ml/min into mm/s

Bob McLean RM12 at swt.edu
Wed Jul 22 03:51:11 EST 1998


At 11:18 AM 7/21/98 -0400, you wrote:
>Dear biofilmers
>
>could anyone provide me with an equation for converting ml/min into 
>mm/s; when using a peristalsic pump and a flow cell.
>
>All literature seems to quote 0.2mm/sec as a usual speed, but I am 
>uncertain as to how they arrive at this figure when pump speeds are in 
>rpm and flow rates are in ml/min.
>
>Surely 0.2mm/s could be highly variable depending on the tube radius?
>
>Help
>
>Sarah Boyle
>Research School of Biosciences
>University of Kent
>CT2 7NJ
>
>01227 764000 ext. 3023
>

Dear Sarah,

We typically calculate our flow rates using a direct approach - we simply
run a liquid such as water through our pump into a graduated cylinder for a
set period of time. 

To calculate a flow rate in distance per second, I would suggest using two
approaches:  1) from your calculations of volume per second (described
above), the volume of liquid in a length of tubing would be pi*r^2*length
(i.e. volume in a cylinder). If for example your flow rate is 2 ml/min and a
2 ml volume occupies 1 cm of tubing, your flow would be expressed as 1
cm/min. The assumption in this first case is that flow is uniform across the
diameter of your tubing. 2) a more accurate approach would be to pump some
small beads through your tubing and microscopically measure the speed at
which they travel. This second approach would be more accurate and would
allow for the effects of tubing surface friction to be measured.

I hope this is helpful,

Bob McLean
___________________________________________________________________________
R.J.C. (Bob) McLean, Ph.D.
Dept. Biology
Southwest Texas State University
San Marcos, Tx 78666
USA
(512)245-3365 phone
(512)245-8713 FAX
Email:  RM12 at swt.edu



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