Development of a new spirometer mouthpiece

*project snapshot:*
*An R&D project to develop a new spirometer mouthpiece*

Spirometers are medical instruments which measure lung function and are used regularly by physicians to help diagnose respiratory problems. To use a spirometer a patient is required to take a deep breath and exhale as vigorously as possible into the portable part of the instrument, which is called the mouthpiece. The mouthpiece is a flow sensor which is connected to the main part of the instrument which houses electronics, data presentation hardware and so on. In view of the fact that some patients may be sick, and for purposes of general hygiene, spirometer mouthpieces are designed for single patient use and are therefore disposable. They are hence manufactured and used in fairly large numbers.

The original spirometer mouthpiece next to the new design. The new mouthpiece is currently in production and is in clinical use.

Our client, a well-established manufacturer of spirometers, currently utilized a proprietary two-stage, concentric orifice plate type differential pressure flow sensor. He wished to upgrade the mouthpiece design with the following requirements:
	a) reduced size so as to make it more suitable for smaller children to use, while remaining practical and comfortable for use by adults; b) increased flow sensitivity while maintaining the current high flow rate measurement capacity; c) improved or similar flow response linearity compared with the existing design; d) reduced manufacturing cost; e) mechanically compatible with the existing instrument design.

The performance of orifice plate type differential pressure flow sensors is a strong function of design and dimension, especially when the upstream flow channel diameter is less than 1”. Achieving the objectives listed above entailed changing the design of the orifice plate profile from a classic square edge to that of a new design. Largely by this means, the linearity performance of the smaller sensor was made similar to that of the existing device, despite the need to substantially increase the orifice beta ratio. The flow sensitivity of the new, smaller design was also enhanced, partly because the new design exhibited a greater degree of aerodynamic stability at moderate to lower flows. It is not possible to accurately predict the detailed performance characteristics of a small DP flow sensor, especially at high beta ratios, due to uncertainties in the precise values of the orifice discharge coefficient. As a result, it was particularly necessary to experimentally test and confirm designs. However, the sensor was to be fabricated by injection molding and it was therefore not economically feasible to test experimental designs made from the final injection molded plastic. Because of this it was necessary to instead perform all testing using experimental SLA prototypes. This gave rise to a practical problem because the surface roughness of the SLA prototypes, although not obviously rough to the touch, was in fact technically much rougher than that of the molded parts to an extent that significantly modified the performance, and it did so in a complex way. Not only that, but to make life more interesting, the surface roughness also sometimes varied between different individual SLA’s of the same overall design. It was therefore necessary to evaluate the effect on performance of this particular type of surface, and learn how to minimize and stabilize it as far as possible so as to obtain consistent results, and then make suitable allowances so that the final tooling, that would instead produce smooth plastic parts, would produce parts having precisely the required performance. This was a challenge and was achieved by a combination of modeling, extremely careful SLA preparation, systematic testing and data analysis. On the subject of modeling, another part of this project was the development of a dynamic mathematical model for predicting the mouthpiece pressure as a function of volumetric flow waveform, gas density and one or two other input parameters. A US patent is anticipated.

Scientific consulting is one of SRC’s R&D services. As such, this project was a product of teamwork between our client and myself. Our client had all of the necessary engineering resources and SRC provided specific technical expertise. We worked together between two different US time zones in the virtual sense; (e-mail, FAX and the occasional phone call). In fact, we have never actually met in person. Nevertheless, a very effective team relationship was established.
SRC:	DW: basic design, specification of all experiments, data analysis and interpretation, mathematical modeling where needed, prototypes and final mouthpiece designs including final tooling dimensions with respect to the key flow channel details and orifices.
Client:	All mechanical engineering, laboratory experimentation utilizing a state-of-the-art waveform generator and test data collection, SLA prototypes preparation, all final drawings and all that was involved in getting the tooling fabricated.

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