Calculator
Formula used
How to use this calculator
- Select a method: ERH (%) or vapor pressure ratio.
- Enter the required values (and pressure unit if needed).
- Optional: enable temperature to estimate p0.
- Click Calculate to view results above the form.
- Use CSV or PDF buttons to export the last result.
Example data table
| Case | Method | Inputs | aw | ERH (%) |
|---|---|---|---|---|
| 1 | ERH | ERH = 75% | 0.7500 | 75.00 |
| 2 | Vapor pressures | p = 2.10 kPa, p0 = 2.80 kPa | 0.7500 | 75.00 |
| 3 | Vapor pressures + temperature | p = 1.90 kPa, T = 20°C (p0 estimated) | ~0.82 | ~82 |
Water activity in physical systems
1) Definition and dimensionless scale
Water activity (aw) is a dimensionless ratio that compares the effective escaping tendency of water in a material to that of pure water. By definition, aw typically lies between 0 and 1 for equilibrium systems, making it convenient for comparing materials across very different compositions.
2) Two practical measurement routes
This calculator mirrors two common routes used in laboratories: (a) headspace equilibrium relative humidity (ERH), where aw = ERH/100, and (b) vapor pressures, where aw = p/p0. Both require equilibrium and the same temperature for the sample and reference.
3) Temperature dependence of saturation pressure
Saturation vapor pressure p0 increases strongly with temperature. As a reference, p0 is about 2.34 kPa at 20°C and about 3.17 kPa at 25°C. If your measured p stays constant while temperature rises, aw decreases because the denominator increases.
4) Useful thresholds for stability comparisons
Many stability discussions use benchmark regions. Values below aw ≈ 0.60 are often treated as “low availability” water. The band from roughly 0.60–0.85 is moderate, and above 0.85 is high. These bands help compare drying, storage, and packaging strategies.
5) Example calculations you can replicate
If ERH is 75%, then aw = 0.75. Using pressures at the same temperature, p = 2.10 kPa and p0 = 2.80 kPa also give aw = 0.75. At 20°C, if p = 1.90 kPa and p0 ≈ 2.34 kPa, then aw ≈ 0.81.
6) Why aw is different from moisture content
Moisture content is a mass fraction; water activity reflects thermodynamic availability. Two materials can have the same moisture content but different aw because solutes, pores, and binding sites change how strongly water is held. That is why aw is often preferred for comparing drying end-points.
7) Instrumentation and uncertainty
Common measurement approaches include chilled-mirror dew point, capacitive humidity sensors, and psychrometric methods. In practice, instrument repeatability can be on the order of a few thousandths (for example, ±0.003 aw) when temperature control and equilibrium time are well managed.
8) Data handling and reporting
For reporting, record method, temperature, unit choices, and the equilibrium condition. This page stores your last calculation so you can export it to CSV for spreadsheets or to PDF for lab notes. When results exceed 1.00, re-check temperature matching and pressure units.
FAQs
1) What is water activity in one sentence?
It is the ratio of a material’s water vapor pressure to the saturation vapor pressure of pure water at the same temperature, reported as a dimensionless value between 0 and 1.
2) Why does the calculator ask for equilibrium relative humidity?
At equilibrium, the headspace ERH tracks water activity closely, so aw = ERH/100. This is a practical measurement path used by many instruments and controlled chambers.
3) Can aw be greater than 1?
In ideal equilibrium systems it should not. Values above 1 usually indicate unit mistakes, a temperature mismatch between p and p0, or non-equilibrium readings.
4) Does temperature matter if I only enter ERH?
Yes, indirectly. ERH itself depends on temperature, and instruments typically control it. If temperature drifts during measurement, the ERH and the inferred aw may shift.
5) Why estimate p0 from temperature?
When you measure only the partial pressure p, you still need p0 at the same temperature. Estimating p0 provides a consistent reference for aw = p/p0.
6) What inputs should I save for traceability?
Save the method, temperature, pressure units, raw p and p0 (or ERH), and the timestamp. These fields let others reproduce the calculation and audit the conditions.
7) How should I interpret the category label?
The label is a quick qualitative guide based on common threshold ranges. It is not a guarantee of behavior; it helps you compare samples, packaging changes, or drying targets consistently.