【正文】 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 or electrical. And control instruments in one location. This makes it practical for a minimum number of operators to run 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 measurement value. This transmitter is mounted close to the point of measurement in the process. The transmitter outputair pressure for a pneumatic transmitteris piped to the recording or control instrument. The standard output range for a pneumatic transmitter is 20 to 100kPa, which is 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 1 to 5V DC for voltage signal. Nowadays, another type of electrical signal, which is being mon, is the digital or discrete signal. The use of instruments and control systems based on puter or 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～ 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. Beside we must measure Temperature Temperature measurement is important in industrial control, as direct indications of system or product state and as indirect indications of such factors as reaction rates, energy flow, turbine efficiency, 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 systems, liquidinglass thermometers, thermocouples, resistance temperature detectors, thermostats, bimetallic devices, optical and radiation pyrometers and temperaturesensitive paints.Advantages of electrical systems include high accuracy and sensitivity, practicality of switching or scanning several measurements points, larger distances possible between measuring elements and controllers, replacement of ponents(rather than plete system), fast response, and ability to measure higher temperature. Among the electrical temperature sensors, thermocouples and resistance temperature detectors are most widely used.單片機(jī)系統(tǒng)廣義地說，微處理系統(tǒng)是用于處理信息的，這種信息可以是電話交談，儀器讀數(shù)或企業(yè)帳戶，但是各種情況下都涉及相同的主要操作：信息處理、存儲和傳遞。 this information may be telephone conversation, instrument read or a pany’s accounts, but in each case the same main type of operation are involved: the processing, storage and transmission of information. in conventional electronic design these operations are bined at the function level。基于單片機(jī)的流量檢測系統(tǒng)的硬件結(jié)構(gòu)框圖如下：數(shù)碼管顯示 單片機(jī) AT89C51 A/D轉(zhuǎn)換器流量傳感器復(fù)位電路報警電路晶振電路電源電路 系統(tǒng)硬件結(jié)構(gòu)框圖 重點(diǎn)和難點(diǎn)：本次設(shè)計的重點(diǎn)和難點(diǎn)是流量信息經(jīng)模數(shù)轉(zhuǎn)換后的電壓信號在送入單片機(jī)后作怎樣的處理才能在LED顯示器上轉(zhuǎn)換顯示成水位數(shù)據(jù)，以及各模塊的軟件編程，并將各硬件電路合理搭接。另外AT89C51在工業(yè)控制上也有著廣泛的應(yīng)用，編程技術(shù)及外圍功能電路的配合使用都很成熟。數(shù)字信號通過數(shù)碼管顯示，從而實(shí)現(xiàn)對流量信息的實(shí)時顯示；另外還可設(shè)置最低和最高警戒流量值，當(dāng)流量不在正常范圍內(nèi)時，報警電路便會識別同時發(fā)出報警聲，提醒流量超出正常范圍，需及時采取措施。人們?yōu)榱丝刂拼髿馕廴?，必須對污染大氣的煙氣以及其他溫室氣體排放量進(jìn)行監(jiān)測；廢液和污水的排放，使地表水源和地下水源受到污染，人們必須對廢液和污水進(jìn)行處理，對排放量進(jìn)行控制。工業(yè)生產(chǎn)中過程控制是流量測量與儀表應(yīng)用的一大領(lǐng)域，流量與溫度、壓力和物位一起統(tǒng)稱為過程控制中的四大參數(shù)，人們通過這些參數(shù)對生產(chǎn)過程進(jìn)行監(jiān)視。流量的測量在熱電生產(chǎn)、石油化工、食品衛(wèi)生等工業(yè)領(lǐng)域具有廣泛的應(yīng)用。這一段時間內(nèi)流體體積流量或質(zhì)量流量的累積值稱為累積流量。新技術(shù)、新器件、新材料和新工藝及新軟件的開