Tel
Add
2025-05-30
Share:
Temperature and humidity test chamber in the moisture and heat test principles and methods
Moisture and heat test technology is mainly used to apply temperature and humidity test chamber to simulate the humid environment, and its uses include:
1, To explore the impact of the humid environment on the product (development, design stage of the research test).
2, The use of temperature and humidity test chamber to identify the moisture-proof performance of the product (development, production stage of the quality inspection or type test).
3, Through the temperature and humidity test chamber to evaluate the safety and reliability of products used in humid environments (safety or reliability test).
The main indicators determined after the test is generally to check the electrical and mechanical properties of the product, but also check the corrosion of certain samples.
Humidity and heat test generally have three types, of which, constant humidity and heat test need to be maintained through the temperature and humidity test chamber with a fixed temperature and humidity (such as 40 ℃ / 93% RH), applicable to general electrical and electronic products, the stress severity level is low, the test equipment requirements are not high.
Alternating humidity and heat test need to rely on the temperature and humidity test chamber programming function to achieve temperature and humidity cycle (such as 30 ↔ 60 ℃), for the environment is more harsh and complex products, for complex environments or may be about to be used in such environments of the military products or communications products.
Alternating humidity and heat or humidity and heat test on the temperature, humidity, duration and cycle cycle requirements are more severe than constant humidity and heat, the military standard of the humidity and heat test is even more severe.
Therefore, if the product has been through the temperature and humidity test chamber to complete the alternating humidity and heat or military standard test, there is no need to repeat the constant humidity and heat test.
Generally important and critical products or military equipment, in the development of reliability test program or prepare a test outline, will not choose the constant humidity and heat test. The order of severity of the three kinds of humidity and heat test, from low to high as “constant humidity and heat” is less than “alternating humidity and heat” is less than “humidity and heat (military standard)”. It should be noted that the severity is not the more items the better.
In the temperature and humidity test chamber, temperature and humidity coupling will trigger the following phenomena:
Temperature and humidity test chamber of water vapor molecules due to van der Waals force adsorption on the surface of the sample, the adsorption amount is affected by the temperature, pressure and the nature of the material. For example, temperature and humidity test chamber in low temperature and high humidity conditions (25 ℃ / 95% RH), the adsorption amount of metal surface up to 3 times the ambient environment.
Gas molecules (in the heat and humid test refers to water vapor molecules) in the temperature and humidity test chamber experimental space movement may collide with the surface of solid materials (samples), when a certain number of molecules continuously touching the surface of the solid, before it comes back to the space, to be in the solid (samples) surface, “stay” a certain length of time.
At this time, the gas in the test chamber on the surface of the concentration is higher than its concentration in space, thus producing condensation. This gas on the solid surface “stay” phenomenon called adsorption.
Therefore, adsorption can also be said to be the gas on the solid surface condensation and evaporation of an intermediate process. According to the experimental results, the amount of gas adsorption is related to the nature of the solid material, the temperature and the pressure of the gas at equilibrium. The lower the temperature and the higher the pressure, the greater the adsorption.
Physical adsorption is caused by van der Waals gravity, and the adsorption layer is generally a multimolecular layer. The adsorption speed is faster, the energy required for adsorption is smaller, and it can be carried out at low temperature. Physical adsorption is the most common phenomenon in the humidity and heat test.
Temperature and humidity test chamber heating stage, the sample due to thermal inertia lags behind the temperature inside the chamber, the surface temperature is lower than the dew point to form condensation.
Condensation is actually water molecules in the sample adsorption phenomenon, but it is generated when the test temperature rises. In the warming phase, the sample surface temperature is lower than the dew point temperature of the surrounding air, water vapor will condense into liquid on the sample surface to form water droplets.
Temperature and humidity test chamber in the alternating heat and humidity test temperature rise phase, due to the thermal inertia of the sample, so that its temperature rise lagged behind the temperature of the temperature and humidity test chamber.
Therefore, the surface of the condensation phenomenon. The amount of this surface condensation, depending on the size of the sample's own heat capacity, as well as the speed of warming and the relative humidity of the warming stage, in the cooling phase of the alternating humidity and heat test, the closed shell of the inner wall of the condensation phenomenon will also occur.
Temperature and humidity test chamber by controlling the absolute humidity (such as 80 ℃ saturated steam) to accelerate the diffusion of water molecules.
Diffusion is a physical phenomenon of molecular movement. In the diffusion process, the molecules always migrate from the concentration of large places to the concentration of small places.
Temperature and humidity test chamber in the humidity and heat test, water vapor in the air to the lower concentration of the speed of diffusion inside the material can be expressed by Fick's law. Therefore, the moisture intrusion caused by diffusion in the humid heat test, in addition to depending on the absolute humidity and temperature in the test conditions, but also related to the material of the sample.
Temperature and humidity test chamber cooling phase (such as from 60 ℃ down to 30 ℃), the closed sample inside the pressure difference due to moisture inhalation.
Water vapor enters inside the material, generally through the gap. The speed of water vapor through the gap depends on the size of the hole. If the size of the pores is smaller than the diameter of the water molecules, water vapor cannot enter.
Since water vapor exists in space mixed with air, its rate of entry is also highly dependent on the mixing ratio of water vapor and air. The ratio of water vapor to air is 1:1, which is equivalent to the amount of water vapor in the saturated state of air at 80°C as the limit. Higher than this limit is called high vapor pressure, lower than this limit is called low vapor pressure, and then the mechanism of water vapor into the void are discussed:
① low vapor pressure water into the mechanism: in the temperature and water vapor pressure are unchanged (equivalent to the use of constant temperature and humidity chamber for constant heat and humidity test), water vapor into the void is mainly due to the diffusion of the speed mainly depends on the void In the air resistance (permeability coefficient) and the size of the gap (the size of the gap, although also affects the rate of entry, but not serious).
When the temperature changes (equivalent to the use of constant temperature and humidity test chamber for alternating heat and humidity test), the water vapor pressure difference between the two sides of the gap to force the air containing water vapor through.
At this time the rate of entry is not only related to the gap resistance and the size of the gap, but also with the water vapor pressure difference between the two ends of the gap is also relevant. It can be seen, even if both tests using the equipment is a temperature and humidity test chamber, but the constant humidity and heat test and alternating humidity and heat test of the role of the mechanism is not the same.
② High vapor pressure conditions, water vapor into the speed and the gap diameter, when the gap diameter is less than the average free travel of water molecules, water vapor into the molecular flow; when the gap diameter is greater than the average free travel, into the speed of viscous flow, the gap diameter in between the two for the transition flow.
Under high vapor pressure, the water vapor entry rate with the gap size change shows that if the temperature is increased to accelerate the moisture entry, for different gap sizes will have different rates, and its acceleration times will be different.
In summary, the entry of water vapor through the absorption phenomenon depends on the temperature and humidity test chamber temperature and water vapor pressure (absolute humidity) and the material of the material.
We will close the sample in the cavity caused by temperature changes in the internal and external air exchanges, called respiration. In the temperature and humidity test chamber for alternating heat and humidity test cooling phase, due to the sharp drop in temperature, caused by the closed cavity air temperature drop or condensation of the inner wall of the cavity will make the pressure inside the cavity to reduce the formation of suction phenomenon, inhalation of humid air in the outside world, so the cooling phase of the respiratory effect of the amount of moisture inhalation how much, with the temperature and humidity test chamber temperature rate of change and absolute humidity related.
This respiratory phenomenon does not only occur in the test temperature alternation, when the sample with a closed shell, such as closed rotary motor in the intermittent movement process, the shell coil heating or cooling repeated alternating changes, will also occur respiratory effect. Motor products used in humid conditions, due to this respiration inhalation of moisture, long-term condensation into water in the shell to accumulate, is also not uncommon.
Sample moisture in the form of two general: a surface moisture, it is usually caused by condensation and surface adsorption; the other is the volume of moisture, it is caused by water vapor diffusion and absorption phenomena.
Sometimes the moisture adsorbed on the surface of the sample reaches a certain level, which also accelerates the rate of volumetric moisture. For the closed type of sample with a cavity, its internal although not in direct contact with high humidity conditions, but due to the temperature and humidity test chamber, the test temperature changes caused by respiration, will make the external moisture through the gap or cracks into the interior, resulting in internal moisture.
At the same time, diffusion and absorption phenomena can also make moisture through the gap into the closed shell. In addition, for the shell of some organic materials, when the diffusion phenomenon caused by the absorption of moisture has reached a stable, moisture can penetrate through the shell into the shell.
Surface and volume of moisture caused by the deterioration of the sample effect, refers to mechanical properties (size and strength) and non-mechanical properties (electrical and other properties); two kinds of changes.
Temperature and humidity test chamber within the humidity test temperature and humidity conditions are generally simulated in the actual environment of the rarer conditions, and the duration of its role than the actual environment is much longer.
Therefore, the use of constant temperature and humidity test chamber for heat and humid test, more severe than natural conditions, the sample is accelerated. According to the mechanism of moisture caused by several physical phenomena discussed above, it can be seen that the test results are not exactly the same for samples of different materials and structures. Therefore, a universal general artificial damp heat test method requires to take a uniform acceleration factor is difficult.
Only for a particular or a single nature of the sample, after analysis and test comparisons, to determine a more appropriate acceleration factor. The classification of the hot and humid environment and the corresponding relationship between the test severity level is a problem that has not been fully resolved over the years.
Temperature and humidity test chamber for artificial damp heat test method of the severity of the grade is composed of test conditions and the number of test cycles. The test conditions generally correspond to the actual use of the sample environmental conditions, while the selection of the number of test cycles is more complex. Usually, the number of test cycles is based on the characteristics of the sample and the main mechanism of humidity and heat on the impact of a comprehensive analysis after the determination.
Generally, it is necessary to compare with the results of natural or on-site operation tests to find out the relationship between them before choosing the appropriate number of cycles.
However, so far, even in the international community has not yet come up with a universally applicable mathematical model to express the relationship between the artificial damp heat test and natural conditions. Therefore, although the preferred number of cycles is recommended in the test method standard, there are still many problems in practical application.
The number of cycles of the damp heat test is the most reliable basis for the long-term storage cycle of the product. Available data show that, especially in stockpiles, the basic and most important factor affecting corrosion is the relative humidity in the warehouse. At low relative humidity, corrosion rates do not increase rapidly with increasing temperature. They follow an empirical equation:
Where: A - degree of corrosion
H - Relative humidity (%)
t - Atmospheric temperature (°C)
k - Constant related to the Types of metal materials related to the constant
According to this equation, the corrosion degree of different metal materials under different conditions can be obtained. According to this equation, when the relative humidity (H) in the atmosphere is 65%, the degree of corrosion A = 0, which means that the metal material will not corrode under such conditions. However, when the relative humidity is greater than 65%, the metal will rust and corrode, and with the increase of humidity and temperature, the degree of rust and corrosion increases sharply.
Whether it is long-term storage or accelerated corrosion testing, another common is pitting matrix corrosion. Most of them are due to dip paint, packaging production process of bumping, melting holes in the process of “inclusions” (most of the iron), stamping process due to bumping, scratches caused by the “ash”, and in the surface treatment before the failure to find the repair of the surface.
Thus, pitting corrosion is also the most difficult to eliminate a source of corrosion. Constant temperature and humidity test chamber to do alternating heat and humidity test, the cooling phase of the respiratory effect, for some types of samples is more obvious, therefore, in the test method especially emphasizes the cooling speed and humidity. Alternating humidity and heat in the larger amplitude of temperature change, cooling higher relative humidity and high humidity continues to act for a long time, will exacerbate the insulation moisture.
Condensation of temperature and humidity test chamber directly leads to metal oxidation (e.g., galvanized parts of the white rust generation rate increased by 10 times) or PCB insulation failure (leakage current rises from 1 μA to 50 μA).
Temperature and humidity test chamber high humidity environment (95% RH) to promote the rubber sealing ring to absorb water expansion (volume increased by 5%), or optical module fiber coupling efficiency decreased by 12%.
Acceleration factor limitations: temperature and humidity test chamber of extreme conditions (such as 85 ℃ / 85% RH) may trigger unnatural failure mode, need to be combined with the actual environment calibration test cycle.
Test process encountered in the sudden power outage and other special reasons caused by the test is forced to interrupt the situation, it is recommended to operate in accordance with the following manner:
(1) If the interruption process in the temperature and humidity test chamber environmental conditions do not exceed the permissible error range, this time the interruption of time should be used as part of the total test time (generally instantaneous power outage after the timely restoration of power box environment);
(2) When the interruption process during the temperature and humidity test chamber test conditions below the lower limit of the allowable error, then the required test environment should be re-attained, and the elimination of the error range outside the test time, has been carried out until the completion of the specified test time;
(3) If the temperature and humidity test chamber has been tested, it is recommended that it is best to stop the test and re-test with a new sample, if after the relevant technical personnel's judgment, it is considered that exceeding the required test conditions will not directly cause damage to the characteristics of the test sample, or the sample is a repairable product, then it can be handled in accordance with the second article, and if there is a failure of the sample in a subsequent test, then it should be considered that the If the sample fails in the subsequent test, then the test result shall be considered invalid.
Test process encountered during the sudden power outage and other special circumstances caused by the test interruption, it is recommended that the following way of operation:
① If the interruption of the process of constant temperature and humidity test chamber in the ambient conditions do not exceed the permissible range of error, the interruption of this time should be used as part of the total test time;
② When the interruption of the process of temperature and humidity test chamber environmental conditions below the lower limit of the permissible error, then it should be restarted from the end of the last valid cycle before the interruption to continue the test (i.e., the cycle where the point of interruption is invalid);
③ If the temperature and humidity test chamber has been tested , it is recommended that it is best to stop the test, re-testing with new samples, if after the determination of the relevant technical personnel, that more than the required test conditions will not directly result in the destruction of the characteristics of the test sample, or the sample is repairable products, then the temperature and humidity test chamber environment can be restored to the required environmental conditions and continue to test, if the sample failure occurs in the subsequent test, then the test results should be considered invalid.
Temperature and humidity test chamber test process encountered a sudden power failure and other special circumstances caused by the test interruption, before the completion of the specified number of cycles (excluding the last cycle), such as the occurrence of not more than one accidental test in the middle of the cycle, you can redo the cycle. If an unplanned test interruption occurs during the last cycle, a further uninterrupted cycle is required in addition to redoing the cycle. Any interruption of more than 24 h requires that the test be redone from start to finish.
Damp heat test, including constant damp heat test, alternating damp heat test, temperature / humidity combination cycle test.
IEC 60068-2-78 constant humidity and heat test, the temperature and humidity test chamber temperature tolerance for 2 ℃.
IEC 60068-2-56 constant humidity and heat test four temperature levels specified temperature tolerance of 2 ℃, relative humidity tolerance of 3%.
IEC 60068-2-56 Alternating humidity and heat test specified upper limit temperature: temperature tolerance is ±2%, relative humidity tolerance is ± 3%; lower limit temperature temperature tolerance is ± 3 ℃; relative humidity requirements of 95%.
IEC 60068-2-38 Combined temperature/humidity cycling test in the humidity exposure cycle of the upper limit temperature, the temperature tolerance for the temperature of 2 ℃, relative humidity tolerance of earth 3%. Relative humidity is a parameter related to temperature, temperature and humidity test chamber temperature is different, the relative humidity is different, the relative humidity difference size is also related to its humidification, wind speed, control accuracy.
Temperature and humidity test chamber humidification and air flow rate is generally fixed, control accuracy can only be guaranteed through good maintenance, maintenance and correct operating procedures. Its effective working space is generally smaller than that of high temperature test, because only small temperature difference and small temperature fluctuation can ensure that the difference in relative humidity is kept at a small value.
IEC 60068-2-78 points out that: in order to make the relative humidity tolerance specified in this standard to remain within the required range, temperature and humidity test chamber workspace temperature difference between any two points, in any instant should not be greater than 1 ℃, short-term temperature fluctuations must be kept within a small range.
Determine the effective space of various kinds of humidity and heat test, also need to be judged by the measurement of relative humidity. In order to ensure that the test specimen is always kept within the specified tolerance range during the various humidity and heat tests.
Service hotline
