The heat, printed circuit board
Definition of PCB temperature
Temperature traditionally defined as the feeling about the changes in temperature in contact with the object.If the two objects together referred to as thermal contact (physical), then the object of high temperature to low temperature heat transfer objects, until the temperature between the two are equal, the thermal equilibrium is reached between the two objects.At this time we will feel the temperature of the two objects are the same.We can define the temperature of the system to reach thermal equilibrium when two objects.
If more in-depth understanding of the two systems, we will find between multiple systems will reach thermal equilibrium with each other.Thermal equilibrium does not depend on the types of objects, but rather in their system.
The second law of thermodynamics tells us that in the two thermodynamic system, if every one is in balance, with a third system, they are in thermal equilibrium state between each other.That is to say, there are only the same temperature in thermal equilibrium system.
Of objects with the hand to feel the cold heat, use eye to pick the heat will feel dark reddish cooler, bright dazzling high temperature.These contact and not contact is the degree of heat and cold, still applied effectively.Either with direct or indirect comparison method measured temperature thermometer design is based on the principle of thermal equilibrium.
2 PCB temperature measurement
Early in the development of resistance thermometer, Callendar to platinum resistance thermometer is studied.Since then, great changes have taken place in resistance temperature measurement method.High precision measurement with the classic style of platinum resistance thermometer, expanding its temperature measurement range.For the most accurate temperature measurement, platinum resistance thermometer is always top priority being chosen thermometer.Pure choose platinum resistance thermometer elements as material is decided by its own nature, good chemical stability, even under the high temperature stability is also very good.Under the condition of high melting point, high purity easy access to filaments;High resistivity (at room temperature about 10 mu Ω. Cm).Ductility is good for pulling into filaments, but will also bring some difficulties at the same time, fully annealed state platinum wire is very soft, may encounter some problems in the installation process.Platinum strong oxidation resistance at high temperature, makes it a measurement of high temperature material of a kind of very good.Platinum is not only a good resistance thermometer material, and it is a kind of very good high-temperature thermocouple.
Platinum resistance thermometer resistance?Temperature relationship, as predicted by theory, extension in upward from the room temperature is a very wide range of temperatures, platinum resistance thermometer resistance are quite close to the temperature of the square function.
Second, the printed circuit board humidity
1. Printed circuit board absolute humidity
Every 1 m3 of water vapor contained in the moist air quality is called the absolute humidity of wet air.Therefore, absolute humidity on numerical T equals in wet air temperature "and water vapor partial pressure of water vapor density rho v Pv.Rho v value can be made of water vapor in Chad, or by calculation:
Type, are my water vapor (kg), the quality of the RV for vapor constant.
2. The printed circuit board relative humidity
The amount of water vapor in the atmosphere, it can change between O and saturation state.Absolute humidity only indicates how much the actual water vapor content in the moist air, and not in the condition of the wet air saturation degree or the size of the water vapor absorption ability.As a result, the relative humidity is commonly used to represent the extent of the wet air humidity.Relative humidity is defined as the absolute humidity of wet air rho with saturated moist air temperature of the ratio of the absolute humidity phi, with symbol "said
Rho Max said when the temperature is t wet water vapor in the air can reach the biggest pressure, namely the rho s.T necessarily rho Max or rho s) has a certain value accordingly.
Type (7-6) shows that the relative humidity can also be used wet air partial pressure of water vapor of rho v, and temperature of water vapor content is close to saturation, therefore, also known as the saturation.The smaller the values of phi, said the more humid air drying, the ability to absorb moisture stronger;Conversely, the greater the values of phi, said the more humid air humidity, the weak ability to absorb water.When the phi equals O, it is dry air;Phi is equal to 1, it is saturated moist air.So, no matter what the temperature of the wet air, by the size of the phi values can be directly see it wet.
Relative humidity is usually with dry and wet ball temperature meter to measure, as shown in figure 7-1.Two of the same type of thermometer, one of which in temperature measurement on the bubble a soak in the water wet gauze, become wet bulb.Put dry wet bulb in ventilated place, make the air continuously through the thermometer, dry bulb thermometer readings on is the air temperature t.Wet bulb and direct contact with wet cloth, the readings shall be the water temperature.If the air is saturated moist air (phi = 1), the water won't boil on wet cloth, the two thermometer readings will be the same.If the air is not saturated moist air (phi < 1), the water will boil when flowing through the wet cloth.Vaporizing latent heat of vaporization, water temperature will decline because of the vaporization heat of forming temperature difference between water and air.The existence of temperature difference, make the hot air heat transfer to the cold water.Water due to evaporation heat release, and because of difference in temperature and the air absorption of heat, such as heat quantity is greater than the heat absorption, water temperature would continue to decline to a certain temperature, the heat and heat absorption capacity are equal, the water temperature is no longer falling, to meet the needs of vaporization heat completely from the air.At this point the reading on the wet bulb is called the wet bulb temperature, expressed as a symbolic Tw.Air temperature is constant value T, contains less water vapor (i.e. far from the more saturated state), the wet bulb temperature is low.Because the evaporation of water is more, when air flows through the wet cloth requires a larger temperature difference in order to draw more heat from the air to meet the needs of the vaporization.
Thus, Tw and actual amount of water vapor in air (or the actual absolute temperature).In addition, the maximum absolute humidity of air depends on the temperature of the air T.Thus phi and should have a certain relationship between T and Tw phi = f (T, Tw).According to this relationship, in the determination of the air T and Tw, can ask the air relative humidity phi.General dry wet bulb will = f (T, Tw) on the list, can be read directly according to T and Tw.