To ensure an accurate measurement of intraocular pressure, a tonometer needs accurate calibration. To check if the calibration is correct, a calibration bar is used. 

Tonometer and a calibration bar.

The following pictures and steps show how the calibration is checked:

1. Attach the calibration bar to the body of the tonometer as shown below.

Attach the bar to the  tonometer.

2. The bar has five markings, the central one is used to check if the tonometer has been 
    calibrated accurately for 0 mmHg, the next markings on either side of the centre are 
    used to check for accurate calibration for 20 mmHg. The last markings nearest to the 
    end are used to check for 60 mmHg (see pictures below). 

3. To check if the tonometer has been correctly calibrated for 60 mmHg, line up the 
    marking on the bar furthest from the centre with the marking on the knob that hold 
    up the bar (as shown in the picture below; to check for 0 and 20 mmHg line up the 
    bar as described in 2 and repeat steps 3 and 4). 
 

4. Now move the knob on the tonometer and note the pressure at which the tonometer 
    tip tilt forward. If the tilt occurs when the pressure is at 60 the tonometer is calibrated 
    correctly. If the tilt occurs below or above 60, the tonometer should be sent for 
    re-calibration.

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Checking the calibration for 60 mm Hg. The end point occurs  when the  tip of the tonometer tilts forward and the bar moves to  horizontal position. If the reading is 60 at this point, the tonometer  is correctly calibrated.

The animation shows a tonometer with an accurate calibration for 20 mmHg.