Spirometers are non-invasive diagnostic devices enabling simple lung function examination, for example in case of respiratory tract and lung diseases such as asthma or chronic-obstructive bronchitis. They measure the respiratory flow rate and calculate the respiratory volume breathed in and out (inspiratory and expiratory lung volume) from this. These devices are usually designed as lightweight, portable systems that can be used conveniently in medical practices and in patients' homes.
Special pneumotachographs (e.g. according to Fleisch or Lilly) that measure the respiratory flow using differential pressure sensors are used in spirometers. The patient's breath flows through a laminated flow element or a screen with a very small flow resistance. A minimal pressure drop results over the element, this representing a measure for the respiratory flow rate (respiratory volume per time). These differential pressures can be logged with highly accurate sensors and converted into an electrical signal. So as not to unnecessarily burden the patient's respiration, it is important to keep the flow resistance as low as possible. Where respiratory flows are about 0.1 l/s during spontaneous respiration and approx. 7 l/s with forced expiration, the differential pressures generated via the flow element are therefore very low – in the range from below 100 pascals to several thousand pascals (100 pascals = 1 mbar). To be able to measure low respiratory flows with accuracies of 1% and, at the same time, ensure measurements over dynamic ranges >10,000, highly sensitive differential pressure sensors are required. In addition, the sensors must fulfill strict specifications regarding the resistance to moisture and dirt.
First Sensor LME and LDE ultra-low pressure sensors are ideally suited for flow rate measurement using the differential pressure method. The innovative sensor layout with a miniaturized flow channel on the chip level enables highly sensitive measurement of ultra-low pressures from 25 Pa (0.25 mbar) full scale with ultra-high resolution and accuracy. In addition, the semiconductor technology allows cost effective yet highly stable and compact designs. The only minimal air flow through the sensor means the LDE is very insensitive to moisture and dirt. Finally, connecting tubes or filters do not typically have any negative effect on the measuring accuracy.
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