What Is the Difference Between a Flow and Volume Spirometer?

Breathing is an activity you may not notice until something is wrong. People with lung diseases such as asthma or emphysema know when they are having difficulty breathing. Doctors need to accurately measure lung function to determine if a disease is present or whether a specific treatment is working. To do this, they use a device called a spirometer.

Flow Spirometer

• One type of spirometer measures the speed at which air flows into and out of the lungs. These devices, called flow spirometers, are smaller than volume spirometers. As a person breathes into and out of the spirometer, a detector measures the air speed 30 to 300 times each second. The air speed can then be used to calculate the volume of air passing through. The detector may measure speed using a small turbine that spins or a heated wire.

Disadvantages and Advantages of Flow Spirometer

• Because these are smaller devices, flow spirometers are portable. Also, the sensors that measure the air speed are disposable, which prevents people from picking up diseases such as colds or flu while using the machine. However, flow spirometers are difficult to calibrate -- a way of verifying that the data collected is accurate. Also, they do not provide real-time measurements.

Volume Spirometer

• Volume spirometers measure the volume of air that is inhaled or exhaled over a certain amount of time. These devices provide real-time measurements of the volume over time. Some volume spirometers work with an accordion-like bellows that moves as the person breathes into the tube. Other types include a seal that is either dry or formed by water.

Disadvantages and Advantages of Volume Spirometer

• Some volume spirometers are portable, although others are heavy and not suitable for carrying. They remain calibrated longer than flow spirometers, but can grow mold or bacteria inside if not cleaned well. With additional electronic parts, these types of spirometers can also produce additional graphs -- called flow/volume curves and loops -- that help show the lung function. Certain types of lung activities, such as coughing or breathing less than maximum, do not show up well with volume spirometers.