How SpiroFit Calibrates & Best Practices for Testing

One of the most common questions practitioners ask before purchasing a portable metabolic analyzer is: how does calibration work?

Anyone who has used a traditional metabolic cart knows the cumbersome routine of setting up a test. Reference gas tanks, multi-point calibration sequences, warm-up periods, environmental adjustments. It can take 20–30 minutes before a single test begins. For a device designed to be used in the field, in a clinic, or between back-to-back clients, that overhead is a barrier.

SpiroFit takes a novel approach. And understanding how and why it works is the foundation of confident, accurate testing.

Why SpiroFit Doesn’t Need External Calibration Gases

Traditional metabolic analyzers calibrate by exposing their sensors to a known reference gas — typically a cylinder of precisely mixed oxygen and carbon dioxide at a certified concentration. The device reads the known gas, establishes its reference point, and uses that baseline for subsequent measurements.

SpiroFit doesn’t need a reference gas cylinder because it already has access to one with every single breath: the atmosphere itself.

The composition of ambient air is remarkably consistent. Oxygen makes up 20.95% of atmospheric air. Carbon dioxide sits at approximately 0.04% — 400 parts per million. These values remain essentially constant regardless of altitude, because while atmospheric pressure changes with elevation, the proportional composition of oxygen and carbon dioxide in the air stays the same. Variations occur only in the parts-per-million range, well within the tolerance of SpiroFit’s sensor system.

Every time you inhale, SpiroFit reads that inhalation against these known, fixed values. The inhaled air is the calibration reference, and it is refreshed with every single breath, 400 times per second.

The Science of Each Sensor

Oxygen (O₂) Sensor: Linear Calibration

SpiroFit’s oxygen measurement is based on a linear sensor curve. Because 20.95% oxygen is a fixed, known value in ambient air, it provides one anchor point on that curve. The second calibration point — 0% oxygen — corresponds to a voltage of zero in the sensor system. With two fixed reference points established, the sensor can accurately determine any oxygen concentration between them.

This linear relationship is precise, stable, and does not require external gases to maintain its accuracy because one of the two reference points is always available in every breath.

Carbon Dioxide (CO₂) Sensor: Thermal Conductivity

SpiroFit’s CO₂ sensor works by measuring the thermal conductivity of air — the rate at which air conducts heat. Carbon dioxide conducts heat differently than the nitrogen and oxygen that make up the vast majority of atmospheric air, allowing the sensor to detect even small changes in CO₂ concentration with high precision.

Because inhaled air contains essentially no carbon dioxide — just 0.04%, which is effectively zero relative to the concentrations found in exhaled breath — each inhalation serves as a fresh reference point for the subsequent exhalation. The sensor measures the difference between inhaled and exhaled air rather than an absolute concentration.

This relative measurement approach carries an important advantage: it eliminates physical drift. Absolute measurement systems — which rely on a fixed calibration point established at the start of testing — are vulnerable to drift as sensor temperature and operating conditions change over time. SpiroFit’s thermal conductivity approach recalibrates with every breath, so drift effects are systematically eliminated regardless of test duration or environmental conditions.

Flow Sensor

The flow sensor measures the volume and velocity of each breath, enabling SpiroFit to calculate total ventilation and distinguish precisely between inhalation and exhalation. At 400 measurements per second, SpiroFit captures breath dynamics that slower sensors cannot resolve, including the humidity content of each inhalation, which is factored into the calculation of gas concentrations to ensure accuracy under real-world conditions.

Factory Testing — Before the Device Ever Reaches You

In addition to automatic in-use calibration, every SpiroFit analyzer undergoes a rigorous factory testing process before it is approved for delivery:

• Medical-grade flow-volume simulator testing — each device is verified against a certified simulator to confirm flow measurement accuracy
• Respiratory gas simulation — a full simulation of respiratory gas conditions is performed on every unit before shipping
• Real-person resting measurements — two resting metabolic rate measurements are conducted by test subjects on each device as a final quality check

The production quality of every sensor is verified during manufacturing and approved at the factory level. No device ships without passing these checks.

Best Practices for Accurate Testing

Understanding SpiroFit’s calibration mechanism also informs the single most important best practice for accurate testing: the warm-up period.

Turn the device on at least 60 seconds before testing away from the subject, in a wind-free environment.

Why this matters:

When SpiroFit is first powered on, its sensors need a brief period to reach thermal equilibrium — a stable operating temperature at which the measurements are most accurate. This is true of all electronic sensors to some degree. During the warm-up period, the device is actively reading ambient air and establishing its calibration baseline. Rushing into a test before this process is complete can introduce unnecessary variability into the early data.

The 60-second warm-up is short, simple, and requires nothing from you except remembering to power the device on before you begin your client intake or preparation process. Build it into your pre-test workflow and it becomes automatic.

Why wind-free matters:

SpiroFit calibrates using the known composition of ambient air. Airflow across the mask or mouthpiece during the warm-up period — from a fan, an open window, an air conditioning vent, or even a client breathing near the device — can temporarily alter the local gas composition the sensor reads. This is a transient effect, but positioning the device away from airflow during warm-up ensures the cleanest possible baseline.

Practical tips for your warm-up protocol:

• Power the device on as you’re greeting your client and reviewing their intake form — by the time you’re ready to begin, the device is ready too
• Set the device on a flat surface away from the client, facing away from any air movement in the room
• In outdoor or field testing environments, position the device in a sheltered location during warm-up if wind is present
• In high-humidity environments, the same 60-second warm-up applies — SpiroFit accounts for humidity in its calculations, but the warm-up period allows the sensor to acclimate to ambient conditions before testing begins

What This Means for Back-to-Back Testing

One of SpiroFit’s practical advantages is its capacity for rapid turnaround between tests. Because there is no external calibration process to repeat between subjects, back-to-back testing is straightforward: power the device off between tests, allow a brief drying period, power back on, and give it at least 60 seconds before beginning the next assessment. We typically recommend turning on the device 5 minutes before beginning your test, and programming in a brief 5-minute warm up for your athlete.

In high-humidity conditions, or after a particularly intense exercise test where the subject’s breath has introduced significant moisture into the system, allowing the device a slightly longer dry-out period before the next test is good practice. While drying cycles typically require no more than 15 minutes in any circumstance, you’ll know you need more drying time if you hold the power button to shut off the device, and it does not power down. This is an indication that the drying cycle is not complete.

A Note on Environmental Conditions

SpiroFit is designed to operate across a wide range of environmental conditions — a key advantage for practitioners working outside controlled laboratory settings. The thermal conductivity CO₂ sensor’s relative measurement approach means temperature variation between testing environments does not compromise accuracy the way it can with absolute sensor systems. While we recommend testing in ranges between __ , SpiroFit has been seen to function and produce accurate results at temperatures outside of this range.

Humidity up to 85–90% is acceptable for standard testing. In extreme humidity conditions, allow the device to equilibrate to the environment during the warm-up period and be mindful of moisture accumulation with back-to-back testing.

Altitude does not meaningfully affect SpiroFit’s accuracy. Because the device measures proportional gas concentrations rather than absolute pressures, the consistent 20.95% oxygen fraction in atmospheric air at any elevation provides a valid calibration reference.

Questions about setup, calibration, or best practices? Reach out to your SpiroFit representative or visit spirofit.de for full documentation and support resources.