How to Significantly Improve Spirometer Precision

Spirometry Sketch

Spirometry Market

Spirometry is a common medical office test used to assess how well a patient’s lungs work by measuring how much air they inhale, how much they exhale and how quickly they exhale. Spirometers are thought to be the most reliable way to test and diagnose several lung conditions that effect breathing such as:

  • Asthma
  • COPD
  • Chronic bronchitis
  • Emphysema
  • Pulmonary fibrosis

If a patient has already been diagnosed with a chronic lung disorder, spirometry may also be used periodically to check how well medications are working and whether breathing problems are under control. Further, spirometry may be ordered before a planned surgery to check if lung function is adequate for the rigors of an operation. Finally, spirometry may be used to screen for occupational-related lung disorders.

Spirometry measures the lung capacity and volume under various test conditions. Some of the most common measurements are:

  1. Forced Vital Capacity (FVC) measures the largest amount of air that you can blow out after you take your deepest breath. A lower than normal FVC reading indicates restricted breathing.
  2. Forced Expiratory Volume (FEV1) measures the amount of air you can blow out of your lungs in the first second of exhalation. This reading assesses the severity of the breathing problem – the lower the FEV1, the more significant the obstruction.
  3. Maximum Voluntary Ventilation (MVV) measures the maximum amount of air that can be inhaled and exhaled within one minute.
  4. Forced Expiratory Flow (FEF) is the flow or speed of air coming out of the lungs during the middle portion of the exhale.
  5. Peak Expiratory Flow (PEF) is the maximum level of flow or speed of air coming out of the lungs during the entire exhalation.
  6. Tidal Volume (TV) is the amount of air inhaled or exhaled when in a resting condition.
  7. Total Lung Capacity (TLC) is the maximum volume of air present in the lungs when inhaling.

The FEV1/FVC Ratio is a critical measure. In healthy adults this should be approximately 70 – 80%, but it does decline with age. In obstructive diseases (asthma, COPD, chronic bronchitis, emphysema) FEV1 is diminished because of increased airway resistance when exhaling; the FVC may be decreased as well, due to the premature closure of airway in expiration, just not in the same proportion as FEV1. This generates a reduced value (less than 70%, often as low as 45%). In restrictive diseases (such as pulmonary fibrosis) the FEV1 and FVC are both reduced proportionally and the value may be normal or even increased as a result of decreased lung compliance.

Picture of a spirometer connected to a computer

Figure 1 – Image of Spirometer connected to PC

The Role of Differential Pressure Sensors in Spirometers

While there are several other types of flow sensing spirometers, such as turbine, thermal and ultrasonic, we are going to focus on differential pressure based spirometers. These types of spirometers are very common and provide accurate measurements.

A differential pressure sensor converts pneumatic pressure values into proportional electrical signals. The pressure sensor typically includes a thin diaphragm. The diaphragm is the most important element for the measurement of the pressure and is equipped with strain-sensitive and compression-sensitive resistance structures. The diaphragm is deflected by air pressure when a patient inhales and exhales from the spirometer. These deflections are then converted into electrical signals (analog output voltages) that are proportional to the applied differential pressure on the sensor as measured by the diaphragm.

However, pressure sensors are sensitive to many external factors including noise, humidity, temperature, atmospheric pressure and physical positioning/orientation of the device. Another challenge for differential pressure sensors when measuring low pressure air flow is the possible need for recalibration, as they tend to drift from their zero reading over time. Differential pressure sensors that reduce the impact of noise, are not susceptible to changes from position/orientation and provide a stable, consistent zero value allow health professionals to better diagnose lung performance and enable spirometers to provide the most accurate readings.

Superior Sensor’s Technology Advantage

Superior Sensors’ proprietary NimbleSense architecture is the industry’s first System-in-a-Sensor integrated platform. Incorporating a highly differentiated advanced pressure sensing system with the ability to integrate optional building blocks enables us to combine the highest accuracy and reliability with spirometry-specific exclusive features. With unique technology deployed in our SP Series of differential pressure sensors, Superior’s products offer many advantages for spirometry and critical care medical devices.

Z-Track Auto Zero

Superior’s proprietary Z-Track technology virtually eliminates zero drift by maintaining minimal zero-point deviation with results that are consistent regardless of elapsed time. For more details on Z-Track technology, read the Z-Track blog post.

Position Insensitivity

Superior’s unique dual-die implementation with the SP210 sensor maintains consistent and highly accurate handheld readings regardless of physical orientation of the spriometry device. Rated with a positional sensitivity to within 0.25 Pa, the SP210 is an industry leader with respect to position insensitivy.

Highest Levels of Accuracy

Sensor accuracy is critical as the difference between an effective and ineffective treatment plan can depend on the precision of the diagnosis. A small difference in a measurement can alter the dosage or even the type of medication a patient is prescribed. Superior’s SP Series spirometry sensors have the industry’s leading accuracy to as close as within 0.05% of the selected range.

Fastest Warm-up and Response Times

For time critical applications, warm-up time of the spirometer is important. The SP Series essentially eliminates warm-up time as the device is ready in just 60 msec. In addition, the amount of time it takes the pressure sensor to update its measurement data is just as vital. The faster you receive updated pressure measurements, the more accurate your spirometry readings. While user configurable, Superior’s sensors support update rates as fast as 2 msec.

Lowest Noise Floor

As mentioned earlier in this article, external noise can have a negative impact on the accuracy and performance of spirometers. Utilizing our integrated advanced digital filtering technology, Superior’s pressure sensors eliminate the noise created by these factors prior to their reaching the sensor sub-system. Thus, the noise is eliminated before it becomes an error signal that can lead to inaccurate lung measurements.

Low Power Consumption

As many spirometers are self-contained handheld devices or handhelds that are connected to a computing device via a USB port, power consumption is another important factor in overall device performance. With power consumption as low as 5 mA, the SP Series will not adversely impact the battery life of even the most sophisticated spirometry equipment.

Spirometry sketch

Figure 2 – Sketch of patient using handheld spirometer


A spirometer is a vital tool used to diagnose and manage many different types of lung diseases.  Spirometry products require high performance differential pressure sensors to accurately diagnose a patient’s lung functions. Additionally, these handheld units must provide highly accurate readings regardless of how the unit is positioned when it is being held.

Superior Sensor’s unique differential pressure sensor technology, based on our proprietary NimbleSense architecture, provides many differentiating features that help medical device makers distinguish their products in a competitive marketplace. For more detailed information about our Spirometry solutions please visit our product page or contact us.

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