Pressure is transmitted undiminished to the manometer, and the fluid levels are no longer equal. Suppose one side of the U-tube is connected to some source of pressure P abs such as the toy balloon in Figure 2(b) or the vacuum-packed peanut jar shown in Figure 2(c). Let us examine how a manometer is used to measure pressure.

If the fluid is deeper on one side, there is a greater pressure on the deeper side, and the fluid flows away from that side until the depths are equal. Atmospheric pressure therefore pushes down on each side equally so its effect cancels. In Figure 2(a), both sides of the tube are open to the atmosphere. Consider the U-shaped tube shown in Figure 2, for example. This aneroid gauge utilizes flexible bellows connected to a mechanical indicator to measure pressure.ДЄn entire class of gauges uses the property that pressure due to the weight of a fluid is given by PВ = hПЃg.

In all mechanical pressure gauges, pressure results in a force that is converted (or transduced) into some type of readout.ДЇigure 1. Figure 1В shows one of the many types of mechanical pressure gauges in use today. Remote sensing is often more convenient than putting a measuring device into a system, such as a personвЂ™s artery. The undiminished transmission of pressure through a fluid allows precise remote sensing of pressures. PascalвЂ™s principle is of major importance in these devices. There are a host of devices for measuring pressure, ranging from tire gauges to blood pressure cuffs. There is no theoretical limit to how large a gauge pressure can be. (A negative absolute pressure is a pull.) Thus the smallest possible gauge pressure is P gВ = в€’ P atm (this makes P abs zero). Fluids push rather than pull, so the smallest absolute pressure is zero.

Gauge pressure is positive for pressures above atmospheric pressure, and negative for pressures below it.ДЄbsolute pressure is the sum of gauge pressure and atmospheric pressure.ДЇor reasons we will explore later, in most cases the absolute pressure in fluids cannot be negative. We therefore define gauge pressure to be the pressure relative to atmospheric pressure. In brief, it is very common for pressure gauges to ignore atmospheric pressure-that is, to read zero at atmospheric pressure. Blood pressure measurements, like tire pressures, are thus made relative to atmospheric pressure. What is important is how much greater blood pressure is than atmospheric pressure. (As noted in PascalвЂ™s Principle, the total pressure in a fluid is the sum of the pressures from different sources-here, the heart and the atmosphere.) But atmospheric pressure has no net effect on blood flow since it adds to the pressure coming out of the heart and going back into it, too. Similarly, atmospheric pressure adds to blood pressure in every part of the circulatory system. Tire gauges are simply designed to read zero at atmospheric pressure and positive when pressure is greater than atmospheric. Why does the gauge read zero? There is no mystery here. In fact, if there were a gaping hole in your tire, the gauge would read zero, even though atmospheric pressure exists in the tire. If you limp into a gas station with a nearly flat tire, you will notice the tire gauge on the airline reads nearly zero when you begin to fill it. Understand the working of aneroid and open-tube barometers.Define gauge pressure and absolute pressure.

780 mmhg to atm