100 Nano-Stories: What’s PSI & Bar Pressure?

Episode #44: PSI & Bar Pressure Explained!

Carlos Manuel Jarquín Sánchez
4 min readMar 8, 2021


Preface! ✨

It’s your favorite material science & nanotechnology enthusiast! A couple of days ago, I explained hybridization in its entirety! Although this is not recommended to read this article, check it out if you want to understand the carbon atoms and how it can lead to alkanes (single bonds), alkenes (double bonds), and alkynes (triple bonds).

But for today’s topic, let’s discuss another aspect of Supercritical Drying of Aerogels: PSI & Bar Pressure!

Wait, you don’t know what it means? Looks like I’ll have to start from the beginning! But that's okay! It should only take about 5 minutes of explaining for you to understand what PSI & Bar Pressure means!

PSI & Bar Pressure Explained! 💡

But What Is PSI & Bar Pressure? 🔑

PSI means Pound-Force per Square Inch!

Bar Pressure is a metric unit of pressure that is used in supercritical drying in aerogels!

However, the International System of Unit for pressure in Pascals, but we will save this fella for another article!

Explaining PSI & Bar Pressure! 🔑

A pound is a unit of mass. One pound is equal to 0.453 kg. Two pounds is equal to 0.907 kg.

To find the PSI of an object, you simply divide the number of pounds of pressure applied to the material/substance by the square inches of the object.

1 bar of pressure is equivalent to approximately 14.5 PSI.

1 PSI is equivalent to 0.0689 Bar of Pressure.

If you want to calculate PSI to Bar or Bar to PSI, just multiply or divide by 14.5038 to convert between bar pressure and PSI.

How Does This Help In Aerogel Technology? 🔑

When we undergo supercritical dying or subcritical drying, we have to reach a certain threshold of pressure (and temperature) to reach the critical point of aerogel. This critical point will allow us to depressurize the gel to remove the majority of the liquid in the gel. The remainder of the supercritical fluid that is left in the gel will revert into gas since the supercritical fluid has both properties of liquid & gas.

However, the pressure must be isothermally depressurized to retain the full aerogel structure! Isothermal means an equal or constant temperature concerning space, volume, or pressure (in this case, we want to focus on the pressure).

But to reach the supercritical pressure of the liquid CO2, we need to reach 74 bar! In PSI, 74 bar is 1073.28 PSI.

If we do not reach or surpass 74 bar/1073.28 PSI, we will only have a subcritical fluid, which means we will be below a certain requirement/threshold, and that is the pressure.

But if we go too far from 74 bar/1073.28 PSI, our supercritical fluid will be too hot, and that will make our supercritical fluid behave more like a gas. If this happens, our gel will no longer be useful to extract the solvent from the pores, and our gel will just be utter crap.

So it’s important that we surpass 74 bar of pressure/1073.28 PSI to reach a supercritical fluid CO2, but not go too far towards 100 bars of pressure/1450.38 PSI.

Closing Thoughts! 💭

Perfect, we explained the bars of pressure, PSI, and its value toward identifying the pressure in supercritical drying!

How are you with that explanation, reader?

Perfect! See you in the next article about an in-depth lesson about Rayleigh Scattering and my idea for Aerogel Technology! 🤫

Vocabulary! 📓

Bar → The metric unit of pressure; equivalent to 100,000 Pascals.

Pascal → The SI Unit of Pressure, and is equal to one kilogram per meter per second squared.

PSI → Pound-Force per Square Inch!

Critical Point → The pressure-temperature curve designates conditions under which a liquid and its vapor allow for properties of both liquid and gas to coexist.

CO2 → Carbon Dioxide.

Supercritical Drying → Liquid in a gel/substance is being transformed into a gas with the absence of the surface tension and the capillary stress of the gel. It’s what allows for a silica gel to transform into an aerogel.

Subcritical Drying → The temperature is below the supercritical temperature of a substance.

Supercritical Fluid → The fluid exhibits particular properties and has a behavioral property between that of a liquid and a gas. The supercritical fluid has liquid-like density and gas-like opposition to flow out of a substance/friction between the molecules and the pores.

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