Question: At what sort of pressure does an isobaric process occur?
About the article "Thermodynamics", section "Commonly considered thermodynamic processes".

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An isochoric process is described by the equation formula_22.
It would be convenient to have a similar equation for isobaric processes.
Substituting the second equation into the first yields formula_23 The quantity "U + p V" is a state function so that it can be given a name.
It is called enthalpy, and is denoted as "H".
Therefore an isobaric process can be more succinctly described as formula_24.
Enthalpy and isobaric specific heat capacity are very useful mathematical constructs, since when analyzing a process in an open system, the situation of zero work occurs when the fluid flows at constant pressure.
In an open system, enthalpy is the quantity which is useful to use to keep track of energy content of the fluid. 
In the cycle, an ideal gas undergoes 1-2: Constant volume (isochoric) heat addition; 2-3: Isentropic expansion; 3-1: Constant pressure (isobaric) heat rejection—compression to the volume at the start of the cycle.
The expansion process is isentropic and hence involves no heat interaction.
Energy is absorbed as heat during the isochoric heating and rejected as work during the isentropic expansion.
Waste heat is rejected during the isobaric cooling which consumes some work. 
An isochoric thermodynamic process is characterized by constant volume, i.e., formula_1.
The process does no pressure-volume work, since such work is defined by formula_2, where "P" is pressure.
The sign convention is such that positive work is performed by the system on the environment.
For a reversible process, the first law of thermodynamics gives the change in the system's internal energy:formula_3Replacing work with a change in volume givesformula_4Since the process is isochoric, formula_5, the previous equation now givesformula_6Using the definition of specific heat capacity at constant volume,formula_7,formula_8Integrating both sides yieldsformula_9Where formula_10 is the specific heat capacity at constant volume, formula_11 is initial temperature and formula_12 is final temperature.
We conclude with:formula_13On a pressure volume diagram, an isochoric process appears as a straight vertical line.
Its thermodynamic conjugate, an isobaric process would appear as a straight horizontal line. 
The following is a list of the four processes that occur between the four stages of the ideal Ericsson cycle: Process 1 -> 2: Isothermal compression.
The compression space is assumed to be intercooled, so the gas undergoes isothermal compression.
The compressed air flows into a storage tank at constant pressure.
In the ideal cycle, there is no heat transfer across the tank walls.
Process 2 -> 3: Isobaric heat addition.
From the tank, the compressed air flows through the regenerator and picks up heat at a high constant-pressure on the way to the heated power-cylinder.
Process 3 -> 4: Isothermal expansion.
The power-cylinder expansion-space is heated externally, and the gas undergoes isothermal expansion.
Process 4 -> 1: Isobaric heat removal.
Before the air is released as exhaust, it is passed back through the regenerator, thus cooling the gas at a low constant pressure, and heating the regenerator for the next cycle.  
Isopycnic means "of the same density."
In particular, an isopycnic surface is a surface of constant density.
This term is a bit more obscure than the similar terms isobaric or isothermal surfaces, which describe surfaces of constant pressure and constant temperature respectively.
It is common in conversational use to hear isopycnic surfaces referred to simply as "iso-density" surfaces, which while strictly incorrect, is nonetheless abundantly more clear.
The term "isopycnic" is commonly encountered in the fluid dynamics of compressible fluids, such as in meteorology and geophysical fluid dynamics, astrophysics, or the fluid dynamics of explosions or high Mach number flows.
It may also be applied to other situations where a continuous medium has smoothly-varying density, such as in the case of an inhomogeneous colloidal suspension.
In general isopycnic surfaces will occur in fluids in hydrostatic equilibrium coinciding with equipotential surfaces formed by gravity.
Isopycnic surfaces occur in geology, especially in connection with cratons which are very old geologic formations at the core of the continents, little affected by tectonic events.
These formations are often known as shields or platforms.
These formations are, relative to other lithospheric formations, cooler and less dense but much more isopycnic.
Compare also with isochoric.
Note that one may discuss isobaric surfaces or isobaric processes; likewise one may discuss isothermal surfaces or isothermal processes.
However, the use of "isopycnic" is typically reserved for surfaces and not processes.
In particular, unless there is a flux of mass into or out of a control volume, a process which occurs at a constant density also occurs at a constant volume and is called an isochoric process and not an isopycnic process.
The term "isopycnic" is also encountered in biophysical chemistry, usually in reference to a process of separating particles, subcellular organelles, or other substances on the basis of their density.
Isopycnic centrifugation refers to a method wherein a density gradient is either pre-formed or forms during high speed centrifugation.
After this gradient is formed particles move within the gradient to the position having a density matching their own (this is in fact an incorrect description of the exact physical process but does describe the result in a meaningful way).
This technique is extremely powerful.