Determine concentration of yeast

haemocytometer

A haemocytometer or counting chamber is a fancy microscopy slide with a grid etched into it, with the depth exactly defined.

Using the dimensions of the grid multiplied by the depth gives a known volume in which cells can be counted, thus making it possible to get to a concentration.

Prepare counting chamber

• carefully clean the chamber with a kim wipe and ethanol (this is a precision instrument with fine engraving we don't want to damage)
• wetten the bars on the side with water ever so slightly
• push the thick coverslip onto the side bars to get rainbow refraction rings

Prepare cells

• make sure the sample is in a homogeneous suspension
• add a dye if necessary
• if the density is too high, consider making a dilution into saline
• take 20-50ul suspension and pipette directly next to the cover slip on the counting surface
• wait till the capillary forces draw the sample in
• you can repeat your measurement on the bottom chamber, or add a different sample if precision is not that important
• let the cells settle for a few minutes

Count

• Focus on the grid lines and try to get the cells into view
• depending on the density, 10X or 20X objectives should give a good magnification
• count cells in enough squares of the appropriate size
• when counting, ignore cells that are touching two of the sides of the square (bottom and right), but keep the ones on the other two sides (top and left)