Report centrifugal force in g units NOT rpm! (Rant)

Curt Ashendel ashendel at aclcb.purdue.edu
Thu Feb 15 22:32:45 EST 1996


On 15 Feb 1996 16:26:18 GMT, 
Robert Roxby  <rr at bmmb01.umesci.maine.edu > wrote:

>So many centrifugation protocols give only rpm and rotor type, even in 
>technical compendia devoted entirely to methods, such as Current 
>Protocols in Molecular Biology.  Rotor and rpm is ambiguous unless you 
>have the same centrifuge equipment as used in the original protocol, or 
>have data for that equipment.  I suggest exercising some rigor in 
>reporting centrifugation conditions in units of g.  It can be 
>easily calculated [formula deleted]

I beg to differ with you. Specifying a rotor and rpm is absolutely 
unambiguous, since it completely specifies (indirectly) the tube 
position relative to the axis of rotation. In contrast, specifying 
the force in g units requires arbitrarily picking a position: the 
top, middle, or bottom of the tube, and the position chosen is not 
usually published. Hence the method you recommend is MORE abiguous, 
not less ambiguous. BTW, the impact on the force generated of the 
actual centrifuge used (not the rotor) is identical (i.e., not 
known/specified) for either method of specification as it only affects 
the rate of accelleration and decelleration, not the force generated 
at the set speed. Of course, this does affect the integral number of 
g x minutes, which is the figure of merit. But without an integrator 
on the equipment, it is not known, and cannot be specified, except 
indirectly by the make and model of centrifuge used. At any rate, for 
most long spins on most makes of centrifuges (which have good 
accelleration), the integral force-time in not substantially different 
for an identical spin (set speed and time) on diferent makes of 
centrifuges.


Curt Ashendel
Purdue University, West Lafayette, IN
ashendel at aclcb.purdue.edu



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