# B Are all fluids incompresible?

1. Jul 5, 2017

### thetexan

Are all fluids incompresible? If not, what is the attribute that allows any compressible fluid to do so while others, such as hydraulic fluid to not?

tex

2. Jul 5, 2017

### Staff: Mentor

No. Air is compressible.

3. Jul 5, 2017

### Staff: Mentor

The word fluid includes liquids and gases. All fluids have some level of compressibility. The term incompressibity is an ideal limit of small, but finite, compressibility, and is often an excellent approximation.

4. Jul 5, 2017

### thetexan

So I should have used the work liquids. So, all liquids have "some" compressibility? Even hydraulic oil?

tex

5. Jul 5, 2017

Sure

6. Jul 5, 2017

### Bandersnatch

Look up tables showing the property called 'bulk modulus' for various liquids. It's the inverse of compressibility. While in mechanical applications higher bulk modulus means you're losing less power on compressing the liquid, it's not always worth the effort of going for exotic substances (I don't think anybody is using mercury in their hydraulics, despite its superior bulk modulus).
You'll find out that most hydraulic oils are not much different than water in terms of how compressible they are. But they do tend to have other desirable properties, such as higher boiling point, anti-corrosive or lubricative properties, etc.

7. Jul 6, 2017

### DrDu

This is also a question of what you are interested in. When you are talking about calculating the aerodynamics of a boat, the assumption of water to be incompressible is certainly excellent. If you are doing chemical reactions in an autoclave at 100 kbar, it might turn out to be rather poor.

8. Jul 6, 2017

### Staff: Mentor

Can you please give a specific reaction example where the fluid dynamics are affected in a liquid phase system?

9. Jul 6, 2017

### DrDu

10. Jul 6, 2017

### Staff: Mentor

11. Jul 6, 2017

### DrDu

May be, I just wanted to indicate that the notion of whether a substance is compressible or not may be different between a naval engineer and a guy simulating nuclear bombs.

12. Jul 6, 2017

### Nidum

The compressibility of some common fluids like water and hydraulic fluid can vary quite a lot in practical applications .

Air can get mixed in with these fluids very easily and the effect of the air is to make the fluids more compressible .

This effect can cause some serious problems in hydraulic systems and in work like boiler pressure testing .