Stationary and portable gas flow and pressure measuring devices are required for the calibration, production and standardization of devices of all types. Devices available to date have the disadvantage that the gas flow measurements are influenced by the environmental conditions, and miscalibrations therefore occur. This is very problematic especially in the case of medical devices, such as respiratory and anesthesia devices. Since the devices are used worldwide, miscalibrations also result from incorrect operation of the devices.
There have to date been several efforts to perform an exact gas measurement. The following measuring principles are known:
Ultrasound
Two combined ultrasonic transmitter/receiver units which are arranged at an angle to the inflow direction. These send an ultrasound pulse at regular intervals and wait for the reception of the pulse of the other transmitter. If ultrasound waves cover a defined distance s in a known medium they require for this purpose a time t dependent on their propagation velocity. The propagation velocity of a wave is thus dependent on the transit time of the signal from one transmitter/receiver to the other.
Volume Counter with Measuring Vanes
A vane wheel is caused to rotate so that its speed is proportional to the mean flow rate.
Wind Pressure Method
The medium flows through a nozzle or aperture. The cross-section is narrowed, which accelerates the flow rate. In order to avoid vortex formation, venturi nozzles are generally used. The mass flow is then calculated from the pressure difference across the nozzle.
Laminar Flow Elements
Like the wind pressure method, laminar flow elements are based on the fact that the flow can be calculated from a pressure difference. In contrast thereto, however, the flow is linear relative to the measured pressure difference. In medicine, the laminar flow elements are known as pneumotachographs.
Hot-Wire Anenometer
A wire or a surface is heated to a temperature which is above the ambient temperature. Molecules which strike this surface and then fly off again absorb kinetic energy there. The heat loss of the heated surface is proportional to the temperature difference between the heated surface and the environment and to the number of molecules striking per unit time.
All these known structures are at their limits, and the exactness of the measurement is restricted. Particularly in the area of medicine, however, it was intended to increase the accuracy further. This is the object of the invention. It is intended to improve the accuracy of gas flow measurement.
The measurement properties of the individual methods differ and, depending on the application, one or other measuring device—optionally optimized—is used.
Particularly in the case of special devices in the area of medicine, such as, for example, respiratory devices, anesthesia devices, etc., however, a very wide range of applications are present in a device, so that it was necessary to date to measure using a plurality of measuring devices, or to be satisfied with a certain accuracy of measurement—for example for standardization or monitoring purposes.