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Tonometer Calibration Tips

Testing Your Device with General Tonometer Calibration Tips

Created more than 50 years ago, Goldmann Applanation Tonometry is still the golden standard for measuring intraocular pressure (IOP) today, which is the only known modifiable risk factor for glaucoma. In addition to being a critical measure for glaucoma management, IOP forms one of the most important factors of a routine evaluation.

In either case, most tonometers are calibrated to measure pressure in millimeters of mercury (mmHG.) The disinfected prism is mounted atop the tonometer head and placed against the patient's cornea. The force applied to the head of the tonometer is then adjusted via a dial that is connected to a range of different tension springs — until the pressure in the eye can be measured from the force applied.

One of the most common misconceptions around goldmann applanation is the "set it and forget it" practice — where some believe the instrument requires no calibration checks or routine maintenance. Practitioners typically only consider tonometer calibration or maintenance when one or two things have gone wrong, such as:

  • The tonometer calibration falls outside of the OEM specifications
  • When the tonometer has been dropped and no longer properly works

In busy clinics, however, these devices have been known to quickly lose accuracy within a few months. When this happens, tonometers are much more likely to vary in the positive range, which results in higher IOP readings. As a result, it's important to check your tonometer on a monthly basis for calibration errors. The following information explains a simple tonometer calibration process so you can ensure the accuracy of your readings.

Tonometer Calibration Procedure

The tonometer calibration procedure is suggested for Goldmann devices. Calibration is completed at dial positions of 0,2, and 6, which is equal to 0 mmHG, 20 mmHG, and 60 mmHG. The following tonometer calibration procedure is designed to verify the accuracy of your instrument. If you determine your tonometer is inaccurate at any of the positions, contact Keeler Ophthalmic Instruments for calibration.  

It's important to note that before you start, you should insert the prism into the prism holder on the head of the tonometer and place the tonometer device onto the slit lamp.

Tonometer Calibration Tips

Tonometer Calibration Procedure at Dial Position 0

When you adjust the tonometer dial to 0, the feeler arm should be able to move freely.

Tonometer Calibration Tips
  1. If your dial is turned backwards a tiny distance to about -.05, the arm will fall toward you — the examiner.
  2. If you turn the dial a tiny bit forward to a position equal to +.05, the arm should move toward the patient you're examining.
  3. If you notice the arm doesn't respond in either of the previously mentioned ways, your tonometer calibration is off at dial position 0.

Tonometer Calibration at Dial Position 2

This is the most important check procedure, since measurement of intraocular pressure in this area is highly significant. We do recommend you check this calibration setting daily. This check is made by using the calibration arm, which is engraved with 5 circles.

  1. The center circle coordinates with drum position 0,
  2. The two gradients to the right and left coordinate to position 2,
  3. The outermost engravings correspond to position 6.
Tonometer Calibration Tips

The arm is only marked on both sides of 0 to maintain symmetry in order to calibrate. Move the bar in the support until one of the position 2 marks on the weight is situated at the support's reference mark. This check can be made as long as the weight of the bar is toward the examiner.

When the drum position is 1.95 and/or 2.05, the sensor arm should move from the free movement area to the corresponding stop.

Tonometer Calibration Tips

Calibration Position 1.95:

Adjust the drum 2 calibration measurement down by the width of a single calibration mark (as demonstrated in Figure 6). Once the sensor arm is in the zone of free movement, it should move against the stop in the examiner’s direction.

Calibration Position 2.05:

Rotate the measurement drum 2 calibration upward by the width of one calibration mark (see figure 7). When the sensor arm is in the free movement zone, it should move against the stop in the patient’s direction.  

Tonometer Calibration Tips

Tonometer Calibration at Dial Position 6

Fortunately, the procedure for dial position 6 is extremely similar to the previously described directions. Instead of 1.95 and 2.05, the checkpoints are 5.9 and/or 6.1. Simply adjust the "6" calibration mark on the drum through ½ interval downward and/or upward — respectively — with respect to the index mark.

Contact Keeler Ophthalmic Instruments for Cutting-Edge Tonometer Devices

For more than 100 years, Keeler Ophthalmic Instruments has been a leader in manufacturing tonometer devices. In addition to offering a full catalog of world-class equipment, we have the understanding and experience to guide you to the best solution your needs and/or practice.

Contact Keeler Ophthalmic Instruments today.

About the Author Eugene VanArsdale

Eugene is the Director of Marketing Communications at Keeler Instruments. He has been with Keeler since 1982 and is co-holder of two patents for the company. Eugene has a true passion for the eye care industry and has dedicated himself to understanding the ins and outs of the optometric and ophthalmic equipment market.