Whenever you have a regular health checkup at your doctor's office, they will likely measure your blood pressure. In that case, a medical tool called the sphygmomanometer is inflated around your arm to temporarily stop the blood flow in an artery.
Explore our app and discover over 50 million learning materials for free.
Lerne mit deinen Freunden und bleibe auf dem richtigen Kurs mit deinen persönlichen Lernstatistiken
Jetzt kostenlos anmeldenNie wieder prokastinieren mit unseren Lernerinnerungen.
Jetzt kostenlos anmeldenWhenever you have a regular health checkup at your doctor's office, they will likely measure your blood pressure. In that case, a medical tool called the sphygmomanometer is inflated around your arm to temporarily stop the blood flow in an artery.
As the pressure from the cuff is slowly released, the artery expands and blood rushes back into the arm. This allows the medical professional to quantify the force exerted by the blood flow against the artery walls, as the heart pumps the blood through the body. This procedure takes no longer than a couple of minutes, yet requires a thorough understanding of fluid mechanics, types of pressure, and their measurements. In this article, we'll focus on gauge pressure (e.g., blood pressure) and absolute pressure, their differences, and applications!
Earth's atmosphere is filled with air which constantly exerts a pressure on all objects and surfaces, as it experiences the gravitational pull toward the planet's surface. This pressure is known as atmospheric pressure.
Atmospheric pressure is the pressure exerted on all surfaces by the air in the Earth's atmosphere.
Sometimes it's also referred to as the barometric or air pressure and has been assigned a special unit of the standard atmosphere (\(\mathrm{atm}\)), where \(1 \, \mathrm{atm}\) describes the average atmospheric pressure at sea level at \(15\,^\circ \mathrm{C}\).
Pressure can be expressed in various units. In SI units, \(1 \, \mathrm{atm}\) is equal to \(101\,325 \, \mathrm{Pa}\) (commonly rounded to \(10^5\,\mathrm{Pa}\)). However, it is also the same as \(760 \, \mathrm{mmHg}\) and \(1.013\, \mathrm{bar}\).
Although we tend to assume atmospheric pressure to be a constant value, it differs across the planet's surface with altitude and temperature. These fluctuations are very minor near the Earth's surface, where most of the measurements we deal with occur, hence the valid assumption for a constant atmospheric pressure value. However, if the calculations deal with sensitive weather patterns or the pressure measured at the top of a mountain or deep in the ocean, the atmospheric pressure value should be adjusted accordingly.
So, we can use the fact that atmospheric pressure is a constant value at sea level to define two types of pressure: one that accounts for this additional atmospheric pressure (absolute pressure), and one that ignores it (gauge pressure).
Gauge pressure is the pressure of a fluid relative to the atmospheric pressure.
Absolute pressure is the pressure of a fluid relative to the zero pressure experienced in a vacuum.
In other words, atmospheric pressure is set as the reference point for the gauge pressure. If the absolute pressure is greater than atmospheric pressure, the gauge pressure will have a positive value. On the other hand, if the absolute pressure is smaller than atmospheric pressure, the gauge pressure will be negative.
The absolute pressure is the total pressure exerted by a fluid and includes the atmospheric pressure in its measurements.
Now that we have established the basic definitions for different kinds of pressure measurements, let's identify the key differences between gauge pressure and absolute pressure.
Returning to the blood pressure example from earlier, the sphygmomanometer used by the doctor will have a scale ranging from \(0 \, \mathrm{mmHg}\) to around \(300 \, \mathrm{mmHg}\). Considering that humans experience atmospheric pressure, shouldn't the scale go up to a minimum of \(1060 \, \mathrm{mmHg}\), as before taking any measurements on a patient, the sphygmomanometer will already display a value of roughly \(760 \, \mathrm{mmHg}\)?
Atmospheric pressure is always present on Earth and is experienced by the sphygmomanometer and the blood flowing through the patient, so keeping a record of this constant measurement doesn't make sense. The interesting value is the blood pressure compared to its surroundings because that value is what the arteries will feel as a net force per unit of artery area. So we are interested in the blood's gauge pressure. Thus, for convenience purposes, it's common to adapt these kinds of instruments to read zero at the atmospheric pressure, which means utilizing the gauge pressure.
On the other hand, if we have a completely sealed system where the atmospheric pressure has an effect on the events occurring, we use absolute pressure. It's commonly used in laboratory settings as well as manufacturing, where even the slightest fluctuation in the setup can alter the results. A great example is vacuum-packaged food, where the longevity of the food depends on the quality of the vacuum seal, or the lack of any pressure, including atmospheric pressure.
Just based on the definitions of both values, it's clear that absolute pressure and gauge pressure are closely related. A visual reference for connecting the two pressures is visible in Figure 3.
We can then use this diagram to come up with a mathematical expression connecting the two types of pressures.
Based on the explanation provided above, the simple version of the relationship between absolute and gauge pressure is
$$P=P_0+P_\mathrm{G}, $$
where \(P\) is the absolute pressure, \(P_0\) is the atmospheric pressure, and \(P_\mathrm{G}\) is the gauge pressure. Considering \(P_0\) is a constant value, we only need to elaborate on the gauge pressure, so let's explain it in more detail.
Pressure in a fluid occurs as the upper layers press their weight onto the lower layers. So let's imagine a closed container filled with a liquid, meaning we can ignore the atmospheric pressure. The pressure \(P\) exerted on the bottom of this container by a vertical, rectangular column of liquid can be expressed mathematically as
$$ P=\frac{F}{A}, $$
where \(F\) is the force the fluid exerts and \(A\) is the area over which the fluid exerts this force. The force in this case is simply the force of gravity acting on the liquid of mass \(m\) with the gravitational acceleration \(g\). Mass can be re-expressed in terms of liquid density \(\rho\) and volume \(V\) as
$$ m=\rho V,$$
where volume is that of the rectangular column, meaning it's equal to the surface area \(A\) multiplied by the column's height \(h\). Now we simply insert all of these values into the pressure equation,
$$ P=\frac{mg}{A}=\frac{\rho gV}{A}= \frac{\rho g \bcancel{A}h}{\bcancel{A}}, $$
to obtain the expression for hydrostatic pressure:
$$ P_\text{fluid}=\rho h g. $$
Hydrostatic pressure is the pressure exerted by a static fluid as a result of the force of gravity acting on it.
This means that a fluid that isn't moving will exert a pressure that only depends on the density of the fluid and its depth.
We can use the hydrostatic pressure equation to complete the absolute pressure formula mentioned earlier to obtain
$$P=P_0+\rho gh. $$
This equation simply tells us that the total pressure exerted on the bottom of a column of fluid that is at sea level is the hydrostatic pressure (which is the gauge pressure) plus the atmospheric pressure. This is logical because both fluid (hydrostatic pressure) and air (atmospheric pressure) exert a force on the bottom of the column of fluid.
Let's look at an example problem applying the absolute and gauge pressure formula!
A cube-shaped container is filled to the top with \(500\,\mathrm{L}\) of water. What are the gauge and absolute pressure exerted by the water on the bottom of this container?
Solution
The equation used to calculate the absolute pressure is
$$P=P_0+\rho gh.$$
In SI units, the volume of the water
$$ 500 \,\mathrm{L}= 500 \,\bcancel{\mathrm{L}} \times \frac{1 \mathrm{m^3}}{1000\,\bcancel{\mathrm{L}}}=0.500\, \mathrm{m^3}$$
represents the volume \(V_\text{cube}\) of the cubed container with a side length of \(a\). We can use this value to find the depth of the fluid \(h\):
\begin{align} V_\text{cube}&= a^3, \\ a&=\sqrt[3]{V_\text{cube}},\\ a&=\sqrt[3]{0.500 \, \mathrm{m^3}}, \\ a&=h=0.794 \, \mathrm{m}. \end{align}
We know that the acceleration due to gravity is \(g=9.8\,\frac{\mathrm{m}}{\mathrm{s^2}}\) and that the water density is \(\rho=1000\,\frac{\mathrm{kg}}{\mathrm{m^3}}\), so we can calculate the gauge pressure:
\begin{align}P_\mathrm{G}&=\rho gh\\&=\left(1000\,\frac{\mathrm{kg}}{\mathrm{m^3}}\right)\left(9.8\,\frac{\mathrm{m}}{\mathrm{s^2}}\right)(0.794 \, \mathrm{m})\\ &=7.7\times 10^3 \, \frac{\mathrm{kg}}{\mathrm{m} \, \mathrm{s}^2} \\ &=7.7\times10^3\, \mathrm{Pa}. \end{align}
Now, we can find the absolute pressure \(P\) by simply adding together the gauge pressure \(P_\text{G}\) we just calculated and the atmospheric pressure (\(P_0=1.013\times 10^5\,\mathrm{Pa}\)) to obtain
$$P=P_0+P_\mathrm{G}= (7.74\times10^3\, \mathrm{Pa}) + (1.013\times 10^5 \,\mathrm{Pa}) = 1.090\times10^5 \, \mathrm{Pa}.$$
The three types of pressure are gauge pressure, absolute pressure, and differential pressure.
Blood pressure is gauge pressure.
The formula for gauge pressure is Pgauge=ρgh=P-P0.
Gauge pressure is the pressure of a fluid relative to the atmospheric pressure, and absolute pressure is the pressure of a fluid relative to the zero pressure experienced in a vacuum.
The difference between gauge pressure and absolute pressure is that atmospheric pressure is set as the reference point in gauge pressure, while the vacuum pressure is the reference point in absolute pressure.
What is the mathematical expression for absolute pressure \(P\) if the gauge pressure is hydrostatic pressure? Here, \(P_0\) is atmospheric pressure, \(\rho\) is the mass density of the fluid, \(g\) is the gravitational acceleration, and \(h\) is the height of the fluid column.
\(P=P_0+\rho gh\).
Blood pressure is an absolute pressure.
False.
Two tanks of water are filled to the same height. The cross-sectional area of tank \(A\) is two times larger than the area of tank \(B\). What is the relation between the pressure on the bottom of these two tanks?
The water pressure will be the same in both tanks.
Which of the following is not an example of a pressure measurement?
\(1 \, \text{atm}\).
What is the difference between gauge pressure and absolute pressure?
The reference point for gauge pressure is the atmospheric pressure, while for the absolute pressure it's zero.
What happens to the pressure exerted by a container filled with water, if the liquid is replaced with oil at the same height?
It decreases.
Already have an account? Log in
Open in AppThe first learning app that truly has everything you need to ace your exams in one place
Sign up to highlight and take notes. It’s 100% free.
Save explanations to your personalised space and access them anytime, anywhere!
Sign up with Email Sign up with AppleBy signing up, you agree to the Terms and Conditions and the Privacy Policy of StudySmarter.
Already have an account? Log in
Already have an account? Log in
The first learning app that truly has everything you need to ace your exams in one place
Already have an account? Log in