How is calibration of the applanation tonometer checked?

The calibration of an applanation tonometer is checked by using a measured weight attached to the tonometer balancing arm at specific points—usually at 0, 2, and 6 grams. This method is essential because it allows for an accurate assessment of the tonometer's performance by ensuring that it applies the proper amount of force needed to flatten the cornea at those calibrated weights. The process of applying known weights simulates the intraocular pressure (IOP) measurements that the tonometer will eventually take in real-world assessments.

Using measured weights provides a clear and objective method for determining whether the tonometer is functioning accurately, as it directly correlates to the principle of applanation tonometry, which is based on the force required to flatten a defined area of the cornea. If the tonometer readings are inconsistent with the expected results from the applied weights, then the device may need recalibration or repair.

In contrast, other methods of checking calibration, such as comparing to a standard gauge, performing visual inspections, or conducting repeat measurements, do not provide the same level of accuracy or reliability as the weight method. These alternative methods may not effectively identify small discrepancies in the device's measurements or could be subjective and prone to operator error.