Spirometry

Spirometry measures lung function by recording how much air a patient can exhale and how quickly. The two most clinically significant measurements are FVC (forced vital capacity) and FEV1 (forced expiratory volume in one second). FEV1, in particular, is a primary diagnostic marker for chronic obstructive pulmonary disease and a key indicator of asthma severity, as it drives treatment decisions that affect medication type, dosage, and disease-monitoring protocols for millions of patients worldwide. The ATS/ERS spirometry standards set strict accuracy requirements: volume accuracy within 3% or 50 milliliters, whichever is greater. These standards exist because an inaccurate reading does not produce a bad data point. It produces a wrong diagnosis.

The central challenge in spirometry pressure sensing is zero drift. A spirometer measures differential pressure across a flow element and integrates the resulting flow signal over time to compute volume. Any constant offset in the pressure zero point accumulates during integration, producing a growing volume error. A zero offset of just 0.1 Pa, sustained throughout a forced exhalation, produces a measurable volume error that can push a marginal measurement across a diagnostic threshold. This problem is compounded in handheld spirometers, where changing the device’s orientation between readings shifts the zero point by an amount that varies with the sensor’s physical characteristics. Conventional spirometers address zero drift by requiring re-zeroing between patients or readings, adding a step to the clinical workflow and creating the risk that tests are performed on a drifted baseline if that step is skipped.

Superior Sensor’s SP Series virtually eliminates sensor-level zero drift through Z-Track™ auto-zero technology, which continuously compensates for drift and maintains minimal zero-point deviation regardless of elapsed time or device orientation. Clinicians using SP Series-based spirometers do not need to re-zero between patients. The SP Series is ready to take accurate measurements within 60 milliseconds of power-on, with update rates as fast as 2 milliseconds to capture peak flow events that occur in the first fraction of a second of a forced exhalation. Multi-Range™ technology supports patient populations from infants to adults in a single device, and position insensitivity ensures accurate readings regardless of the angle at which the clinician holds the device during testing.

Why Choose Superior Sensor for Spirometry

Spirometry accuracy is not determined by how well the sensor performs on a clean bench in a temperature-controlled lab. It is determined by how the sensor performs in a clinic, after sitting between patients for an hour and being held at whatever angle a technician happens to grip it. The SP Series was designed specifically for these conditions, with capabilities that address the real failure modes of clinical spirometry rather than the ideal case.

Recommended Sensors

Common Device Features: 3.3V supply
Long-Term Stability is measured after first 12 months
Short-Term Error Band (STEB) is measured over 24 hours, after auto-zero

Common Specifications

Spirometry FAQ

How does a spirometer measure lung function using pressure?

Spirometers measure lung function by placing a flow element, typically a pneumotachograph or mesh screen, in the patient’s breathing path. As the patient exhales through the mouthpiece, air flows through the element, creating a differential pressure proportional to the flow rate. The pressure sensor measures this differential pressure, and the spirometer converts the reading into a flow rate using a calibration factor. The flow rate signal is then integrated over time to compute the volume of air moved during the breathing maneuver. The accuracy of all derived measurements, including FVC, FEV1, and peak expiratory flow, depends on the accuracy of the differential pressure measurement at each point in the breathing maneuver.

What is zero drift, and why does it matter in spirometry?

Zero drift is a shift in the sensor’s output when no pressure is applied, that is, when the pressure difference across it is zero. In an ideal sensor, this reading is exactly zero. In a real sensor, it varies over time due to temperature changes, mechanical stress, and the properties of the sensing element. In spirometry, zero drift matters because volume is computed by integrating the flow signal over time. A constant offset at the zero point means the flow signal carries that offset throughout the measurement, and when integrated over the duration of a forced exhalation, it accumulates into a volume error. A zero error of 1 Pa sustained during six seconds of forced exhalation can produce a volume error that may be clinically significant. Zero drift also changes between readings as the sensor’s state changes, making the error inconsistent and difficult to correct after the fact.

What spirometry measurements are most affected by sensor accuracy?

FEV1, forced expiratory volume in one second, is the measurement most sensitive to sensor accuracy because it is calculated from the first second of a forced exhalation, when flow rates change most rapidly. An inaccurate sensor reading during this critical window produces an incorrect FEV1 value that the rest of the maneuver cannot correct. FVC integrates the flow signal over the full exhalation, so zero drift continuously accumulates in the volume calculation throughout the maneuver. Peak expiratory flow requires a sensor fast enough to capture the maximum instantaneous flow rate, which occurs in the first fraction of a second; a sensor with a slow update rate misses the true peak and underreports PEF. Each measurement has direct clinical implications for the diagnosis and staging of obstructive and restrictive lung disease.

How does spirometer position affect accuracy?

Handheld spirometers change orientation as clinicians and patients grip and position them during testing. Conventional pressure sensors produce a different zero-point output depending on the orientation of the sensing element relative to gravity, because the weight of the sensing diaphragm exerts a pressure proportional to the sensing element’s orientation relative to gravity. In a flow measurement application where the full-scale pressure range is only a few hundred pascals, a gravitational zero shift of several pascals represents a significant percentage error. The SP Series position-insensitivity specification, rated to within 0.25 pascal regardless of orientation, ensures that repositioning the device between readings or holding it at different angles during a test does not introduce orientation-dependent measurement error into the spirometry result.

What accuracy standards must spirometers meet?

The American Thoracic Society and the European Respiratory Society publish joint technical standards for spirometry accuracy that clinical spirometers are expected to meet. The current ATS/ERS standards specify volume accuracy of plus or minus 3% of the reading or 50 milliliters, whichever is greater, and flow accuracy of plus or minus 5% of the reading or 200 milliliters per second, whichever is greater. The standards also specify requirements for back-extrapolated volume, linearity across the measurement range, resistance, and the flow measurement pathway’s frequency response. Regulatory bodies, including the FDA, apply these standards when evaluating spirometer submissions, and manufacturers use them to validate device performance across the range of patients, testing conditions, and clinical environments where the device will be used.