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Calibration of Micro Medical spirometers
There are different types of transducer available to measure lung volumes and flow rates. The type of transducer used is responsible for the stability of the Spirometry measurement and many transducers require regular re-calibration to compensate for the effects of drift. This drift may derive from physical contamination, changes in ambient temperature, altitude, or humidity(1)
The Micro Medical Spirometers use a digital volume transducer that exhibits exceptional stability (2) and remains unaffected by these conditions.
The transducer consists of an acrylic tube with a freely rotating vane supported on jewelled bearings positioned between a fixed swirl plate and cross bar. As air is passed through the transducer, a vortex is created by the swirl plate, which causes the vane to rotate. The number of rotations is proportional to the volume of air passed through the turbine and the rate of rotation is proportional to the flow rate. The rotation of the vane depends solely on the velocity of air passing through the turbine and the geometry of the swirl plate and has been shown to be extremely stable. The calibration of the turbine will only change if the swirl plate or the jewelled bearings supporting the vane have been physically damaged. This may easily be verified by visual inspection.
Re-calibration by the user is not recommended for two reasons.
Firstly, re-calibration of a transducer would mask an underlying problem of physical damage or contamination that should be dealt with. If damage has occurred then transducer replacement and factory calibration is recommended.
Secondly, due to the exceptional stability of the digital volume transducer, the errors arising from differences in calibration techniques would be greater than any change in the transducer response. For example, in the trial by Asger Dirksen cited below re-calibration of the Spirometers would have caused incorrect calibration to have occurred and would have masked the problem of drift with the calibrator.
After several months of use, or if physical damage is noticed, then a calibration check using a syringe of at least 3 litres is recommended. This accuracy verification may be performed more regularly should protocol demand.
(1) Prevention of thermal and condensation errors in pneumotachographic recordings of the maximal forced expiratory manoeuvre.
M.R. Miller, T. Sigsgaard
European Respiratory Journal 1994, Vol 7, p 198-201.
(2) Long term performance of a hand held spirometer.
Asger Dirksen, Flemming Madsen, Ole Find Petersen, Anne Mette Vedel, Axel Kok-Jensen. Thorax, October 1996, Vol 51, No 10, p 973-976.