Several improvements have been made in the constant pressure tonography device. Minimum corneal applanation for uniform pressure sensing has been established at 24.6 sg mm or 2.6 mm radius. In the average eye the minimal corneal dome flattened has a volume of 6.5μl. Displacement of 6.5μl raises intraocular pressure so that initial tonography must be carried out, in general, at least 8 mm Hg above Po.
Tonography with areas of applanation greater than 4.2 mm radius tend to show progressive decrease in facility of outflow. It is not known whether the reduction in outflow facility is the result of the large area of applanation or of the heavy tonometer weight (more than 20 gm) necessary to produce the large applanation area. The volume change available to the tonographic process at constant pressure is 8μl or more.
MOSES RA. Constant Pressure Applanation Tonography With the Mackay-Marg TonometerII. Limits of the Instrument. Arch Ophthalmol. 1967;77(1):45-49. doi:10.1001/archopht.1967.00980020047010