Calculator Inputs
Example Data Table
| Case | Airflow | Volume | Indoor | Outdoor | Loss Rate | Estimated Rate |
|---|---|---|---|---|---|---|
| Small lab | 450 m³/h | 180 m³ | 6 mg/m³ | 1 mg/m³ | 0.10 h⁻¹ | 2340 mg/h |
| Classroom | 900 m³/h | 320 m³ | 4 mg/m³ | 0.8 mg/m³ | 0.05 h⁻¹ | 2931 mg/h |
| Workshop | 1400 m³/h | 500 m³ | 9 mg/m³ | 1.5 mg/m³ | 0.15 h⁻¹ | 11062.5 mg/h |
Formula Used
The steady state mass balance is:
G = (Qe + kV) × (Cin - Cout)
The transient buildup balance is:
G = V × dC/dt + (Qe + kV) × (C - Cout)
Here, G is the contaminant generation rate.
Qe is effective ventilation airflow.
V is room volume.
k is first order loss, decay, or deposition rate.
Cin and Cout are indoor and outdoor concentrations.
Air changes per hour are calculated with:
ACH = Qe / V.
Required airflow is estimated by rearranging the steady balance for a target indoor concentration.
How to Use This Calculator
- Select steady state when the concentration is stable.
- Select transient buildup when concentration changes over time.
- Enter airflow from a fan, hood, duct, or air balance report.
- Add room volume and concentration readings.
- Use outdoor concentration as the background level.
- Enter a decay or deposition rate if known.
- Use molecular weight for ppm or ppb conversions.
- Press calculate and review the result above the form.
- Export the result using the CSV or PDF button.
Article: Understanding Generation Rate Ventilation Calculations
Why Generation Rate Matters
A generation rate describes how fast a source releases a contaminant into indoor air. It may describe carbon dioxide, dust, vapor, odor, smoke, or another measured substance. The value is useful because concentration alone does not show the full source strength. A room with high airflow can hide a strong source. A room with weak airflow can show high concentration from a smaller source.
Mass Balance Idea
The calculator uses a room mass balance. The room is treated as a mixed air volume. Contaminant enters from an indoor source. It also leaves through ventilation. Some material may disappear through decay, settling, filtration, or surface loss. The calculator joins these effects in one practical equation. This makes the result more useful than a simple airflow multiplication.
Steady and Transient Conditions
Steady state means the indoor concentration is not changing much. This is common during a long test with stable activity and airflow. In that case, the source must equal the removal rate. Transient mode is different. It includes the storage term. This term accounts for concentration rising or falling during the test. It is helpful for short experiments, startup periods, or changing occupancy.
Reading the Result
The main result is shown in milligrams per hour. The tool also gives grams per hour, daily mass, air changes, and per person rate. A negative value usually means the inputs conflict. It can happen when indoor concentration is lower than outdoor concentration. It can also happen when the concentration falls quickly during transient testing. Always check units, sensor calibration, sampling time, and air mixing.
Practical Use
Engineers can use the result for source control, ventilation checks, and exposure review. Facility teams can compare rooms. Students can study indoor air physics. The output should guide decisions, not replace field judgment. Real rooms may have poor mixing, short circuits, open doors, or changing source strength. Good measurements make the estimate stronger.
FAQs
1. What does generation rate mean?
Generation rate means the amount of contaminant released per unit time. This calculator reports it mainly in milligrams per hour.
2. When should I use steady state mode?
Use steady state mode when indoor concentration is stable. It works best after enough time has passed for ventilation and source release to balance.
3. When should I use transient mode?
Use transient mode when concentration changes during the test. It includes the room storage term and is better for buildup or decay experiments.
4. Why do I need outdoor concentration?
Outdoor concentration is the background level. The calculator subtracts it from indoor concentration to estimate the part caused by indoor sources.
5. What is first order loss rate?
It represents removal beyond ventilation. Examples include deposition, filtration, chemical decay, settling, and absorption on surfaces.
6. What molecular weight should I enter?
Use the pollutant molecular weight when using ppm or ppb. For carbon dioxide, use about 44.01 grams per mole.
7. Why is my result negative?
A negative result can mean outdoor concentration is higher, concentration is falling, or units are mixed. Recheck all inputs before using the result.
8. Can this replace professional testing?
No. It is a calculation aid. Use calibrated sensors, field judgment, and qualified review for health, safety, or compliance decisions.