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1、1,Lesson 2.2 Level Measurements,Topics discussed in this unit are as follows:The formulas used in level measurementsThe difference between direct and indirect level measuring devicesThe difference between continuous and single-point measurementsThe various types of instruments available for level me
2、asurements,1,2,1.Introduction,This unit discusses the measurement of the level of liquids and free flowing(自由流动)solids in containers(容器).The detector is normally sensing the interface(界面)between a liquid and a gas,a solid and a liquid,or possibly the interface between two liquids.Sensing liquid leve
3、ls fall into two categories;firstly,single-point sensing(单点检测)and secondly,continuous level monitoring(监测).,2,这一单元讨论容器内液体和自由流动固体的液面测量.,探测器通常检测液体和气体，固体和液体，或者两种液体之间的界面.,检测液体液面可分为两类：一类是单点检测，第二类是对连续液面进行监测.,3,1.Introduction,In the case of single-point sensing the actual level of the material is detected
4、when it reaches a predetermined(预定，预置)level,so that the appropriate action can be taken to prevent overflowing(溢出)or to refill(再充填)the container.,3,在单点检测的情况下，当物质的实际液面被检测出达到预定液面时，（控制器）采用适当的动作来防止溢出或再次填充容器.,4,1.Introduction,Continuous level monitoring measures the level of the liquid on an uninterrupte
5、d(不间断的，连续的)basis.In this case the level of the material will be constantly monitored and hence,the volume can be calculated if the cross-sectional(横截面)area of the container is known.Level measurements can be direct or indirect;examples of these are using a float technique or measuring pressure and c
6、alculating the liquid level.Free flowing solids are dry powders(粉末),crystals(晶体),rice,grain and so forth.,4,5,2.Level Formulas,Pressure is often used as an indirect method of measuring liquid levels.Pressure increases as the depth increases in a fluid.The pressure is given byp=hwhere p=change in pre
7、ssure=specific weight(比重)h=depthNote the units must be consistent(一致的),i.e.,pounds and feet,or Newtons and meters.,5,6,2.Level Formulas,Buoyancy(浮力)is an indirect method used to measure liquid levels.The level is determined using the buoyancy of an objet partially immersed(浸没，沉入)in a liquid.The buoy
8、ancy B or upward force on a body in a liquid can be calculated from the equationB=areadWhere area is the cross-sectional area of the object and d is the immersed depth of the object.,6,浮力是测量液面的一种间接方法.液位通过一个部分沉浸在液体中的物体的浮力而确定得出.浮力或液体中物体受到的向上的力B可以用以下方程计算,其中面积是指物体的横截面积，d指物体浸没的深度.,7,2.Level Formulas,The
9、liquid level is then calculated from the weight of a body in a liquid WL,which is equal to its weight in air(WA-B),from which we getThe weight of a container can be used to calculate the level of the material in the container.In Figure 2.2.1(a)the volume V of the material in the container is given b
10、y V=areadepth=r2dWhere r is the radius of the container and d is the depth of the material.,7,因此，液体的液位可以通过物体在液体中的重量WL计算得出，其中WL等于物体在空气中的重量(WA-B),由该关系我们得出上式,8,2.Level Formulas,The weight of material W in a container is given by W=VCapacitive probes(电容电极)can be used in nonconductive(非导电的)liquids and fr
11、ee flowing solids for level mesurement.Many materials,when placed between the plates of a capacitor,increase the capacitance by a factor called the dielectric constant(介电常数)of the material.For instance,air has a dielectric constant of 1 and water 80.Figure 2.2.1(b)shows two capacitor plates partiall
12、y immersed in a nonconductive liquid.The capacitance(Cd)is given by,8,电容电极可以用于非导电液体和自由流动固体的液面测量.许多材料被放置于电容板之间时，以因子来增加电容量，被称作该材料的介电常数.比如，空气的介电常数为1，水的介电常数为80.图2.2.1(b)表示了部分浸入非导电液体的两个电容板.,9,2.Level Formulas,Where Ca=capacitance with no liquid=dielectric constants of the liquid between the plates r=heig
13、ht of the plates d=depth or level of the liquid between the platesThere are large variations in dielectric constant with temperature so that temperature correction may be needed.In Eq.(2.2.6)the liquid level is given by,9,其中，Ca表示没有液体时的电容，表示电容板之间液体的介电常数，r表示电容板的高度，d表示电容板之间液体的液面或深度。,10,3.Level Sensing
14、Devices,There are two categories of level sensing devices.They are direct sensing,in which case the actual level is monitored,and indirect sensing where a property of the liquid such as pressure is sensed to determine the liquid level.,10,液面测量仪器可以分为两类.一类是直接测量，监测当前的液面，,另一类是间接测量，通过测量液体的性质，比如压力，来计算出液体的
15、液面.,11,3.Level Sensing Devices,Direct Level SensingSight glass(open end/differential)or gauge is the simplest method for direct visual reading.As shown in Figure 2.2.2 the sight glass is normally mounted(安置)vertically adjacent(邻近的)to the container.The liquid level can then be observed directly in th
16、e sight glass.,11,直接液面测量（法）玻璃液面计或玻璃液面表是最为简单的方法，可以直接视觉读取数据.,如图所示，通常玻璃液面计垂直放置在容器旁边.,那么液体的液面可以通过玻璃液面计直接观察到.,12,3.Level Sensing Devices,The container in Figure 2.2.2(a)is closed.In this case the ends of the glass are connected to the top and bottom of the tank,as would be used with a pressurized(增压的)con
17、tainer(带压容器，带压储罐)(boiler，锅炉，蒸发器)or a container with volatile(易挥发的),flammable(易燃的),hazardous(危险的),or pure liquids.In cases where the tank contains inert(惰性)liquids such as water and pressurization is not required,the tank and sight glass can both be open to the atmosphere as shown in Figure 2.2.2(b).
18、,12,图2.2.2(a)中的容器是封闭的.在这种情况下，玻璃液面计的两端与容器的顶部和底部相连，用于加压储罐(比如锅炉)，或者带有易挥发、易燃、危险、或提纯液体的容器.,13,3.Level Sensing Devices,The top of the sight glass must have the same pressure conditions as the top of the liquid or the liquid levels in the tank and sight glass will be different.In cases where the sight glas
19、s is excessively long,a second inert liquid with higher density than the liquid in the container can be used in the sight glass(see Figure 2.2.2(c).,13,玻璃液面计的顶端必须与容器中液体顶端具有同样的压力条件，否则容器的液位和玻璃液面计的液位将不一致.,当玻璃液面计过长时，可以在玻璃液面计内使用第二种惰性液体，其密度比容器内液体密度更高(如图2.2.2(c)所示).,14,3.Level Sensing Devices,Floats(浮体，浮标)
20、(angular arm or pulley，角臂或滑轮)are shown in Figure 2.2.3.The figure shows two types of simple float sensors.The float material is less dense than the density of the liquid and floats up and down on top of the material being mesured.In Figure 2.2.3(a)a float with a pulley is used;this method can be use
21、d with either liquids or free flowing solids.With free flowing solids,agitation(搅拌)is sometimes used to level the solids.,14,图所示为浮体(浮力传感器)(角臂式或滑轮式).该图给出了两种简单的浮力式传感器.,浮体材料的密度小于液体密度，并且在被测材料顶部上下浮动.,15,3.Level Sensing Devices,An advantage of the float sensor is that it is almost independent of the densi
22、ty of the liquid or solid being monitored.If the surface of the material being monitored is turbulent(扰动的，湍流),causing the float reading to vary excessively,some means of damping might be used in the system.,15,浮力式传感器的一个优点是它几乎不受监测液体或固体密度的影响.,如果被测物质的表面是扰动的，使得浮标读数变化过大，那么在系统中要采取一些阻尼方法.,16,3.Level Sensin
23、g Devices,In Figure 2.2.3(b)a ball float is attached to an arm;the angle of the arm is measured to indicate the level of the material(an example of the use of this type of sensor is the monitoring of the fuel level in the tank of an automobile).Although very simple and cheap to manufacture,the disad
24、vantage of this type of float is its nonlinearity as shown by the line of sight scale in Figure 2.2.4(a).The scale can be replaced with a potentiometer(电位计)to obtain an electrical signal that can be linearized for industrial use.,16,在图2.2.3(b)中，浮球连接到一个臂上，通过测量臂的角度来指示物质液面(使用这种传感器的一个例子，是测量汽车油箱里的燃料液面).,
25、尽管这种浮力式传感器制造起来既简单又便宜，但它的缺点在于其非线性特性，如图2.2.4(a)中的视觉刻度线所示.,这种刻度可以采用电位计来代替，用以得到在工业使用中可以线性化的电信号。,17,3.Level Sensing Devices,Figure 2.2.4(b)shows an alternative method of using pulleys to obtain a direct visual scale that can be replaced by a potentiometer to obtain a linear electrical output with level.P
26、robes(探针，探测器)for measuring liquid levels fall into three categories,i.e.,conductive(传导的，导电的),capacitive(电容的),and ultrasonic(超声波).,17,图2.2.4(b)是另一种滑轮式方法（浮力传感器），可直接获得视觉刻度，该刻度可采用电位计来代替，以获得液面的线性电信号输出.,检测液面的探针可以分为三类，导电式，电容式，和超声波式.,18,3.Level Sensing Devices,Conductive probes are used for single-point mea
27、surements in liquids that are conductive and nonvolatile as a spark(火花)can occur.Conductive probes are shown in Figure 2.2.5(a).Two or more probes as shown can be used to indicate set levels.If the liquid is in a metal container,the container can be used as the common probe.,18,导电电极(传感器)用于单点液位测量，液体是
28、导电的并且不可挥发，以防止电火花的产生.导电电极如图2.2.5(a)所示.,两个或更多的电极可用于显示液位.如果液体在一个金属容器中，那么该容器可以用作普通电极.,19,3.Level Sensing Devices,When the liquid is in contact with two probes the voltage between the probes causes a current to flow indicating that a set level has been reached.Thus,probes can be used to indicate when the
29、 liquid level is low and to operate a pump to fill the container.Another or a third probe can be used to indicate when the tank is full and to turn off(关闭)the filling pump.,19,当液体与两个电极均有接触时，两个电极之间的电压产生电流，说明液面已经达到了设定值.,因此，电极可用于测试液位较低的情形，驱动泵的运行来填充容器.,另外或第三个电极可以用于显示容器填充满的时候，以关闭泵的运行.,20,3.Level Sensing
30、Devices,Capacitive probes are used in liquids that are nonconductive and have a high and can be used for continuous level monitoring.The capacitive probe shown in Figure 2.2.5(b)consists of an inner rod(连杆)with an outer shell(壳);the capacitance is measured between the two using a capacitance bridge.
31、,20,电容电极用于不导电并且具有很高的介电常数的液体，可以用于连续液面监测.,图2.2.5(b)所示的电容电极包括一个内杆和一个外壳；两者之间的电容值用电容电桥测量.,21,3.Level Sensing Devices,In the portion out of the liquid,air serves as the dielectric between the rod and outer shell.In the immersed section,the dielectric is that of the liquid that causes a large capacitive ch
32、ange,if the tank is made of metal it can serve as the outer shell.,21,在液体以外的部分，空气作为内杆和外壳之间的电介质.,浸入液体的部分，液体作为电介质并产生较大的电容变化，如果容器是用金属做的，那么容器也可以作为外壳.,22,3.Level Sensing Devices,The capacitance change is directly proportional to the level of the liquid.The dielectric constant of the liquid must be known
33、for this type of measurement.The dielectric constant can vary with temperature so that temperature correction may be required.,22,电容变化与液位变化直接比例相关.在这种测量办法中，液体的介电常数必须已知.,在介电常数随温度变化，因此需要进行温度校正.,23,3.Level Sensing Devices,Ultrasonics can be used for single point or continuous level measurement of a liqu
34、id or a solid.A single ultrasonic transmitter(发送器)and receiver(接收器)can be arranged with a gap(间隙，缺口)as shown in Figure 2.2.6(a)to give single-point measurement.As soon as liquid fills the gap,ultrasonic waves from the transmitter reach the receiver.,23,超声波(传感器)可以用于液体或固体的单点液位测量或连续液位测量.,一套超声波发送器和接收器，在
35、安装时两者之间留有一定的间隙，来实现单点液位测量，如图2.2.6(a)所示。,当液体充满间隙时，超声波可以从发送器传送到接收器.,24,3.Level Sensing Devices,A setup for continuous measurement is shown in Figure 2.2.6(b).Ultrosonic waves from the transmitter are reflected by the surface of the liquid to the receiver;the time for the waves to reach the receiver is
36、measured.The time delay gives the distance from the transmitter and receiver to the surface of the liquid,from which the liquid level can be calculated knowing the velocity of ultrasonic waves.,24,图2.2.6(b)给出了连续(液位)测量的装置.发送器发出的超声波被液体表面反射到接收器；测量超声波传递到接收器所用的时间.,25,3.Level Sensing Devices,As there is n
37、o contact with the liquid,this metod can be used for solids and(腐蚀的)and volatile liquids.In a liquid the transmitter and receiver can also be placed on the bottom of the container and the time measured for a signal to be reflected from the surface of the liquid to the receiver to measure the depth of the liquid.,25,由于该装置与液体没有接触，所以这个方法可以用于固体和腐蚀性和挥发性的液体.,在液体中，发送器和接收器也可以安置于容器底部，信号被液体表面反射到接收器的时间可以测量液体的深度.,
