# Formulas that everybody (well...) needs and nobody knows

### Centrifugation: Calculation of g-value from the rpm value on your display

I got this from a centrifuge manual, so it should be right.

rcf = relative centrifugational force. This is the value you want.
("Relative" probably is relative to g =acceleration due to earth's gravity.)

r = radius in cm.

n = rpm-value

rcf = 11.18 * r * n² * 10^{-6}

or

rcf = 0.00001118 * r * n²

Alternatively, you may want to measure the radius r in m (not cm) and get rid of some zeros. Then rcf = 0.001118 * r * n²

### 3D-distance between 2 points in a 3D-image stack

What you need:

- The 3D-coordinates in *microns* (or some other usefull length unit),
but *not in pixels*. (Exception: if your pixels should have the
same size in all 3 directions you can calculate in pixels. But for 3D microscopy
this is very unusual.)

- Example: point 1: X_{1}=5.67 µm, Y_{1}=10.56
µm and Z_{1}=4.5 µm

- Correction for chromatic abberation of the microscope lens must be made
before this calculation.

The distance between the 2 points then is calculated as follows

### Cold temperature landmarks

assembled from a number of Web-sites:

-78.5 °C : sublimation point of dry ice

-114 °C: freezing/melting point of Ethanol

-183 °C boiling point of Oxygen

-195,8 °C boiling point of Nitrogen

more to come, eventually...

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