Difference Between Gauge Pressure and Absolute Pressure

Pressure is the force per unit area applied toward a course perpendicular to the outer layer of an item. Numerically, it is represented with a ‘P’. To keep it precise, it is an amount of force following up on a unit area. The simple equation for pressure is P = F/A where P is Pressure, F is Force, and A is Area. The SI unit for pressure is measured in Pascals (Pa). Other non-SI units are bar and PSI. There are two types of references to quantify pressure,

• Gauge Pressure
• Absolute Pressure

Gauge Pressure

The most well-known pressure reference is Gauge Pressure which is connoted by a ‘g’ after the pressure unit, for example, 33 psi g. It is the pressure relative to barometric or atmospheric pressure; it is positive for pressures above atmospheric pressure and negative for pressures that are below atmospheric pressure. This shows that the pressure estimated is the ‘Total pressure (minus) Atmospheric pressure. Atmospheric pressure is not taken into account in this pressure reference. As such, gauge pressure utilizes atmospheric pressure (14.7 PSI), as its zero point. At times, it is alluded to as ‘Overpressure’. 

Gauge Reference Pressure

Machines that use gauge pressure are Well pumps, Air compressors, and Tire gauges. There are two kinds of Gauge reference pressure: 

1. Vented Gauge (vg) 
2. Sealed Gauge (sg)

Converting 33psi g to an absolute pressure

Example: If the gauge pressure is 33psi what would be the corresponding absolute pressure?

Answer:

A gauge pressure can be changed over into an absolute pressure by including it in the ongoing perusing of atmospheric pressure. For instance on the off chance that the atmospheric pressure is 14.7psi absolute, then the 33 psi gauge would be converted to 47.7 psi absolute.

Absolute Pressure

An Absolute pressure estimation is one that is alluded to as a perfect or an ideal vacuum. The best illustration of an absolute referenced pressure is the calculation of Barometric pressure. To deliver an absolute pressure sensor, one strategy is for a maker to seal a high vacuum behind the detecting diaphragm. Consequently assuming you vent the interaction pressure association of an absolute pressure transmitter to ambient air pressure, it will peruse the nearby barometric pressure.

Absolute Pressure

Difference between Gauge Pressure and Absolute Pressure

	Gauge Pressure	Absolute Pressure
1.	Gauge pressure is defined as the pressure relative to barometric or atmospheric pressure.	Absolute pressure is defined as the pressure of having no matter inside a perfect vacuum of space.
2.	It doesn’t take atmospheric pressure into account. It is equal to Absolute pressure minus atmospheric or ambient pressure.	It is the addition of atmospheric pressure and gauge pressure. It is used for the calculation of atmospheric pressure
3.	It uses atmospheric pressure (14.7 PSI) as its Zero point.	Calculations that took in absolute pressure and use this absolute zero as their reference point.
4.	It cannot be used in gas law calculations as it doesn’t reflect accurate pressure but rather a difference in pressure.	It gives accurate barometric or atmospheric pressure when sealed in a high vacuum behind the detecting diaphragm.
5.	It is greatly influenced by changes in altitude or weather conditions in atmospheric pressure.	It doesn’t get influenced by any external changes in atmospheric pressure.
6.	Formula of Gauge pressure is given by, pg = ps – patm ,   (Where pg is gauge pressure, ps is system pressure and Patm is atmospheric pressure)	Formula of Absolute pressure is given by: pa = pg + patm  (Where pa is absolute pressure, pg is gauge pressure and Patm is atmospheric pressure)
7.	It is used in machines like Well pumps, Air compressors, and Tire gauges.	It is used for leak testing in a totally sealed container, smartwatches, and navigation systems for altimeter measurements.
8.	A manometer is a device used to measure the gauge pressure.	Mercury Barometer is the device used to measure atmospheric pressure.

Sample Questions

Question 1: Explain sealed gauge and vented gauge?

Answer: 

• Vented Gauge pressure: A vented gauge pressure transmitter lets in the outside air strain to be exposed to the negative side of the pressure sensing diaphragm in order that it continually measures with regards to the ambient atmospheric pressure. Consequently when the pressure process connection is held open to atmospheric air a vented gauge pressure sensor reads zero pressure
• Sealed Gauge Pressure: A sealed gauge reference is practically the same with the exception of that barometric pressure is sealed on the negative side of the diaphragm. This is normally taken on high pressure applications, for example, estimating water driven pressures where atmospheric pressure changes will affect the precision of the sensor. It is the pressure estimated through a sealed gadget in which the zero point is set. This set point is anything that the pressure within the gadget was prior to sealing, which the maker of the sealed pressure gauge chooses.

Question 2: How do you have any idea when to quantify absolute pressure or when to quantify gauge pressure?

Answer:

This isn’t generally clear however for the most part if you have any desire to quantify or control a pressure that is impacted by changes in barometric pressure, similar to the degree of fluid in an open tank for instance; you would pick Vented gauge pressure as you are keen on the pressure perusing minus the barometric pressure component.

If you have any desire to quantify pressures that are not impacted by changes in barometrical pressure, for example spill testing a totally sealed non-adaptable container, you would utilize an Absolute pressure sensor. Assuming a gauge pressure sensor was utilized rather to quantify the container pressure, and the atmospheric pressure changed, then the sensor’s reading would change, regardless of the way that the pressure in the container continues as before.

Question 3: If the atmospheric pressure is 14 psi and a pressure gauge measures the reading as 39 psi. Calculate the absolute pressure that corresponds to this gauge pressure reading.

Answer:

From above parameters:

Atmospheric pressure P_atm = 14 psi

Gauge pressure, P_g = 39 psi

According to the formula,

Absolute pressure P_abs = P_atm + P_g

P_abs = 14 psi+39 psi

P_abs = 53 psi

Hence Absolute pressure is 53 psi.

Question 4: Calculate the corresponding gauge pressure if an absolute pressure instrument reading is given as 38.8 psi and at atmospheric pressure of 15 psi.

Answer:

From the given readings,

Atmospheric pressure P_atm = 15 psi

Absolute pressure P_abs = 38.8 psi

From the known formula

P_g= p_abs – p_atm

Now the Gauge pressure calculated as

P_gauge = 38.8 psi – 15 psi  = 23.8 psi.

Hence Gauge pressure calculated is 23.8 psi.

Question 5: At a depth of 11 m from the free surface what is the absolute pressure of water if P_atm is 102.213 kN.m²?

Answer:

As we know from Hydrostatic law, at any point in a static liquid, the local specific weight of liquid must be equal to the vertical rate of increase of pressure.

Therefore, P_g = p ×g × h

Where, P_g is gauge pressure, p is density of water, g is acceleration due to gravity (9.8 m/s²), h is the height below the static liquid surface

Absolute pressure P_abs = P_g + P_atm

From given data,

h= 11m,  P_atm = 102.213 kN/m²  ,  p= 1000 kg/m³

P_g = p g h = 1000 × 9.8 ×11 = 107800 N/m²= 107.8kN/m²

Applying the equation

P_abs = 102.213 + 107.8 = 210.013 kN/m²

Hence the absolute pressure calculated is 210.013 kN/m² 

Question 6:  What will be the gauge pressure at a depth of 990 m in the ocean when the given density of seawater is 1.02 x 10³ kg/m³? Consider gravitational acceleration g is 9.8m/s².

Answer:

From the given data we know that,

h = 990 m,  p = 1020 kg/m³, g = 9.8m/s²

At a depth h, Gauge pressure P_g = p g h / 10⁵ atm

Where, p is the density of water, g is acceleration due to gravity, h is the depth of the water

Applying the values,

P_g = 1020 ×9.8 × 990 / 10⁵

Therefore P_g = 98.96 atm