Pressure can be described as the force applied to an area.
There are many different systems of pressure measurement, with absolute
pressure and sealed gauge pressure being two of the most common. There are many
differences between these two measurements of pressure that have significant
effects on their use and measurement. Depending on why you are measuring
pressure, determining whether you need gauge or absolute reference pressure is
as important as selecting the pressure range itself, particularly for low
pressure. If you get it wrong it could create huge errors in your measurements.
The simplest way to explain the difference between the two
is that absolute pressure uses absolute zero as its zero point, while gauge
pressure uses atmospheric pressure as its zero point. Due to varying
atmospheric pressure, gauge pressure measurement is not precise, while absolute
pressure is always definite.
Gauge
Pressure
The most common pressure reference is gauge pressure which
is signified by a ‘g’ after the pressure unit e.g. 30 psig. Gauge pressure is
measured in relation ambient atmospheric pressure. Changes of the atmospheric
pressure due to weather conditions or altitude directly influence the output of
a gauge pressure sensor. A gauge pressure higher than ambient pressure is
referred to as positive pressure. If the measured pressure is below atmospheric
pressure it is called negative or vacuum gauge pressure.
Gauge pressure sensors only usually have one pressure port.
The ambient air pressure is directed through a vent hole or a vent tube to the
back of the sensing element. A vented gauge pressure transmitter allows the
outside air pressure to be exposed to the negative side of the pressure sensing
diaphragm so that it always measures with reference to the ambient barometric
pressure. Therefore a vented gauge pressure sensor reads zero pressure when the
process pressure connection is held open to atmospheric air.
A sealed gauge reference is very similar except that atmospheric
pressure is sealed on the negative side of the diaphragm. This is usually
adopted on high pressure applications such as measuring hydraulic pressures
where atmospheric pressure changes will have only a slight effect on the
accuracy of the sensor. The definition of sealed-gauge pressure is the pressure
measured through a sealed device in which the zero point is set. This set point
is whatever the pressure inside of the device was before sealing, which the
manufacturer of the sealed pressure gauge decides.
Absolute
Pressure
The definition of absolute pressure is the pressure of
having no matter inside a space, or a perfect vacuum. Measurements taken in
absolute pressure use this absolute zero as their reference point. The best
example of an absolute referenced pressure is the measurement of barometric
pressure. In order to produce an absolute pressure sensor the manufacturer will
seal a high vacuum behind the sensing diaphragm. Therefore if you hold open the
process pressure connection of an absolute pressure transmitter to the air it
will read the actual barometric pressure.
So how do you know when to measure absolute pressure or
when to measure gauge pressure?
This is not always straightforward but generally if you
want to measure or control a pressure that is influenced by changes in
atmospheric pressure, like the level of liquid in an open tank for example; you
would choose vented gauge pressure as you are interested in the pressure
reading minus the atmospheric pressure component.
If you want to measure pressures that are not influenced
by changes in atmospheric pressure, e.g. leak testing a completely sealed
non-flexible container, you would use an absolute pressure sensor. If a gauge
pressure sensor was used instead to measure the container pressure, and the
barometric pressure changed, then the sensor’s reading would change, despite
the fact that the pressure in the container remains the same.
Absolute,
Gage, Vacuum, And Atmospheric Pressures
Absolute pressure –
The actual pressure at a given position is called the absolute pressure, and it
is measured relative to absolute vacuum (i.e., absolute zero pressure).
Gage pressure –
Gage pressure is the pressure relative to the atmospheric pressure. In other
words, how much above or below is the pressure with respect to the atmospheric
pressure.
Vacuum pressure –
Pressures below atmospheric pressure are called vacuum pressures and are
measured by vacuum gages that indicate the difference between the atmospheric
pressure and the absolute pressure.
Atmospheric
pressure –
The atmospheric pressure is the pressure that an area experiences due to the
force exerted by the atmosphere.
1 atm
= 33.91 ft H2O = 10.34 m H2O = 14.696 lb/in2 =29.921 in. Hg =760 mmHg =101.3
kPa = 1.013 bar
0
absolute pressure = 1 atm vacuum = -1 atm gauge pressure
atmospheric pressure = 0 vacuum = 0 vacuum = 0 gauge pressure
10.696 lb/in2 = 4 lb/in2 vacuum = -4 lb/in2 gauge pressure
atmospheric pressure = 0 vacuum = 0 vacuum = 0 gauge pressure
10.696 lb/in2 = 4 lb/in2 vacuum = -4 lb/in2 gauge pressure
Absolute
pressure versus gauge pressure
The
absolute pressure of any system is the gauge pressure of the system plus the
local atmospheric or ambient pressure.
In
technical writing, If the atmospheric pressure is 101 kPa, it would be written
as a gauge pressure of 520 kPa or as an absolute pressure
of 621 kPa. Where space is limited, such as on pressure gauge dials,
table headings or graph labels, the use of a modifier, such as kPa
(gauge) and kPa (absolute) or kPa-gauge and
kPa-absolute or kPa (A )or kPa (G), is strongly
encouraged. Gauge pressure is also sometimes spelled gage pressure. This
discussion of modifiers also applies to the other pressure units such as bar,
torr, atmosphere, etc.
520
kPa (G) = 621 kPa (A)
Negative
or vacuum pressures
If the atmospheric pressure is 101 kPa, point B in the
diagram might be expressed as being 45 kPa of vacuum, meaning it is 45 kPa
below atmospheric pressure. It might also be expressed as 56 kPa (absolute)
meaning that it is 56 kPa above zero pressure. In technical writing, it is very
important to make clear how any pressures below atmospheric pressure are
expressed.
45 kPa vacuum = -45 kPa gauge = 56 kPa absolute
