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A common way of measuring the level of liquid in a tank places a low-pressure sensor at the bottom. The pressure readout is the product of the liquid column height times the density of the liquid. As long as liquid density remains constant, the measured pressure is directly proportional to the liquid level. This method works with all low-vapor-pressure, low-viscosity liquids and, of course, water.

传统的测量罐体内液位的高度变化的方法，通常是在罐体底部安装一只压力传感器，读取的压力值即是液面高度与液体密度之积。只要液体的密度不变，测出的压力将与液位高度成正比。这种方法很适用于检测低气压、低粘度的液体，也就是水。

The difficulty lies in inserting the pressure sensor at the right location in the tank to make the measurement. The sensing of lowdensity liquids or of small level changes may require special, lowpressure transducers that are expensive and hard to find. The use of a float attached to a nearly friction-free linear-variable-displacement transducer (LVDT) offers a reasonable replacement. LVDTs can measure liquid-level changes from a few inches to several feet.

问题在于，将液压传感器装在罐内合适的位置检测液位高度，并不简单。检测有些低密度或者高度变化细微的液体，还需配置特殊的液压传感器，该产品往往是价格不菲且不易寻找。在无滑动触点的差动变压式(LVDT)位移传感器的探杆上连接一个浮球，就可以测量1英寸（毫米级）到数英尺(分米)的液位高度，达到微米级的精度，从而提供了一个很好的解决方案。

In the typical LVDT level sensor, a stainless-steel float coupled to a nonmagnetic stainless-steel rod is attached to the high-permeability armature core of the LVDT. The 4 to 20-mA loop-powered LVDT position transmitter senses the position of the core and, thus, the level of the float. Sensitivity to the change in level depends on the length of LVDT. The most common units measure level changes from 0 to 2, 12, or 20 in. (50, 300, or 500 mm) with the shorter lengths providing greatest sensitivity.

标准的LVDT液位传感器，是将LVDT电枢铁芯通过非磁不锈钢连杆连接到浮球。LVDT液位传感器通过检测铁芯的位置变化，进而检测到浮球的液位高度，输出4-20MA的标准工业信号。这类LVDT原理的液位传感器的量程范围通常是零到2英寸、12英寸、20英寸（50毫米、300毫米、500毫米）等几种规格。量程越小，精度也就越高（精度=满量程*线性度）





As water level changes, the float moves up or down, bringing the LVDT core with it. A threaded stainless-steel rod protrudes from the other end of the core. It carries two jam nuts to adjust the position sensor output at the desired low water level. The nuts also prevent the core from falling out of the LVDT housing if the liquid level drops too low.

随着液位的变化，浮球上下移动，LVDT的铁芯也随之浮动。在铁芯末端突出一根不锈钢螺杆, 上面配有两只安全螺母，一个用来调节低水位时位置传感器的输出 (即零点), 另一个防止液位降得过低时铁芯从LVDT的主体中脱落。

A nonmagnetic clamp block attaches the stainless-steel body of the LVDT to a flat hook. The hook hangs the level-sensor assembly over the tank edge.

LVDT通过一块温度膨胀系数小的非磁夹具悬挂在扁平钩上，借此固定在罐体边缘。

Core position is sensed using magnetic induction. In fact, the electronics of the LVDT are hermetically sealed inside its stainless-steel housing. A Teflon liner inside the bore minimizes any friction to core motion. There are no springs to fatigue or parts to wear out. And, LVDT transmitters come in agency-listed versions for use in Class 1, Div. 1 or 2, or Zone 2 hazardous locations.

LVDT的铁芯定位是运用了电磁感应原理。事实上，LVDT的电子电路是完全密封在不锈钢壳体内的。不锈钢管内的铁氟龙衬垫避免了铁芯在运动中的摩擦。里面也没有弹簧或其他可损部件。因而，LVDT位移传感器已位列EX（ia）IIA防爆级别。