Micro Medical Spiro USB Service Manual

Spiro USB

Service Manual

075-12

Revision 1.0 February 2004

Micro Medical Limited, P.O. Box 6, Rochester, Kent ME1 2AZ

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Spiro USB - System Overview (Fig. 1)

The Spiro USB is a PC connected spirometer dedicated to work with SPIDA 5
spirometry software.
It consists of a removable digital volume transducer (1) and a housing (2) containing
a microprocessor control circuit and USB driver.
When testing a subject the transducer is inserted into the housing, which is plugged
into a USB socket of a PC. The digital volume transducer is used to measure the
subjects expired flow and volume in accordance with the operating manual.

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2

2

Transducer (Fig. 2)

The Micro Medical digital volume transducer consists of an acrylic tube with a
vane positioned between two swirl plates. The low inertia vane is attached to a
stainless steel pivot which is free to rotate on two jewelled bearings mounted at the
centre of the swirl plates. As air is passed through the transducer a vortex is created
by the swirl plates which causes the vane to rotate in a direction dependant upon the
direction of air flow. The number of rotations is proportional to the volume of air
passed through the transducer and the frequency of rotation is proportional to the
flow rate. The transducer housing consists of a main body which contains a pair of
light emitting diodes (LED’s) and phototransistors. The transducer is fixed to the
mouthpiece holder which pushes into the main body and is captured by an “O” ring
seal. The LED’s produce infra red beams which are interrupted by the vane twice
per revolution. This interruption is sensed by the phototransistors. The output from
the collector of each phototransistor will be a square wave with a phase difference
between the two of + or - 90 degrees depending upon the direction of flow.
There is no routine maintenance required for the transducer other than cleaning
according to the instructions in the operating manual.

Micro Medical Digital Volume Transducer

Volume proportional to the number of pulses
Flow proportional to the puse frequency

Rotating

vane

Volume = k X No. of pulses

Infra red

emitter

Infra red
detector

Swirl
plate

Flow = k / pulse period

Jewelled

bearing

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