Spirometer Working Principle

Spirometer is the most widely used instrument for the measurement of various lung capacities and respiratory volume.


Working Principle:

As told the spirometer is an apparatus for accurately measuring the volume of air inspired and expired by the lungs. The standard spirometer consists of a movable bell inverted over a chamber of water. To balance the bell jar we use a weight to maintain the gas inside the atmospheric pressure. The height of the bell jar above the water will be proportional to the amount of gas inside it. A breathing tube is connected to the mouth of the patient with the gas under the bell.

When no one is breathing into the mouth piece, the bell will be at rest with a fixed volume above the water level. When the patient expires the pressure inside the bell increases above the atmospheric pressure causing the bell to rise. Similarly when the patient inspires, the pressure inside the bell decrease below the atmospheric pressure and the bell drops down.

As the change in bell pressure changes the volume inside the bell, the position of the bell jar is varied with respect to the inspiration and expiration. As the bell position varies, the position of the weight which balances the bell jar also varies. A pen is attached to the weight in order to record the volume changes in a piece of graph paper. The chart recorder is called spirograph or kymograph and it is a rotary drum. The graph obtained corresponding to breathing is called spirogram.

Some spirometers also have the provision to offer an electrical output that is analog equivalent of the spirogram. Here we connect the pen and weight assembly to a linear potentiometer. If we connect certain positive and negative potentials to the end of the potentiometer, then the resulting electrical output can provide the same data as the pen. When the patient is not breathing the output will be zero. When the patient inspires the output will have one polarity and it will be of opposite polarity during expiration.

Spirometers are one of the primary equipments used for PFT meaning Pulmonary Function Tests. It is a useful test for assessing the health conditions of the patient’s lungs. In addition, it is often used for finding the cause for shortness of breath, analyzing the effects of contaminants on lung functions, effect of medication, and progress for disease treatment.

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