100 °C), it is recommended to use +15 °C. After adjustment, reperform the 125 °C test for 48 consecutive hours without interruption. Concurrently, verify the overtemperature protection setpoints for all test procedures, and compile a "Protection Threshold Setting Reference Table" to prevent similar false triggers.

Case 10 | Uneven temperature variation across multiple test samples; some samples failed the inspection

[Failure Observation]Five plastic-cased test specimens from the same batch were placed on three tiers of shelves for testing at 85 °C/85 % RH. Upon completion of the test, the two test specimens on the upper tier exhibited color change and were judged as qualified; whereas the three test specimens on the lower tier showed slight deformation and more pronounced color changes, and were judged as unqualified. Significant differences in test results were observed among test specimens located at different positions within the same test chamber.

[Root Cause Analysis]The air supply and circulation system, comprising multiple-bladed fans paired with a 90 W motor, directs airflow from the rear wall of the cabinet, passes it through the test specimens, and then recirculates it through the front wall. When the density of shelf placement is excessive, the airflow channels between test specimens become obstructed, leading to a temperature gradient between the upper and lower layers. Experimental measurements show a temperature of 85.1 °C in the upper layer and 83.2 °C in the lower layer, yielding a temperature difference of 1.9 °C; when combined with the humidity gradient, the environmental stress experienced by the test specimens in the lower layer becomes more severe. Additionally, the lower-layer specimens are positioned closer to the evaporator air outlet, and uneven mixing of hot and cold airflow also contributes to this phenomenon.

[Solution]First, adjust the spacing between test specimens: the distance between adjacent specimens shall be no less than 50 mm, and the distance between each specimen and the chamber walls shall be no less than 30 mm, ensuring unobstructed airflow circulation within the duct system. Second, utilize eight adjustable spacing clips to modify the shelf layer spacing from uniform intervals to a "sparse lower, dense upper" configuration—increasing the spacing between lower shelves to compensate for airflow attenuation. Third, incorporate a 10-minute "temperature homogenization phase" into the test protocol (maintain this phase for 10 minutes after reaching the target temperature before starting the timer) to ensure full uniformity of the temperature field within the chamber. After treatment, the temperature difference between upper and lower shelves was reduced to below 0.4 °C, and the test results from the five specimens were consistent.

Case 11 | Frequent start-stop cycles of the compressor, abnormal noise, and questionable service life

[Fault Symptoms]During the constant temperature holding phase (when active cooling or heating is not required), the compressor cycles on and off repeatedly every 3–4 minutes, accompanied by a distinct "humming-click" noise. After approximately one week of continuous operation, the compressor housing temperature becomes elevated (feels hot to the touch), and the noise level increases significantly compared to the new unit.

[Root Cause Analysis]During the constant-temperature holding phase, the ideal operating mode for the compressor is low duty cycle operation (e.g., 30 seconds of operation followed by 120 seconds of shutdown), rather than high-frequency start-stop cycling. A start-stop frequency of 3–4 cycles per minute is considered excessive for two reasons: ① The proportional band (P) in the P.I.D. parameters is set too narrow, causing the controller to over-respond to minor temperature deviations and frequently issue start-stop commands; ② The start-stop pressure differential setting of the Saginomiya pressure controller is set too small, resulting in insufficient separation between the threshold for initiating operation at the low-pressure side and the threshold for terminating operation at the high-pressure side, which leads the refrigeration system to repeatedly act within a narrow pressure differential range. Frequent start-stop cycling poses the greatest risk to fully enclosed compressors—each start instant draws a current 5–7 times greater than the normal operating current, and such repeated stress accelerates the insulation aging of the motor windings.

[Solution]Step 1: Execute the P.I.D automatic calculation to widen the proportional band, thereby reducing the controller's excessive response to minor deviations. Step 2: Consult professional technicians to adjust the start/stop pressure differential settings of the Saginomiya pressure controller (appropriately lower the low-pressure start threshold and appropriately raise the high-pressure stop threshold to expand the operating range and reduce start/stop frequency). Step 3: In the program settings of the TEMI880, add a "temperature hold dead band"; within a setpoint range of ±0.3 °C, no cooling or heating action will be triggered, allowing the system to leverage the thermal inertia of the cabinet to maintain the temperature. After these adjustments, the compressor start/stop frequency was reduced to once every 15–20 minutes, operation became stable, and the cabinet temperature returned to normal levels.

Case 12 | Limescale blockage in the humidification system: extremely slow increase in humidity

[Fault Symptoms]After six months of operation, the rate of humidity increase has significantly slowed down—the time required for the humidity to rise from 40% RH to 85% RH has increased from the initial 8 minutes to over 25 minutes. During operation of the humidification tube, an abnormal "gurgle" noise is audible, and white scale deposits can be observed on the inner walls of the wet-bulb tank and the surface of the humidification tube.

[Root Cause Analysis]The humidification system utilizes ordinary tap water as its water source; during the heating process in the humidification tubes, calcium and magnesium ions in the water precipitate to form calcium carbonate/magnesium carbonate scale. This scale deposits on the surface of the 1 kW humidification tubes, creating an insulating layer that reduces heating efficiency. Simultaneously, scale fragments obstruct the internal water flow channels of the humidification tubes, leading to a decrease in humidified steam output. Scale also accumulates on the inner walls of the wet-bulb water tank, impairing the capillary water-absorption performance of the gauze. This constitutes a systemic efficiency degradation attributable to water quality issues