Pascal’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 g = − 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.Ībsolute pressure is the sum of gauge pressure and atmospheric pressure.įor 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 Pascal’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.

Atmospheric pressure psi