1. Equipment selection Based on the natural environmental factors and the types of induced environmental factors that exist on the surface of the Earth and the atmosphere, it is not yet possible to calculate an exact number, among which factors affecting the use and life of engineering products (equipment). There are no more than a dozen. Engineers engaged in the study of environmental conditions of engineering products classify the environmental conditions induced by nature and human activities into a series of test standards and specifications to guide the environmental and reliability tests of engineering products. For example, instructing military products to conduct environmental tests GJB150 - National Military Standard of the People's Republic of China "Environmental Test Methods for Military Equipment", GB2423 for guiding environmental testing of electrical and electronic products - National Standard of the People's Republic of China "Guidelines for Environmental Test Methods for Electrical and Electronic Products", etc. . Therefore, the main basis for our selection of environmental and reliability test equipment is the test specifications and test standards for engineering products.
Secondly, in order to standardize the tolerance of environmental test conditions in test equipment and ensure the control accuracy of environmental parameters, the national technical supervision agencies and various industrial departments have also formulated a series of verification procedures for environmental test equipment and instruments. Such as the People's Republic of China national standard GB5170 "Electrical and electronic products environmental testing equipment basic parameters verification method", and the National Technical Supervision Bureau issued JJG190-89 "Electric Vibration Test Bench System Trial Verification Regulations" and so on. These verification procedures are also an important basis for selecting environmental and reliability test equipment. Test equipment that does not meet the requirements of these verification procedures is not allowed to be put into use.
Second, the basic principles of equipment selection Environmental and reliability test equipment selection should follow the following five basic principles:
1. Reproducibility of Environmental Conditions It is an elusive thing to completely and accurately reproduce the environmental conditions in nature in the laboratory. However, within a certain tolerance range, people can correctly and correctly simulate the external environmental conditions experienced by engineering products during use, storage, transportation and the like. This passage is summarized in the language of the project, that is, "the environmental conditions (including the platform environment) created around the test equipment created by the test equipment should meet the environmental conditions and tolerances specified in the product test specifications." For example, the temperature box used for military product testing should not only meet the requirements of different uniformity and temperature control accuracy in the national military standard GJB150.3-86 and GJB150.4-86. Only in this way can the reproducibility of environmental conditions in environmental tests be guaranteed.
2. Repeatability of environmental conditions One environmental test equipment may be used for multiple tests of the same type of product, and one engineered product may be tested in different environmental test equipment, in order to ensure the same product The comparability of the test results obtained under the environmental test conditions specified in the same test specification necessarily requires the environmental conditions provided by the environmental test equipment to be reproducible. That is to say, the stress level (such as thermal stress, vibration stress, electrical stress, etc.) applied to the product under test by the environmental test equipment is consistent with the requirements of the same test specification.
The repeatability of the environmental conditions provided by the environmental test equipment is guaranteed by the national metrological verification department after passing the verification according to the verification procedures formulated by the national technical supervision agency. To this end, environmental testing equipment must be required to meet the requirements of various technical indicators and accuracy indicators in the verification procedures, and the time limit specified in the verification period should not exceed the time limit. If the use of the very common electric vibration table meets the technical indicators such as excitation force, frequency range, load capacity, etc., it must also meet the requirements of the accuracy index such as the lateral vibration ratio, the uniformity of the table surface acceleration, and the harmonic distortion specified in the verification regulations. And the use period after each verification is two years, more than two years must be re-qualified before they can be put into use.
3. Measureability of environmental condition parameters The environmental conditions provided by any environmental test equipment must be observable and controllable, not only to limit environmental parameters to a certain tolerance, but also to ensure test conditions. Reproducibility and repeatability requirements, and from the safety of product testing is also necessary in order to prevent the environmental conditions from running out of control leading to damage to the test product, resulting in unnecessary losses. At present, the accuracy of the parameter test generally required in various test specifications should not be less than one third of the error allowed by the test conditions.
4. Exclusivity of environmental test conditions Each time an environmental or reliability test is carried out, the categories, magnitudes and tolerances of environmental factors are strictly regulated, and environmental factors required for non-tests are excluded from penetration in order to be tested. Or when the product failure and failure mode are judged and analyzed after the end of the test, the exact basis is provided. Therefore, the environmental test equipment is not allowed to attach other environmental stress interference to the test product except the specified environmental conditions. For example, the leakage magnetic field of the table, the acceleration signal-to-noise ratio, and the ratio of the total rms value of the in-band and out-of-band acceleration are defined in the electric vibration table verification procedure. Accuracy indicators such as random signal test and harmonic distortion are all verification items to ensure the uniqueness of environmental test conditions.
5. Safety and reliability of test equipment Environmental test, especially reliability test, long test period, the test object is sometimes a high value military product. During the test, the tester often has to conduct inspection or test work around the site. Therefore, environmental test equipment is required to have the characteristics of safe operation, convenient operation, reliable use and long working life to ensure the normal operation of the test itself. The various protection, warning measures and safety interlocking devices of the test equipment should be perfect and reliable to ensure the safety and reliability of the test personnel, the tested products and the test equipment itself.
Third, the choice of temperature and humidity box
1. Selection of volume When the test product (components, components, components or complete machine) is placed in the climatic environment box for testing, in order to ensure that the atmosphere around the test product can meet the environmental test conditions specified in the test specification, the climate chamber works. The following points should be observed between the dimensions and the dimensions of the product to be tested:
a) The volume of the test product (W×D×H) shall not exceed 20% to 35% of the effective working space of the test chamber (20% recommended). It is recommended to use no more than 10% for products that generate heat during the test.
b) The ratio of the windward area of ​​the test product to the total area of ​​the test chamber on the section is not more than (35 to 50)% (35% is recommended).
c) The distance between the outer surface of the test product and the wall of the test chamber shall be at least 100~150mm (150mm is recommended).
The above three points are actually interdependent and unified. Taking a 1 cubic meter cube box as an example, the area ratio is 1: (0.35 to 0.5), which corresponds to a volume ratio of 1: (0.207 to 0.354). The ratio of the volume to the volume of 100 to 150 mm from the wall of the tank is 1: (0.343 to 0.512).
In summary, the above three points stipulate that the working chamber volume of the climatic environment test chamber should be at least 3 to 5 times the outer volume of the tested product. The reasons for this provision are as follows:
1) After the test piece is placed in the box, the smooth passage is occupied, and the narrowing of the passage will result in an increase in the flow rate of the air flow. Accelerate the heat exchange between the gas stream and the test piece. This is inconsistent with the reproduction of environmental conditions, because in the relevant standards, the temperature flow around the test specimens in the test chamber should not exceed 1.7 m/s, so as to prevent the test specimens and the surrounding atmosphere from being produced. Heat transfer. When the average wind speed in the test chamber is vacant at 0.6-0.8 m/s and does not exceed 1 m/s, the wind speed of the flow field may increase when the space and area ratio specified by the two requirements of a) and b) are met. ～100)%, the average zui high wind speed is (1 to 1.7) m/s. Meet the requirements of the standard. If the volume of the test piece or the wind-breaking area is increased without restriction in the test, the airflow velocity will increase to exceed the high wind speed specified in the test standard during the actual test, and the validity of the test results will be suspected.
2) The accuracy index of the environmental parameters in the working chamber of the climate chamber (such as temperature, humidity, salt spray settling rate, etc.) is the result of the test under no-load condition. Once the test piece is placed, the working environment of the test chamber The uniformity of the parameters will have an effect, and the greater the space occupied by the test piece, the more serious this effect will be. The measured test data shows that the temperature difference between the windward and leeward sides in the flow field can reach 3-8 °C, and in severe cases it can be as large as 10 °C. Therefore, the two requirements a) and b) must be met as much as possible to ensure the uniformity of the environmental parameters around the product under test.
3) According to the principle of heat conduction, the temperature of the airflow near the tank wall is usually 2 to 3 °C from the center temperature of the flow field, and may reach 5 °C at the upper and lower limits of the high and low temperature. The temperature of the tank wall is different from the temperature of the flow field near the tank wall by 2~3 °C (depending on the structure and material of the tank wall). The greater the difference between the test temperature and the external atmospheric environment, the greater the above temperature difference, therefore, the distance from the tank wall The space within the distance (100 to 150 mm) is an unusable space.
2, the choice of temperature range At present, the range of foreign temperature test chamber is generally -73 ~ +177 ° C, or -70 ~ +180 ° C. Most domestic manufacturers generally have -80 to +130 ° C, -60 to +130 ° C, -40 to +130 ° C, and high temperatures to 150 ° C. These temperature ranges usually meet the needs of most military and civilian products for temperature testing in the country. Unless there are special needs, such as products with heat sources close to the engine, do not blindly increase the upper temperature limit. Because the higher the upper limit temperature, the greater the temperature difference between the inside and outside of the tank, the worse the uniformity of the flow field inside the tank. The available studio volume is also smaller. On the other hand, the higher the upper limit temperature value, the higher the heat resistance requirement for the heat insulating material (such as glass wool, etc.) in the tank wall interlayer. The higher the sealing requirements of the cabinet, the higher the manufacturing cost of the cabinet.
3, the choice of humidity range The humidity indicators given by the environmental test chambers at home and abroad are mostly 20 ~ 98% RH or 30 ~ 98% RH, if the damp heat test box does not have a dehumidification system, the humidity range is 60 ~ 98%, this category The test chamber can only be tested for high humidity, but it is much cheaper. It is worth noting that the corresponding temperature range should be indicated after the humidity indicator, or the low dew point temperature of zui should be given. Because the relative humidity is directly related to the temperature, for the same absolute moisture content, the higher the temperature, the smaller the relative humidity, such as the absolute moisture content of 5g / Kg (refers to 5 grams of water vapor in 1 kg of dry air) ), when the temperature is 29 ° C, the relative humidity is 20% RH, the temperature is 6 ° C, the relative humidity is 90% RH, when the temperature drops below 4 ° C, the relative humidity exceeds 100%, condensation will appear in the box phenomenon.
To achieve high temperature and high humidity, it is only necessary to spray water vapor or atomized water droplets into the air of the cabinet to humidify. Low temperature and low humidity are relatively difficult to control because the absolute moisture content at this time is very low, and sometimes it is much lower than the absolute moisture content in the atmosphere. It is necessary to dehumidify the air flowing in the tank to make the air dry. At present, most of the temperature and humidity chambers at home and abroad use the principle of refrigeration and dehumidification. A set of cooling tubes is added to the air pre-conditioning room of the cabinet. When the humid air passes through the cold pipe, its relative humidity will reach 100% RH, and the air will become drier due to air saturation on the light pipe. This dehumidification method can theoretically reach a dew point temperature below zero, but when the surface temperature of the cold spot reaches 0 ° C, the water droplets on the surface of the light pipe will freeze, thereby affecting the heat exchange on the surface of the light pipe, and the dehumidification ability is lowered. And because the box can not be absolutely sealed, the humid air in the atmosphere will penetrate into the box, so that the dew point temperature will rise. On the other hand, the humid air flowing between the light pipes is saturated with water at the moment of contact with the light pipe (cold spot), so that the dehumidification method is difficult to make the dew point temperature in the case below 0 °C. The actual zui low dew point temperature is 5 to 7 °C. The dew point temperature of 5 ° C corresponds to an absolute moisture content of 0.0055 g/Kg, and the temperature corresponding to a relative humidity of 20% RH is 30 °C. If the required temperature is 20 ° C and the relative humidity reaches 20% RH, the dew point temperature is -3 ° C at this time. It is very difficult to dehumidify by means of cooling, and it must be achieved by using an air drying system.
4. Selection of control mode The temperature and humidity test chamber has two conditions: constant test chamber and alternating test chamber.
Ordinary high and low temperature test chambers generally refer to constant high and low temperature test chambers, which are controlled by setting a target temperature and the ability of the test chamber to automatically constant temperature to the target temperature point. The control method of the constant temperature and humidity test chamber is similar. Set a target temperature and humidity point. The test chamber has the ability to automatically adjust the temperature to the target temperature and humidity point. The high and low temperature alternating test chamber has the program of setting one or more high and low temperature changes and circulations. The test chamber has the ability to complete the test process according to the preset curve, and can be accurate within the range of Zui's large temperature rising and cooling rate capabilities. By controlling the rate of temperature rise and temperature drop, the temperature rise and temperature drop rates can be controlled according to the slope of the set curve. Similarly, the high and low temperature alternating heat and humidity test chamber also has a preset temperature and humidity curve, and the ability to control according to presets. Of course, the alternating test chambers all have the function of a constant test chamber, but the manufacturing cost of the alternating test chamber is higher, because the alternating test chamber needs to be equipped with a curve automatic recording device, a program control device, and the test chamber must be solved in the working chamber. When the temperature is high, the refrigerator is turned on. Therefore, the price of the alternating test chamber is generally 20% higher than the price of the constant test chamber. Therefore, we should be realistic and take the test method as the starting point, choose a constant test box or an alternating test chamber.
5, the choice of variable temperature rate Ordinary high and low temperature test chamber does not have the index of cooling rate, the time from the ambient temperature to the nominal zui low temperature is generally 90 ~ 120min. The high and low temperature alternating test chamber and the high and low temperature alternating wet heat test chamber all have the requirements of variable temperature speed, and the temperature change rate generally requires 1 ° C / min, and the speed is adjustable within the range of this rate. The rapid temperature change test chamber has a faster temperature change rate, and the temperature rise and temperature drop rate can reach 3 ° C / min ~ 15 ° C / min, in some temperature ranges, the temperature rise and temperature drop rate can even reach 30 ° C / min or more.
The temperature range of the rapid temperature change test chamber of various specifications and speeds is generally the same, that is, -60 to +130 °C, but the temperature range of the evaluation of the cooling rate is not the same. According to the different test requirements of the test, the rapid temperature change test The temperature range of the box is -55 to +80 °C, and some are -40 to +80 °C.
There are two ways to change the temperature change rate of the rapid temperature change test chamber. One is the average temperature rise and fall speed, and the other is the linear temperature rise and fall speed (actually the average speed per 5 min). The average speed of the whole process refers to the ratio of the difference between the high temperature of zui and the low temperature of zui in the temperature range of the test chamber. At present, the technical parameters of the variable temperature rate provided by the foreign environmental test equipment manufacturers refer to the whole average rate. The linear temperature rise and fall speed refers to the variable temperature rate that can be guaranteed in any 5 min period. In fact, for the rapid temperature change test chamber, the difficulty of ensuring the linear temperature rise and fall speed is the key to the cooling rate of the test chamber in a 5 min period after the cooling stage Zui. From a certain perspective, the linear temperature rise and fall (average speed per 5 min) is more scientific. Therefore, the test equipment has the two parameters of the average average temperature rise and fall speed and the linear temperature rise and fall speed (average speed per 5 minutes). In general, the linear temperature rise and fall (average speed per 5 min) is 1/2 of the average average temperature rise and fall.
6. According to the relevant standards of wind speed , the wind speed in the temperature and humidity chamber should be less than 1.7m/s when conducting environmental tests. For the test itself, the wind speed is as small as possible. The wind speed is too high to accelerate the heat of the flow of the test piece and the flow inside the box. Exchange, the authenticity of the test is unfavorable. However, in order to ensure the uniformity of the test chamber working chamber, it is necessary to have circulating air in the test chamber. However, the rapid temperature change test chamber and the multi-factor comprehensive environmental test chamber such as temperature, humidity and vibration, in order to pursue the temperature change rate, the flow velocity of the circulating airflow in the tank must be accelerated, and the wind speed is usually 2 to 3 m/s. Therefore, the wind speed limit is different for different purposes.
7. Temperature fluctuations Temperature fluctuations are a relatively easy to implement parameter. The actual temperature fluctuations of most test chambers produced by all environmental testing equipment manufacturers can be controlled within ±0.3 °C.
8. Temperature field uniformity In order to more accurately simulate the actual environmental conditions suffered by the product in nature, it is necessary to ensure that the periphery of the tested product is at the same temperature and environmental conditions in the environmental test. For this reason, the temperature in the test chamber must be Gradient and temperature fluctuations are limited. In the national military standard GJB150.1-86 military equipment environmental test method general provisions clearly stipulate "the temperature of the measurement system near the test sample should be within ± 2 ° C of the test temperature, the temperature of the work does not exceed 1 ° C / m or the total zui The value is 2.2 ° C (test sample does not work).
9. Accuracy Control of Humidity Most of the humidity measured in the environmental test chamber is dry and wet ball method, and the manufacturing standard of environmental test equipment is required by GB10586. The relative humidity deviation should be ±23% RH. In order to meet the requirements of humidity control accuracy, the temperature control accuracy of the humidity test chamber is high, and the temperature fluctuation is generally less than ±0.2 °C. Otherwise it is difficult to meet the requirements of humidity control accuracy.
10. Cooling method selection If the test box has a refrigeration system, the cooling system needs to be cooled. The test chamber is available in both air-cooled and water-cooled versions. .
Air - cooled water-cooled <br>Usage conditions The equipment is easy to install and can be used only when the power is turned on. The ambient temperature should be lower than 28 °C. If the ambient temperature is higher than 28 °C, the cooling effect will be affected. (The air conditioning is well equipped) The circulating cooling water system needs to be configured.
Poor heat transfer (relative to water cooling) Stable, good impact on the environment can cause the temperature of the air around the chamber to rise. Does not affect the temperature around the test chamber.
Larger noise (relative to water cooling) Small conclusions Which cooling method should be chosen should be based on the actual conditions of the laboratory. When conditions permit, priority is given to the choice of water cooling. However, if the cooling system of the selected test chamber is small, it can also be given priority.
Food Additives refer to the chemical synthesis or natural substances added to food to improve the quality, color, aroma and taste of food, as well as the need for preservation and processing technology. Food additives are a term that refers to the chemical synthesis or natural substances added to food to improve the quality, color, aroma and taste of food, as well as the need for preservation and processing technology. Food additives are generally not food and may not have nutritional value, but they must conform to the above definition, that is, they do not affect the nutritional value of food, and can prevent food spoilage, enhance food sensory properties or improve food quality.
Common food additives in spices, candy and chocolate in general have essential oils, flavors, powder flavor extract several types. Each type has numerous varieties, such as in candy and chocolate, according to the aroma type can be divided into fruit type, nut type, frankincense type, flower type, wine type and other different varieties.
1. preservatives: mainly used in carbonated drinks, fruit paste, jam, pickled fruits, preserves, pickles, and sauce.
Oil, vinegar, juice drinks, meat, fish, eggs, poultry food, commonly used are: benzoic acid, sodium benzoate, sorbic acid, potassium sorbate and so on.
2. coloring agent: mainly used in carbonated drinks, fruit juice drinks, confectionery, pastry color, sugar.
Fruit, hawthorn products, pickled vegetables, ice cream, jelly, chocolate, cream, instant coffee and other foods. Commonly used are: amaranth, carmine, lemon yellow, sunset yellow, caramel pigment and other synthetic pigments. Some natural edible pigments, such as sodium copper chlorophyllin, are mainly extracted from plant tissues, but their pigment content and stability are generally not as good as synthetic pigments.
3. sweeteners: sweeteners for food. Commonly used are: saccharin sodium (also known as saccharin), sodium cyclohexylaminosulfonate (cyclamate), maltitol, sorbitol, xylitol and so on. There are many foods that use sweeteners. Like: beverages, pickles, cakes, biscuits, bread, ice cream, preserves, candy, seasonings, canned meat and other almost everyday foods will be added with different kinds of sweeteners.
4. Puffing agent: some candy and chocolate products, as well as some fried products, puffed food, fermented flour products. The commonly used leavening agents are sodium bicarbonate, ammonium bicarbonate and compound leavening agents.
5. Thickener: It is a kind of hydrophilic polymer compound with stable, emulsified or suspended state, can form gel or improve food viscosity, so it is also called gel, gelling agent or emulsified stabilizer.
6. Emulsifier: A kind of surfactant whose molecule usually has hydrophilic group (hydroxyl) and lipophilic group (alkyl). It is easy to form an adsorptive layer at the water-oil interface, thus changing the surface activity between the phases of the emulsifier to form a uniform emulsifier or dispersion, so as to improve the structure, taste and appearance of food.
7. Bulkiness agent: Food additives with grain flour as the main raw material, which produce gas during the processing (heating process) and make the structure become homogeneous and dense porous structure, and make the food loose and crisp.
8. Defoamers: In the process of food processing, with the ability to eliminate and suppress liquid surface bubbles, so that the operation can proceed smoothly. Anticoagulant prevents aggregation or agglomeration of powdery or crystalline foods.
9. Colorants: Food additives that promote people's appetite and increase the value of food products.
10. Antioxidants: Food additives that prevent and delay the oxidative deterioration of food surfaces by inhibiting the activity of oxidases by hydrogen atoms, blocking oxidative chain reactions, or forming complexes with deoxygenated groups in the molecules of oxidizable components of food.
11. Tissue improver: A Food Additive that improves the appearance or feel of food by retaining water, bonding, plasticizing, thickening, and improving rheological properties.
12. Flour improver: a kind of additives to improve the quality of flour, can improve the yield, improve flour whiteness and gluten strength.
13. acidity regulator: it has the function of improving food quality, and is more widely used in all kinds of foods. A considerable number of candy and chocolate products use acidic agents to regulate and improve the aroma effect, especially fruit-based products. Commonly used are: citric acid, tartaric acid, lactic acid, malic acid and so on.
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Food Additive
Additives In Food,Sodium Dehydroacetate,Methyl Cyclopentenolone,3 4-Dihydroxybenzoic Acid
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