Water Vapor Transmission Rate (WVTR) Calculator
.wvtr-calculator-container {
max-width: 800px;
margin: 0 auto;
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
background: #f9f9f9;
padding: 30px;
border-radius: 12px;
box-shadow: 0 4px 15px rgba(0,0,0,0.1);
}
.wvtr-calculator-title {
text-align: center;
color: #2c3e50;
margin-bottom: 25px;
font-size: 28px;
}
.wvtr-form-grid {
display: grid;
grid-template-columns: 1fr 1fr;
gap: 20px;
}
.wvtr-input-group {
margin-bottom: 15px;
}
.wvtr-input-group label {
display: block;
margin-bottom: 8px;
color: #34495e;
font-weight: 600;
}
.wvtr-input-group input, .wvtr-input-group select {
width: 100%;
padding: 12px;
border: 1px solid #bdc3c7;
border-radius: 6px;
font-size: 16px;
box-sizing: border-box;
transition: border-color 0.3s;
}
.wvtr-input-group input:focus {
border-color: #3498db;
outline: none;
}
.wvtr-input-helper {
font-size: 12px;
color: #7f8c8d;
margin-top: 4px;
}
.wvtr-btn-container {
grid-column: 1 / -1;
text-align: center;
margin-top: 20px;
}
.wvtr-btn {
background-color: #3498db;
color: white;
border: none;
padding: 14px 30px;
font-size: 18px;
border-radius: 6px;
cursor: pointer;
transition: background-color 0.3s;
}
.wvtr-btn:hover {
background-color: #2980b9;
}
.wvtr-result-section {
grid-column: 1 / -1;
margin-top: 30px;
background-color: #fff;
padding: 25px;
border-radius: 8px;
border-left: 5px solid #3498db;
display: none;
}
.wvtr-result-header {
font-size: 20px;
color: #2c3e50;
margin-bottom: 15px;
border-bottom: 1px solid #eee;
padding-bottom: 10px;
}
.wvtr-result-value {
font-size: 36px;
font-weight: bold;
color: #27ae60;
margin-bottom: 5px;
}
.wvtr-result-unit {
font-size: 16px;
color: #7f8c8d;
}
.wvtr-article {
max-width: 800px;
margin: 40px auto;
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
line-height: 1.6;
color: #333;
}
.wvtr-article h2 {
color: #2c3e50;
margin-top: 30px;
}
.wvtr-article p {
margin-bottom: 15px;
}
.wvtr-article ul {
margin-bottom: 15px;
padding-left: 20px;
}
.wvtr-article li {
margin-bottom: 8px;
}
.wvtr-formula-box {
background: #f0f4f8;
padding: 15px;
border-radius: 6px;
font-family: "Courier New", Courier, monospace;
margin: 20px 0;
border-left: 4px solid #34495e;
}
@media (max-width: 600px) {
.wvtr-form-grid {
grid-template-columns: 1fr;
}
}
Water Vapor Transmission Rate Calculator
function calculateWVTR() {
// Get Inputs
var wInitial = parseFloat(document.getElementById('initialWeight').value);
var wFinal = parseFloat(document.getElementById('finalWeight').value);
var duration = parseFloat(document.getElementById('duration').value);
var timeUnit = document.getElementById('timeUnit').value;
var area = parseFloat(document.getElementById('sampleArea').value);
var areaUnit = document.getElementById('areaUnit').value;
var resultSection = document.getElementById('resultSection');
// Validation
if (isNaN(wInitial) || isNaN(wFinal) || isNaN(duration) || isNaN(area) || duration <= 0 || area <= 0) {
alert("Please enter valid positive numbers for Time and Area, and valid weights.");
return;
}
// 1. Calculate Weight Change (G)
// Absolute value used because depending on method (desiccant vs water), weight can increase or decrease.
var weightChange = Math.abs(wFinal – wInitial);
// 2. Normalize Time to Days (t)
var timeInDays = 0;
if (timeUnit === 'hours') {
timeInDays = duration / 24.0;
} else if (timeUnit === 'days') {
timeInDays = duration;
} else if (timeUnit === 'minutes') {
timeInDays = duration / 1440.0;
}
// 3. Normalize Area to Square Meters (A)
var areaInM2 = 0;
if (areaUnit === 'm2') {
areaInM2 = area;
} else if (areaUnit === 'cm2') {
areaInM2 = area / 10000.0;
} else if (areaUnit === 'in2') {
areaInM2 = area / 1550.0031; // 1 m² = 1550.0031 in²
}
// 4. Calculate WVTR
// Formula: WVTR = G / (t * A)
var wvtr = weightChange / (timeInDays * areaInM2);
// 5. Display Results
document.getElementById('wvtrResult').innerText = wvtr.toFixed(4);
document.getElementById('weightChangeResult').innerText = weightChange.toFixed(4);
document.getElementById('areaResult').innerText = areaInM2.toFixed(6);
document.getElementById('timeResult').innerText = timeInDays.toFixed(4);
resultSection.style.display = 'block';
}
How to Calculate Water Vapor Transmission Rate (WVTR)
Water Vapor Transmission Rate (WVTR), also known as Moisture Vapor Transmission Rate (MVTR), is a critical metric in materials science, packaging engineering, and construction. It measures the rate at which water vapor permeates through a substance—such as a plastic film, fabric, or building barrier—over a specific period.
Understanding WVTR is essential for ensuring product shelf life in food packaging, breathability in sportswear, and moisture control in building insulation.
The WVTR Formula
The standard calculation for WVTR is based on the gravimetric method (often referenced in standards like ASTM E96). The fundamental formula determines the mass of moisture passed through a specific area over a specific time.
WVTR = G / (t × A)
Where:
- G = Weight change in grams (g). This is the difference between the final weight and initial weight of the test assembly.
- t = Duration of the test in days (or hours converted to days).
- A = Surface area of the material being tested in square meters (m²).
Step-by-Step Calculation Guide
1. Prepare the Sample
A sample of the material is sealed over a test dish. The dish contains either a desiccant (to absorb moisture) or water (to release moisture). The setup ensures that water vapor can only pass through the test material.
2. Measure Initial Weight ($W_i$)
Weigh the entire assembly (dish + contents + sample) at the start of the experiment using a precision balance. Record this as the initial weight.
3. Run the Test
Place the assembly in a controlled environment chamber with specific temperature and humidity settings (e.g., 38°C and 90% RH). Let it sit for a specific duration ($t$).
4. Measure Final Weight ($W_f$)
After the time has elapsed, weigh the assembly again.
Note: If using the desiccant method, the weight will increase. If using the water method, the weight will decrease.
5. Calculate the Result
Determine the absolute weight change ($G = |W_f – W_i|$). Divide this weight change by the product of the test duration (in days) and the sample area (in square meters).
Example Calculation
Imagine you are testing a biodegradable food packaging film.
- Initial Weight: 150.000 g
- Final Weight: 150.150 g (Gained 0.15g)
- Sample Area: 50 cm² (0.005 m²)
- Duration: 24 hours (1 day)
Step 1: Calculate Weight Change ($G$) = 150.150 – 150.000 = 0.15 g
Step 2: Convert Area to m² = 50 / 10,000 = 0.005 m²
Step 3: Apply Formula = 0.15 / (1 × 0.005) = 30 g/(m²·day)
Why Units Matter
While the standard unit is $g/(m^2 \cdot day)$, some industries use different metrics. For example, in the United States, you might see $g/(100 in^2 \cdot day)$. Always ensure you convert your Area and Time inputs to standard units before dividing to get accurate results.