Instructables
Centrifuges are common, useful tools in the modern laboratory, especially in the biology lab.

I happened to acquire a small version (a microcentrifuge, or microfuge), and this is how I use it.

Caveat: I'm not saying this is the correct way to use a microfuge, since I have never had any actual training in same, there does not seem to be much in the way of advice online, and manufacturers I have contacted have decided not to respond to my requests for advice (after all, who is going to be in the position of acquiring a piece of delicate laboratory equipment without acquiring the appropriate skills? Apart from me, I mean.)
 
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Step 1: What is a centrifuge?

Centrifuges are, at heart, simple devices - samples are put in and spun. Fast.

Through centripetal forces, this subjects the sample to artificially-high "gravity" (actually acceleration, but it amounts to the same thing), often thousands of times the gravity acting on you as you read these words.

It is exactly the same effect you exploit when you spin a bucket of water over your head - if the water wasn't subjected to slightly over one gravity of acceleration ("1g"), then it would pour out of the bucket and all over you.



Sinking and Floating

As you already think you know, heavy things sink and light things float. To be more accurate, denser substances tend to sink in less dense substances. If the difference is great enough, and the particles large enough, the sinking happens at visible speeds, say stones in water.

However, if the particles are very small, or the difference in densities is very slight, or the liquid very viscous, then the sinking can be infinitesimally slow, or even non-existent. The perpetual, random motion of the particles of liquid can constantly re-mix the solid into the liquid, or the liquid's viscosity can simply trap particles and hold them in suspension.

Increase the gravitational forces, though, and even tiny differences in density can be exploited to separate mixtures into layers. That is what a centrifuge does - it uses centripetal forces to increase the apparent gravity acting on the sample, which makes things sink or float more quickly.



Centripetal versus Centrifugal.

We often talk about centrifugal force that pushes things outwards as they spin. This is an intuitive concept (after all, we can feel the force pushing us sideways when we corner in a car), but it is wrong.

But let us remind ourselves of Newton's First Law of Motion:

Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.

What? Your Latin's a bit rusty? OK:

Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.

That is - nothing changes speed or direction unless there's a force acting on it.

So, for our high-speed sample to curve away from it's straight line into the circle of the centrifuge, there has to be a push from the outside or a pull from the middle. Since there is nothing outside the circle, it must be the pull.



The forces involved

There is a simple calculation for the g-forces generated by a centrifuge:

RCF = 0.0001118rN2

RCF = Relative Centripetal Force (the "g" forces exerted)
r = the radius of the centrifuge in centimetres
N = the rotational speed (revolutions per minute)
Hey i know this hasnt had a comment in a while so i just wanted to say Thanks helped me alot i actually have a instructablein the planning stages of making a microcentrifuge
Kiteman (author)  stephenniall5 years ago
Cool, let me know when it's done.
dsman1952766 years ago
not bad, i still have to go back and read it all but it looks like a good well done instructable. i wonder, maybe there is a way to make a centrifuge. most likely not with knex because the motors would just be to slow, even with gear ratios. but maybe with something like a drill. maybe.
PKM dsman1952766 years ago
A friend in primary school built a LEGO gear train that geared up something like 10:1 or 20:0.. then chucked the end into an electric drill and span it up. Another, slightly less sensible friend decided to touch the end gear (rotating at 20 times electric drill speed >_<) with the tip of one finger. The lego broke apart, the cogs flew everywhere and the not-sensible friend had a small cauterised burn/cut in the end of his finger. A cautionary tale against using power tools and plastic construction sets together.
dsman195276 PKM6 years ago
lol. man that must have been spinning fast. thats one advantage of K'Nex, they connect differently so they don't just fly apart. the one thing i would be worried about if i made one of these would be that by using gear ratios it would have not torque.
it make me sick that people use legos for useful things at all. legos are TOYS, not tools. and yes, I have a LOAD of legos in my shed.
Kiteman (author)  dsman1952766 years ago
I believe Killerjackalope has a centrifuge ible in the planning stages, and one or two others have made noises along those lines. I'm looking forward to their work.
ok. centrifuges seem really cool to me, but i don't have the money to buy one.
static6 years ago
I know where there still may be a hand cranked centrifuge, in an oil field pump house. I need to remember and ask the owner of the land where it should be I can go ahead and swipe it.
balisticjoe6 years ago
Where can I get a pack of those tubes.
Kiteman (author)  balisticjoe6 years ago
ebay is a good bet.
Gavabc1236 years ago
i want one
M F6 years ago
Has anyone made an centrifuge Instructable? I can get motors pretty easily
Kiteman (author)  M F6 years ago
Not yet. I believe a couple of members have it on their list of planned projects, but nobody has published yet.

Even if they do, I'd invoke Kiteman's Law anyway (search for notes on the zeroth law).
tiuk6 years ago
Just out of curiosity, what does something like that centrifuge cost?
Kiteman (author)  tiuk6 years ago
About $150, plus various taxes, shipping etc from this link.
tiuk Kiteman6 years ago
Neat, thanks.
awoodcarver6 years ago
well done we used them at work to test lead and drug levels as well as other things ours were a bit bigger and when they closed the place they threw 2 away...I took the suction machines and a pump never saw a use for a Centrifuge...sighs and slaps myself in the head ...had a plethora of test tubes in different sizes and pipettes till a friend decided he would open a head shop and made pipes out of them ...I think I could make one with a few spare parts the balance would be the problem
LinuxH4x0r6 years ago
Nice job! You should do another session of tests
Patrik6 years ago
"What? You're Latin's a bit rusty?"

You're English seems a little rusty as well! ;-)

Sorry - I couldn't help myself... :-D
Nice job! Larger centrifuges that are spun at super high speeds for hours on end have platinum (i think) innards, and are the size of dishwashers. If they are even slightly unbalanced (that's why upipetter skills are needed), they can vibrate with enough force to break through walls. If a centrifuge ever does begin to move, the safest way to stop it is to unplug the entire machine.
Oh..the next fun tool you should get is a vortex mixer =]