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Calibration check using a 3 litre syringe
Calibration tests at different temperatures reveal that the turbine response is independent of temperature in the range 15 to 37 degrees C and therefore no BTPS correction is required. This is due to the speed that the air passes through the transducer, which is designed to present a small surface area of low thermal conductivity to the air passing through.
There is, however, an effect due to humidity of about 2%. This effect occurs only near to saturation and is taken into account with factory calibration. When checking calibration with a syringe the measurements will appear to be 2% high.
A three-litre syringe will therefore produce a nominal measurement of 3.06 litres. With a +/- 3% tolerance (as stated in the ATS 1994 recommendations) added, the measurement should be in the range 2.97 to 3.15 litres. When the calibration of the spirometer is verified using an ATS pulmonary waveform generator, using the spirometry routine, the unit will appear to over read by 2% unless the latest ATS generator is used. The latest generator uses air heated to 37°C saturated with water vapour.
During the design of the latest MicroLab and MicroLoops (MK6) it was decided to take this 2% correction into account in the calibration check routine such that a three-litre syringe would produce a nominal 3.00 litre reading. With a tolerance of +/- 3% readings in the range 2.91 to 3.09 should be obtained. In the interests of consistency it was decided to also implement this in the SuperSpiro. This has come into effect with Serial No.1115 However, if the calibration of the spirometer is verified using an ATS pulmonary waveform generator the unit will appear to over read by 2% unless the latest ATS generator is used as before.
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C.P.Lawson
Technical Director