Fluid Pressure Measurement

Since static fluid pressure is determined by the fluid density and depth, the depth or height difference of a given liquid is commonly used for pressure measurement: 由于静止流体压力由流体密度和深度决定，因此通常使用某一液体的深度或高度差进行压力测量：

The fact that the liquid levels are the same in these three manometers shows that the pressure in the glass manifold above them is uniform. This is under static conditions with no air flow through the system so that all parts of it are at atmospheric pressure. The fact that open liquids will seek a common level is the principle behind liquid levels for construction purposes. 这些三个液位计中液面高度相同的事实表明，玻璃总管上方的压强是均匀的。这发生在静止状态下，系统中没有空气流动，因此其所有部分都处于大气压下。开放液体会趋向于达到共同液面的高度这一原理，是液位在建筑用途中的基础。

The flow in this system is from left to right, driven by a high pressure air supply. The system is said to have a positive gauge pressure exerted by the air supply. This pressure acts to push the manometer levels down, so the minimum height in the left manometer indicates that it is subjected to the highest pressure. 该系统的流动方向是从左到右，由高压气源驱动。系统被认为受到气源施加的正表压。这种压力作用于推动 manometer 水平下降，因此左侧 manometer 的最低高度表明它所受的压力最高。

The fact that the center manometer has a higher level under these conditions of rapid air flow indicates that the pressure has been lowered in the constriction by the Bernoulli effect. Note that the liquid level in the right hand tube is slightly higher than the left tube, indicating that the pressure there is slightly less than that at the left hand tube. This is indicative of the normal pressure drop in a flowing fluid from Poiseuille's law since the diameters of the top tube are the same at the left and right tubes.

Fluid Pressure Calculation

Discussion 讨论

Fluid column height in the relationship is often used for the measurement of pressure. After entering the relevant data, any one of the highlighted quantities below can be calculated by clicking on it. Pressure difference = density x g x height 压强差 = 密度 × g × 高度 If the fluid density is 若流体密度是 ρ = gm/cm3 = kg/m3 and the column height is 并且柱高是 h = m = x 10^ m h = ft then the pressure difference is 那么压力差是 ΔP = kPa ΔP = lb/in2 ΔP = mmHg= inches Hg ΔP = atmos ΔP = inches water= cm water 在关系中，流体柱的高度常用于压力的测量。输入相关数据后，点击下面突出显示的任一量即可计算。ρ = gm/cm 3 = kg/m 3 h = m = x 10^ m h = ft ΔP = kPa ΔP = lb/in 2 ΔP = mmHg= inches Hg ΔP = atmos ΔP = inches water= cm water 中文译文中的待填/计算数值依次对应：1：d 2：dk 3：h 4：hb 5：hp 6：hc 7：pk 8：pc 9：pm 10：pmc 11：pa 12：piw 13：pcw。实际数值以上方原输入框为准。

Atmospheric Pressure

The surface of the earth is at the bottom of an atmospheric sea. The standard atmospheric pressure is measured in various units:

The fundamental SI unit of pressure is the Pascal (Pa), but it is a small unit so kPa is the most common direct pressure unit for atmospheric pressure. Since the static fluid pressure is dependent only upon density and depth, choosing a liquid of standard density like mercury or water allows you to express the pressure in units of height or depth, e.g., mmHg or inches of water. The mercury barometer is the standard instrument for atmospheric pressure measurement in weather reporting. The decrease in atmospheric pressure with height can be predicted from the barometric formula.

The unit mmHg is often called torr, particularly in vacuum applications: 760 mmHg = 760 torr

For weather applications, the standard atmospheric pressure is often called 1 bar or 1000 millibars. This has been found to be convenient for recording the relatively small deviations from standard atmospheric pressure with normal weather patterns.

Mercury Barometer

The mercury barometer and other manometer devices are reliable pressure measurement devices since fluid pressure depends only on fluid density and depth. The atmospheric pressure provides the force necessary to push the mercury up the evacuated tube. Even though it acts downward in the illustration, by Pascal's principle we know that the pressure acts in all directions and can force the mercury up the tube until its weight is enough to equal the force of the atmosphere.