# 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: X1=5.67 µm, Y1=10.56 µm and Z1=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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