If fine wire gauze is introduced into a stream of air, the passage of air through the gauze results in back pressure being developed which is related to the velocity of air, and to its viscosity. If the flow is laminar, the relationship is linear. (All G-M-Instrument's current range have linearity of 5% or better in the normal range). This velocity-dependent pressure - typically a few millimetres or water-applied to a suitable differential pressure transducer yields a velocity/dependent voltage. A sensitive transducer, together with one or more integrators to derive volume, is built into each of G.M. Instrument's Electronic Spirometers.
One of the problems that has been encountered with pneumotachographs is condensation from expired air. This can be prevented by fitting a non-return valve and measuring only inspiration or alternatively by heating the flowhead, but viscosity errors may arise which, in the first few breaths especially, preheat the inspired air most uncomfortably. In this range of heads the problem is approached from a fresh angle. By mounting a fine stainless steel gauze in plastic rings the thermal inertia is greatly reduced. The gauze therefore rapidly equilibrates in temperature with passing air and condensation is minimal. Each of the heads described below produces a similar pressure drop at their rated flows. This enables each head to be rapidly and accurately calibrated with any of G-M. Instrument's range of spirometers allowing flows from 3000 to 0.05 litres/minute and tidal volumes from 30 to 0.0005 litres to be measured. Instrument recalibration consists of moving the number on a calibrated dial from the number marked on one head to the number marked on the new head. Heads may be sterilised by gas or vapour sterilisers, by wiping with alcohol or by wet or dry heat not exceeding 75°C. If liquids are used, care must be taken to allow the head to dry out thoroughly, as beads of moisture trapped in the airways will cause the spirometer to give bizarre results.
These flowheads, although large in size are light in weight. The F3000L cones are made from acrylic, while the F1000L's casings are formed entirely in ABS, a lightweight plastic. The construction is such that they can be dismantled with ease to give good access to the interchangeable gauze assembly. The F3000L is intended for measurement of respiration in ponies and horses and the F1000L is suitable for adults during exercise and for certain types of lung function test.
This head is injection moulded in ABS plastic and comes as a complete single unit. Although it will stand frequent washing, if the head does become excessively fouled (or infected) it can be discarded altogether and replaced for little cost. It is intended to cater for most adult applications with the exception of hard exercise and certain lung function tests.
These heads are machined from acetal to give good stability with low weight and have found application in paediatrics and in the respiration measurement of animals such as dogs, cats, rats and mice. As with head types F3000L and FI000L interchangeable gauze assemblies are available.
|Flowhead Type||Linear Range L/min||Flow for 10mm H²O||Tube Outside Diameter||Length||Weight|
|F1L||± 1||1.2 L/min||5mm||40mm||13.8 grammes|
|F10L||± 10||12 L/min||8mm||54mm||21.8 grammes|
|F100L||± 100||90 L/min||16mm||54mm||37.7 grammes|
|F300L||± 300||125 L/min||29.5mm||91mm||36.8 grammes|
|F1000L||± 1000||485 L/min||29.5mm||198mm||260 grammes|
|F3000L||± 3000||1100 L/min||29.5mm||260mm||600 grammes|