The maximum relative centrifugal force (RCF) in terms of gravity (g) is calculated from:
(1.118 X 10^-5) X radius from the centre of rotation to the bottom of the tube (cm) X RPM^2.
The length of time required to give the equivalent centrifugation when using an alternative rotor is thus:
Time (alternative rotor) = Time X RCF (original rotor)/RCF (alternative rotor).
So Jim is correct as r will have been used to calculate the g in his example.
The same calculation applies whether a fixed angle or swing out head is used. The g force at the surface of the liquid, being closer to the centre of rotation, will be less than that at the bottom of the tube.
An angled head will not give as satisfactory a separation of blood as a swing out head as the cells will be forced to the side rather than pack at the bottom of the tube.
Regards,
Jeff Davies
>>> Dr Henry Chandler <[log in to unmask]> 30/08/2007 17:43:45 >>>
If my memory of A Level Physics is still OK what seems to have been forgotten is r the radius.
Centrifugal force = m x w^2 x r
Where m = mass
w = omega (as I can't find it in this character set) = angular velocity
& r = the radius
So the force varies down the sample tube and it also varies with the rotor radius, ie the size of the particular centrifuge, given the speed (angular velocity).
That is to say that life is slightly more complex than it would seem anyway.
Henry Chandler
Long retired
----- Original Message -----
From: James J Miller
To: [log in to unmask]
Sent: Thursday, August 30, 2007 12:01 PM
Subject: 3 centrifugation question
1. This may sound simplistic, but I haven't been able to find documentation. I think the relative centrifugal force (RCF) times the time of centrifugation can be used to calculate equivalent degrees of centrifugation.
For example, centrifuging at 1500 g for 10 min is equivalent to centrifuging at 5000 g for 3 min. Right?
2. Centrifuging whole blood to obtain the serum or plasma at too high of a RCF might damage cells during the process. Besides the possibility of some hemolysis leading to artifactual increases in K+, LD, etc., are there any other chemistry analytes that might be affected?
3. I always thought satisfactory centrifugation could be accomplished more quickly in fixed angle rotors than in swinging bucket rotors. However, BD stipulates a longer centrifugation time for fixed angle than for swinging bucket. (This might only apply to gel separator tubes.) Did I always think wrong (on this issue, not to mention many more).
Thanks. -Jim
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