CO₂ Decay Ventilation Analyzer
Estimate air changes from measured carbon-dioxide decline over time.
Your measured ventilation inputs
Requires a controlled, approximately well-mixed decay period with no continuing source and a stable measured background.
Measured ventilation report
3 ACH · fitted decay estimate
Unoccupied, well-mixed single-zone assumption
Full result table
| Elapsed min | CO₂ ppm | Above background ppm | ln excess | Fit ln excess | Log residual |
|---|---|---|---|---|---|
| 0 | 1,420 | 1,000 | 6.907755 | 6.907755 | 1.8234e-9 |
| 15 | 892.36655 | 472.36655 | 6.157755 | 6.157755 | -3.6467e-9 |
| 30 | 643.13016 | 223.13016 | 5.407755 | 5.407755 | 1.8234e-9 |
Requires a controlled, approximately well-mixed decay period with no continuing source and a stable measured background. Sensor quality and mixing affect the result. A high fit statistic does not certify adequate ventilation or establish infection risk.
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How to use CO₂ Decay Ventilation Analyzer
- Estimate air changes from measured carbon-dioxide decline over time.
- The logarithm uses measured CO₂ minus the background.
- Ordinary least squares fits the logarithmic excess concentration.
Worked example
For C(t) = 420 + 1000 exp(−0.05t), the fitted slope is −0.05 per minute and estimated ACH is three per hour.
Formula, accuracy & limitations
Fit ln(C(t) − background) = intercept + slope × minutes by ordinary least squares. ACH = −60 × slope. Outdoor-air flow estimate(m³/hour) = ACH × room volume. R² describes agreement with the exponential-decay assumption.
Requires a controlled, approximately well-mixed decay period with no continuing source and a stable measured background. Sensor quality and mixing affect the result. A high fit statistic does not certify adequate ventilation or establish infection risk.
The primary result is rounded to two decimal places. Calculation tables show up to four and supporting metrics up to six. Very small nonzero values use scientific notation. Display rounding is not fed back into the arithmetic.
Official source and method context ↗Use a real decay experiment
Estimate air changes from measured carbon-dioxide decline over time. Record a stable background and at least three increasing elapsed times during an approximately unoccupied period. A busy room with changing occupants or continued combustion does not match this simple decay model.
Check every reading against the background
The logarithm uses measured CO₂ minus the background. Values at or below that background cannot be used by this fit and are rejected rather than clamped to a small positive number. A wrongly assumed background can alter the slope even when the plotted decline looks smooth.
Inspect the fitted curve and residuals
Ordinary least squares fits the logarithmic excess concentration. The result includes a slope, R² and an observed-versus-modelled table. The fitted slope must represent decay. Fit quality describes this dataset’s consistency with the assumed curve; it does not prove that the room was well mixed or the sensor calibrated.
Avoid converting ACH into a personal risk score
Multiplying estimated ACH by room volume gives a flow estimate under the model. It cannot identify every air pathway, certify code compliance or calculate a person’s infection probability. Use the measurement context and professional ventilation assessment when those decisions matter.
Frequently asked questions
Can I use CO₂ readings from an occupied meeting?
A continuing source violates the simple decay assumption. Use an appropriate controlled decay period instead.
Why must background be measured?
The fitted logarithm depends on excess concentration above that background, so an incorrect baseline changes the estimate.
Does R² close to one prove accuracy?
It indicates a close curve fit, not correct sensors, mixing or experimental conditions.
Is filtered-air CADR included?
Particle filtration does not remove CO₂ in the same way, so its CADR is not inferred from this experiment.
