It has been noted in this laboratory1 that tonograms starting with scale readings less than 3 gave values for the intraocular pressure (Po) and outflow facility (C) which were often higher than anticipated from the clinical findings in these patients. Furthermore, Po and C values obtained on the same eye with a greater plunger load and a larger initial scale reading gave more reasonable, lower estimates of C and Po. Kronfeld independently pointed out2 that at low scale readings (high intraocular pressure) comparative measurements with the 5.5 and 7.5 gm. weights give results inconsistent with the 1955 Friedenwald tables. Clinically this problem has been resolved in most tonography laboratories and offices by using a sufficient plunger load to obtain an initial scale reading greater than 3.
These findings stimulated us to search for the source of the discrepancies, and on theoretical grounds it was decided
MOSES RA, HAHN KA. The Effect of the Tonometer Footplate Hole on Scale Reading. AMA Arch Ophthalmol. 1958;60(1):36-48. doi:10.1001/archopht.1958.00940080050008