Density and Viscosity

Question 1

Define a gas?

Answer 1

A gas is a state of matter in which particles move freely without a defined shape or volume. Allowing them to fill any container.

Question 2

What is Avogadro’s law?

Answer 2

At a given temperature and pressure there are the same number of molecules in any given volume of gas.

Question 3

What is the formula for density?

Answer 3

p=m/v

p = density in rho
m = mass in Kg
v = volume in m3

Question 4

What is the relationship between pressure and density?

Answer 4

They are directly proportional.
As pressure increases density increases.

(Think about halving the volume of a fixed number molecules of gas, the pressure will increase as will the density)

Question 5

What is the relationship between temperature and density?

Answer 5

They are inversely proportional.
As you increase temperature density decreases.

(As you increase temperature a gas has more kinetic energy and therefore moves more and spreads out decreasing the density.)

Question 6

What is the specific gravity?

Answer 6

Specific gravity is the density of the gas divided by that of air (1.2 kg/m3).

If a gases specific gravity is greater than 1 then it is more dense than air.

If its specific gravity is less than 1 then it is less dense than air.

Question 7

Define viscosity?

Answer 7

Viscosity is the tendency of fluid to resist flow. It can also be thought of as fluid friction.

Question 8

What is a Newtonian fluid?

Answer 8

A Newtonian fluid is one where the viscosity of the fluid is unaffected by the shear or tangential stresses inflicted upon it. The viscosity is dependent solely on temperature and pressure.

Question 9

What is the relationship of temperature and pressure on viscosity?

Answer 9

As you increase temperature viscosity is reduced. (Inversely proportional)

As you increase pressure the viscosity increases.
(Directly proportional)

Question 10

What is Poiseuille’s equation?

Answer 10

Formula describing laminar flow.

Laminar Flow = ΔPr4π / 8lη

Question 11

What factors affect turbulent flow?

Answer 11

Turbulent flow is directly proportional to r2 and the square root of ΔP.

It is inversely proportional to length of tube and density of the fluid.

Question 12

What is a wright’s respirometer, how can it be used to measure flow?

Answer 12

A wright’s respirometer is a device which measures volume of gas.

E.g you could take a breath through the respirometer and it would give you a volume of that breath.

If you timed that breath then you could calculate flow as flow is equal to volume over time.

Question 13

What is the difference between a tube and an orifice?

Answer 13

In a tube the length is greater than the diameter, the flow through a tube is laminar.

An orifice is a tube in which the diameter is greater than the length, the flow through an orifice is turbulent.

Question 14

What is Reynaulds number?

Answer 14

Reynauld’s number is used to predict whether flow will be lamina or turbulent. A Reynauld’s number or greater than 2000 predicts that flow is more likely to be turbulent.

Question 15

What is the formula for Reynauld’s number?

Answer 15

Reynauld’s number = vρd/η

v=linear velocity
ρ= density of the fluid
d= diameter of the tube
η= viscosity of the fluid

Memory tip:
DVD/V (denisty x velocity x diameter/viscosity)

Question 16

Describe how an oxygen flowmeter (rotameter) works?

Answer 16

A flowmeter consists of:
Flow control needle valve.
Tapered flowmeter tubing.
Bobbin (little metal ball)

Flow control needle valve: as the knob is turned it is connected to a needle which moves backwards allowing gas to flow into the flowmeter tubing. The more the knob is turned the more space is opened up between the needle and gas supply allowing increased flow.

The bobbin rises upwards with increased gas flow and gravity exerts a downward force. When the forces are balanced the bobbin stops. As the bobbin floats there is a constant pressure difference across the bobbin.

The bobbin has slits cut across it this allows it to rotate as gas flows past it, this prevents the bobbin from sticking to the tube wall due to dirt/static.

At the base of the flowmeter tubing the bobbin will be close to the walls of the flowmeter, therefore the length around the bobbin will be greater than the diameter, this therefore favours laminar flow.

At the top of the flowmeter it is wider and therefore the bobbin will be further away from the walls of the flowmeter, the diameter here will be greater than the length and therefore this will favour turbulent flow.

Question 17

Describe how a pneumotachograph works?

Answer 17

Flow of air comes through the ‘in’ tubing there is then small diameter tubes connecting to the ‘out’ tubing.

The small diameter tubing creates laminar flow.

The is a pressure transducer connecting to the in and out tubing. The pressure difference can be used to derive the flow as per Poiseullie-Hagen’s equation.

Calibration of the pressure difference is necessary by using a known flow rate, and must be repeated when a different gas is used (as the change in viscosity and density of different gases will affect the results).

Question 18

How does a peak flow meter work?

Answer 18

Peak expiratory flow meters (PEFM) are constant pressure variable orifice devices.

PEFM consists of a tube with a piston which must be blown down, uncovering a linear slot or orifice

Question 19

What is the boiling point of O2?

Answer 19

-183degrees celsius