The material to be examined should be a pulp. Mix a small quantity with water until the solids of the diluted pulp are between 8.37% and 9.37%. This corresponds to an Abbe refractometer reading at 20°C of 1.3460.
Spread a small drop of the well-mixed sample with the end of a glass rod over the counting chamber.
Place the cover glass onto the counting chamber and carefully press down the shoulder of the chamber until Newton's rings are visible.
Prepared samples containing air bubbles beneath the cover glass or an over-full moat should be discarded.
If using a compound microscope, examine using the X10 eyepiece and the X10 objective.
Systematically examine all 25 fields and note those with a presence or absence of mould filaments (hyphae).
The field is regarded as positive if the aggregate length of not more than three filaments present exceed one sixth of the diameter of the field.
This is a general description of how a Howard Cell is used. The results are interpreted as a percentage of positive fields observed in all the fields examined.