[The safety of metrological control of applanation tonometers by technical investigations on transfer standards].
BACKGROUND
According to the German medical devices act and German ordinance on operators of medical devices, human eye tonometers have to be retested every two years. Independent verification laboratories carry out the metrological controls for tonometers which are used by ophthalmogists. Applanation tonometers are frequently used for the accurate measurement of intraocular pressure. For metrological controls these measurement devices are retested by means of test devices (transfer standards) used by verification offices and by private test agencies. PROCEDURE AND METHOD: 25 test devices for applanation tonometers (15 cross-beam balances and 10 force transducer devices), applied for metrological controls by verification offices and by private test agencies, were checked by means of comparative measurements with regard to their quality of measurement and their handling. An applanation tonometer of the type AT 870 (manufacturer: Haag-Streit, Switzerland) served as reference device. Within a measurement range from 9.81 mN to 78.45 mN, upward and downward measurements were performed for the mechanical force of applanation tonometers and for the determination of the reverse span.


RESULTS
The investigations for all test devices of applanation tonometers show the compliance within permissible error limits in the observed measuring range for the reverse span with the tonometer AT 870. Some instruments exceed the permissible error limits for test devices for up- and downward force measurements. A comparison concerning the measuring accuracy of two different types of test devices (cross-beam balance and force transducer device) shows that the force transducers tend to have smaller measurement uncertainties.


CONCLUSIONS
The high metrological demands on test devices for applanation tonometers for the measurement of mechanical force, a prerequisite for accurate calibration and adjustment of this tonometer type, require much experience and special training of the operator handling these standards, in order to minimize subjective error sources. Schoolings with practical trainings for user are recommended to minimize these errors. At least every three years the test devices have to be maintained and recalibrated, in order to guarantee proper function and accuracy of the measurement.
Schwenteck
T
T
Physikalische Technische Bundesanstalt, Berlin.
Thiemich
H-J
HJ
D001699:Biometry / Q000379:methods*; D004869:Equipment Safety / Q000592:standards*; D005858:Germany; D009885:Ophthalmology / Q000592:standards*; D017410:Practice Guidelines as Topic; D014065:Tonometry, Ocular / Q000295:instrumentation* / Q000592:standards*