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Close, Viables, Basingstoke, United Kingdom) and a GM electrospirometer (GM Instruments
Limited, Kilwinning, United Kingdom) then gave an electrical output that could be sent to the
chart recorder. The pneumotachograph was initially calibrated by passing flow from an air
cylinder set at known flow rates (100, 250, 500, 750 and l0o0mls/min) and was linear over
this range but then seemed to read slightly but consistently high after 1l0omls/min. In initial
self trials (myself and Carolyn Busst) the response from human exhalation seemed unsteady
or 'noisy'. This appeared to be the effect of the small but repeated voluntary alterations in
expiratory flow by the subject (one of us) trying to maintain a flow without adequate
feedback. It disappeared when the steady flow of compressed air was put through the
rotameter and the pneumotachograph at designated flow rates. In view of this, we then added
the rotameter as part of the routine exhalation. Subsequently, all the subjects were directed to
watch the rotameter during exhalation and aim to maintain a set flow rate by keeping a
'bobbin' at a set number on the upright scale. This improved the steady quality of the signal.
The rotameter was a Platon glass tube c6 with stainless steel float 6D (Flowmeter Model
GTV - CT Platon Ltd, address above).
Figure 5.2: An example of tracing from the testing
sec9-
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The first thing to note is that the chart record is read from right to left which is how it was generated
from the chart recorder. Note the offset of the traces from the different analysers which were displayed
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