The accidental discovery of a hundred per cent. error in the depth of a recently purchased chamber of the Helber type led to an examination of the accuracy of others and to inquiry at the Bureau of standards as to the accuracy of hemacytometers in general.
The gross error referred to was discovered by an enthusiastic student who used the Helber chamber in making a count of red blood corpuscles. In calculating the number of corpuscles from the data obtained he found twice the number of corpuscles that one of his companions found in the same blood. He repeated the work with a similar result. The conclusion was obvious that the Helber chamber was about 40 microns deep instead of the labeled 20. Probably other counting chambers are on the market or in use as carelessly labeled as is this one.
Before it was learned that the Bureau of Standards
Lyon MW. INACCURACIES IN THE SIZE OF HEMACYTOMETER CHAMBERS AND PIPETS. JAMA. 1917;LXVIII(9):709-710. doi:10.1001/jama.1917.04270030041017