Calculator Input
Example Data Table
| Sample | Mass Loss (g) | Time (h) | Area (m²) | Thickness (micron) | Temp (°C) | RH High / Low (%) | MVTR (g/m²/day) |
|---|---|---|---|---|---|---|---|
| Coated PET | 0.18 | 24 | 0.05 | 50 | 38 | 90 / 50 | 86.40 |
| Barrier Laminate | 0.08 | 24 | 0.05 | 75 | 23 | 75 / 35 | 38.40 |
| Medical Pouch Film | 0.03 | 24 | 0.05 | 60 | 40 | 90 / 10 | 14.40 |
These rows are example values for reference only. Real results depend on actual test setup, humidity control, seal quality, and chamber stability.
Formula Used
1) Mass Loss
Mass Loss = Initial Mass − Final Mass
or use the directly measured mass loss.
2) Moisture Vapor Transmission Rate
MVTR = (Mass Loss × 24) / (Exposure Time in Hours × Exposed Area in m²)
3) Saturation Vapor Pressure
Psat = 0.61078 × exp[(17.2694 × T) / (T + 237.29)]
4) Vapor Pressure Difference
Δp = Psat × |RHhigh − RHlow| / 100
5) Permeance
Permeance = MVTR / Δp
6) Permeability
Permeability = Permeance × Thickness
The calculator normalizes units before solving. Permeance and permeability are most useful when the humidity gradient and temperature are accurately controlled during testing.
How to Use This Calculator
- Enter a sample name and choose the test method.
- Provide direct mass loss, or enter initial and final mass.
- Select the correct mass unit for the recorded measurement.
- Enter exposed area and choose its matching unit.
- Enter exposure time and select minutes, hours, or days.
- Input material thickness and choose the correct thickness unit.
- Enter test temperature and the high and low humidity values.
- Press Calculate Transmission to show results above the form.
- Use the CSV or PDF buttons to download the report.
FAQs
1. What does MVTR mean?
MVTR means moisture vapor transmission rate. It shows how much water vapor passes through a material over a defined area and time.
2. What is the difference between permeance and permeability?
Permeance describes whole-sample vapor flow under a pressure difference. Permeability adjusts permeance by thickness, helping compare different materials more fairly.
3. Why are temperature and humidity required?
They define the vapor pressure gradient. Without that gradient, permeance and permeability lose meaning because the driving force for diffusion is unclear.
4. Can I use initial and final mass instead of direct loss?
Yes. Enter both values and the calculator subtracts final mass from initial mass. That difference becomes the mass loss used in the rate formula.
5. Why does exposed area matter so much?
A larger test area allows more vapor passage. MVTR normalizes by area so you can compare different specimen sizes consistently.
6. Is a lower MVTR always better?
Not always. Lower MVTR is good for strong barriers, but breathable products may need higher transmission to release moisture safely.
7. Which units does this calculator support?
It supports grams, milligrams, kilograms, several area units, multiple time units, and thickness in millimeters, microns, mils, or inches.
8. Why might my lab result differ from a datasheet?
Differences can come from method choice, chamber control, seal leakage, conditioning time, specimen defects, and rounding or unit conversion practices.