Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1998-07-17
2004-05-11
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
C073S204180, C073S861950
Reexamination Certificate
active
06732581
ABSTRACT:
TECHNICAL FIELD
This invention refers to the fields of spirometry measurements and in particular to thermostatic control of pneumotacographs. Furthermore, it can also be conveniently used in both micro-calorimetry and in the measurement of flow rates of a fluid along a pipe.
BACKGROUND ART
It is common practice to monitor lung function by analysing sample breath performance. As humans live breathing, it is easy to observe that our breaths are not all exactly the same and that even lung function may change depending on a wide range of variable factors. This is especially true in the case of particular environments or of special reactions to certain volatile or suspended agents, or of respiratory pathologies in progress.
Lung volume is one of the most informative measurements that can be performed from the breath. Variations of volume allow estimating and predicting the profile of the lung functionality and are therefore crucial towards a proper and prompt treatment.
Since lung volume is on the order of several liters, it is impractical to measure it directly. The flow can be measured instead—e.g. from the pressure drop across a resistance—and then integrated to obtain volume readings. This can be done reliably only to the extent in which the flow-measuring device is linear and accurate. Measuring flow instead of volume allows reducing the size and weight of spirometric devices. However, measuring flow entails a variety of technical problems mainly related to thermostatic regulation, which we examine below.
In 1925 A. Fleisch proposed what is still one of the most accurate methods to perform spirometry by means of measuring laminar flows. To that end, Fleisch used a honeycomb structure. The most common form of honeycomb structure consists of a cylinder made by coupling and rolling up two thin metal sheets, one corrugated and the other smooth, around a tiny hub.
The metal sheets are made of brass and the dimensions of the resulting cylinder can be of diameter=42 mm, height=32 mm (Fleisch No. 4), the pressure drop is measured around the outer ring of the cylinder at a distance of 20 mm.
The honeycomb structure is one of the many possible physical filters capable of maintaining a laminar flow in a conduit, so that pressure drop and flow rate values are linearly correlated. Following the Fleisch method, several other different structures have been introduced. The most widely known is that of the fine mesh screen structure. This is lighter than the honeycomb structure but “frequent cleaning and re-calibration are considered essential to preserve accuracy in measurements”(
1
).
Notes:
1
)
Office Spirometry
, P. L. Enright, R. E. Hyatt Lea & Febiger, 1987
The temperature of the air inside human lungs is 37° C. and saturated with water vapor. When exhaled, air condenses and releases its vaporisation enthalpy.
To maintain a temperature of 37° C., Fleisch and his successors chose to heat their structures by means of an external heater. As a matter of fact, the approximately 6.5 Watts required to heat a classical size 4 Fleisch cylinder, which is also due to heat inefficiency factors and to the high thermal inertia of the whole assembly, has up until recently made it difficult to produce a truly portable heated pneumotach system.
As portable instruments cannot conceivably feature such high values of power consumption, in recent years several non-heated spirometry instruments and pneumotacographs have been proposed.
One particularly serious difficulty that has arisen with the widespread use of non-heated flow transducers is vapor condensation.
This factor slightly affects flow cooling, as condensing vapor releases its vaporisation enthalpy, though the main point is that turbulence occurs and this means that the flow is no longer linear. As turbulence implies a quadratic relationship between the pressure drop and flow rate, flow readings can be seriously flawed. This source of error makes the non-heated systems unreliable in a very subtle way.
Spirometry is a test consisting of several repeated trials and for each trial condensing vapor releases its vaporisation enthalpy. After a small number of maneuvres referring to a single patient's test, the instrument tends to accumulate water vapor, hence the air flow becomes turbulent, and measurement overestimation increases. According to the rules suggested by the American Thoracic Society and the European Respiratory Society, the highest value achieved in a series of maneuvres has to be kept as the spirometry measurement result. So any overestimation error occurring during the test leads to globally wrong results. The recent increase of unreliable spirometric results is strongly related to the widespread use of non-heated pneumotach systems.
Ceramic honeycombs had been introduced in an attempt to solve this problem, though without success, as these tended to absorb water vapor (
2
). They were also quite heavy and temperature had to be measured with extremely delicate, lightweight elements, besides creating difficulties indisinfecting.
2
) U.S. Pat. No. 5,277,196 January 1994 Hankinson et al. 128/725
The present state-of-the-art provides a wide choice of solutions that exhibit technical drawbacks, which have heavily delayed the production of a simple, reliable, small, lightweight, low-power, easy-to-clean and truly portable pneumotach system.
Similar problems also occur with those instruments which measure heat exchanges in physical (fluid flow rate measurement) and/or chemical processes especially in gas-solid interactions (Palladium-Hydrogen interaction).
In fluid flow rate measurements, with constant direct measurement techniques, problems arise when flow rate changes rapidly and the fluid temperature is variable (e.g. like the case of blood flowing through a vessel (
3
)). On the other hand, techniques that separate the heat source (emitter) from the measuring device (sensor) have evident disadvantages in that the sensor collects only a small fraction of the signal emitted.
3
) Baxter Int. Inc., WO 94/28788 December 1994
Another case where thermostatic regulation is crucial is in the interaction Palladium-Hydrogen. In this case, the measurement concerns the enthalpy release accompanying the absorption of hydrogen into a Palladium layer and (on the basis of literature data) the determination of the amount of hydrogen absorbed by the sample (
4
). In this case, the calorimeter used to enclose the environment where the reaction takes place tends to be rather bulky, features a slow response and its cost is proportional to the precision to be achieved by the measurement. On the other hand, the use of small elements as thermistors(
5
), platinum wires(
6
) or macromachined structures, exhibits difficulties related to the surface where the reaction takes place. Whenever a large surface is needed as in measurements of gas adsorption enthalpies, accurate measurement becomes problematic. These determinations can in fact involve very low thermal effects and low thermal conductivity substrates whereby punctual measurements cannot be applied.
4
) Lewis F. A.,
The Palladium Hydrogen System
, Academic Press 1967
5
) U.S. Pat. No. 3,645,133, (SIMETH) 29 Feb. 1972
6
) WO, A, 95/10 980, (MEDTRAC) 27 April 1995
DISCLOSURE OF INVENTION
The purpose of this invention is to produce a device with at least one active element featuring low heat capacity, that may be used especially as a space-saving instrument for measuring and controlling heat exchanges.
The invention relates, as defined in claim
1
, to a device based on an element of low heat capacity, suited for monitoring and controlling the temperature of a certain environment. Power is supplied in pulses to this element while heat relaxation analysis during the intervals between pulses is performed on the element itself.
When it is set up for spirometry and calibrated for normal body temperatures, the device becomes a pocket-size instrument featuring high precision and low power consumption. With appropriate adaptations the same device may also be used either for high precision flow
Biomedin
Hoffman Wasson & Gitler
Patel Harshad
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