DZ225多功能手機充電器的研制

DZ225多功能手機充電器的研制,dz225,多功能,手機充電器,研制 Pressure & Temperature Measurement pressure and Pressure TransmittersPressure arises when a force is applied over an area. Provided the force is one Newton(N)and uniformly over the area of one square meter(m2),the pressure has been designated one Pascal(Pa=N/m2). Pressure is a universal processing condition. It is also a condition of life on this planet: we live at the bottom of an atmospheric ocean that extends upward for many miles. This mass of air has weight, and this weight pressing downward causes atmospheric pressure. Water, a fundamental necessity of life, is supplied to most of us under pressure. In the typical process plant, pressure influences boiling point temperatures, condensing point temperatures, process efficiency, costs, and other important factors. The measurement and control of pressure, or lack of it-vacuum-in the typical process plant critical.The working instruments in the plant usually include simple pressure gauges, precision recorders and indicators, and pneumatic and electronic pressure transmitters. A pressure transmitter makes a pressure measurement and generates either a pneumatic or electrical signal output that is proportional to the pressure being sensed.In the process plant , it is impractical to local the control instruments out in the place near the process. It is also true that most measurements are not easily transmitted from some remote location. Pressure measurement is an exception ,but if a high pressure of some dangerous chemical is to be indicated or recorded several hundred feet from the point of measurement, a hazard may be from the pressure or from the chemical carried.To eliminate this problem, a signal transmission system was developed. This system is usually either pneumatic (air pressure ) or electrical. Using the transmission system, it will be possible to install most of the indicating, recording, and control instruments in one location. This makes it practical for a minimum number of operation to the plant efficiently.When a pneumatic transmission system is employed, the measurement signal is converted into pneumatic signal by the transmitter scaled from 0 to 100 percent of the measured value. This transmitter is mounted closed to the point of measurement in the process. The transmitter output---air pressure for a pneumatic transmitter---is piped to the recording or control instrument. The standard output range for a pneumatic transmitter is 20 to100kPa, which is a almost universally used.When an electronic pressure transmitter is used, the pressure is converted to electrical signal that may be current or voltage, Its standard range is from 4 to 20mA DC for current signal or from 1to 5V DC for voltage signal. Nowadays, another type of electrical signal, which is becoming common, is the digital or discrete signal. The use of instruments and control systems based on computers or microprocessors is forcing increased use of this type of signal.Sometimes it is important for analysis to obtain the parameters that describe the sensor/transmitter behavior. The gain is fairly simple to obtain once the span is known. Consider an electronic pressure transmitter with a range of 0～600kPa. The gain is defined as the change in output divided by the change in input. In this case, the output is electrical signal (4～20mA DC) and the input is process pressure (0～600kPa). Thus the gain .Kr＝(20mA-4mA)/(600kPa-0kPa)＝16mA/600kPa＝0.027mA/kPaTemperature Measurement Temperature measurement is important in industrial control, as direct indications of system or product state and lubricant quality. Present temperature scales have been in use for about 200 years. The earliest instruments were based on the thermal expansion of gases and liquids. Such filled systems are still employed, although many other types of instruments are available. Representative temperature sensors include: filled thermal detectors (RTD) , thermistors, bimetallic devices, optical and radiation pyrometers and temperature-sensitive paints.Advantages of electrical systems include high accuracy and sensitivity, practicality of switching or scanning several measurement points, larger distance possible between measuring elements and controllers, replacement of components ( rather than complete system ), fast response, and ability to measure higher temperature. Among the electrical temperature sensors, thermocouples and resistance temperature detectors (RTDs) are most widely uded.ThermocouplesThermoelectricity was discovered by Seebeck in 1821. A thermocouple consists basically of tow dissimilar metals, such as iron and xonstantan wires, joined to produce a thermal electro-motive force when the junctions are at different temperature. The measuring, or hot, junction is inserted into the medium where the temperature is to be measured. The reference, or cold, junction is the open end that is normally connected to the measuring instrment terminald. The e. m. f. of a thermocouple increases as the difference in junction temperature increase. Therefore, a sensitive instrument, capable of measuring e. m. f. , can be calibrated and used to read temperature directly.To make accurate temperature measurements with thermocouples, the reference junction temperature must remain constant. If it varies, suitable compensation for these variations must be provided. Should there will be a corresponding change in millivolt with a resultant error in temperature measurement.When used in the laboratory and for other checking purposes, the thermocouple reference junction can be place in a vacuum bottle filled with shave ice saturated with water. This method provides close temperature control ( within a fraction of a degree) and permit accurate reading. To ensure accurate readings, most thermocouples are mow installed with instruments that provide automatic reference junction compensation. In most instruments, this is accomplished by passing current through a temperature-responsive resistor, which measures the variations in reference temperature and automatically provides the necessary compensating e. m. f. by means of the voltage drop produced across it.Resistance Temperature DetectersResistance thermometry is based on the change of electrical conductivity with the temperature. Therefore, a coil of wire can act as a temperature sensor, with a direct relationship established between resistance and temperature. Standard curve are available, with certified accuracy within 0.1 °F or °C. Platinum RTDs used as laboratory standards can be obtained with tolerances well within this limit, and are capable of practice temperature measurement up to 1650°F or 900°C. If a RTD is adjusted to conform to its curve, it may be interchanged with other RTDs calibrated according to the some curve.One of the most popular circuits used with RTDs is Wheatstone bridge. Figure 22.1 圖shows a simplified circuit diagram of Wheatstone bridge. The resistance temperature detector (RTD) is one arm of a Wheatstone bridge excited by a DC power supply. Point A and point B form the input to amplifier. When the temperature changes, the bridge and creates an error signal between points A and B , which is amplified by the amplifier. With all resistance-temperature measurements, the use of three conductor RTD cable is recommended. The effect of ambient temprtature variations on the cable is thereby minimized. One of the conductors is common to both sides of the bridge while the other two connect one to each side of the bridge. Any change in cable temperature will be cancelled as both sides of the bridge are changed a like amount. In practice, a RTD may be used with as much as 500 feet of three-conductor cable without creating a perceptible error.