Water Vapor Transmission Rate Calculation

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Water Vapor Transmission Rate Calculator

Calculate the Water Vapor Transmission Rate (WVTR), also known as Moisture Vapor Transmission Rate (MVTR), based on gravimetric testing data (e.g., ASTM E96). This tool determines the rate at which water vapor passes through a material per unit area per day.

Circular (Dish Method) Square/Rectangular
Enter the inner diameter of the test dish mouth.
Please enter valid numerical values for all fields.
WVTR Result
0.00 g/(m²·day)
Mass Change: 0 g
Test Area: 0 m²

Understanding Water Vapor Transmission Rate (WVTR)

Water Vapor Transmission Rate (WVTR), frequently referred to as Moisture Vapor Transmission Rate (MVTR), is a measure of the passage of water vapor through a substance. It is a critical metric in industries such as packaging, textiles, and construction materials.

A high WVTR indicates that the material is breathable and allows moisture to escape, which is desirable for activewear fabrics. Conversely, a low WVTR indicates a good moisture barrier, which is essential for food packaging to prevent spoilage or for vapor retarders in building walls.

The Formula

The standard calculation for WVTR determines the mass of moisture transmitted through a unit area over a unit of time. The formula used in this calculator is:

WVTR = Δm / (A × t)
  • Δm (Delta m): The change in mass (g), calculated as the absolute difference between final and initial weight.
  • A: The surface area of the test specimen (m²).
  • t: The time duration of the test (days).

The standard unit is g/(m²·day).

How to Perform the Test

The most common method for measuring WVTR is the Gravimetric Method (e.g., ASTM E96).

  1. Preparation: A desiccant (drying agent) or water is placed inside a test cup.
  2. Sealing: The material to be tested is sealed over the opening of the cup using wax or a gasket.
  3. Weighing: The initial weight of the entire assembly is recorded.
  4. Exposure: The cup is placed in a controlled environment (temperature and humidity chamber).
  5. Final Weighing: After a set duration, the cup is weighed again to measure the gain (desiccant method) or loss (water method) of moisture.

Factors Affecting WVTR

  • Thickness: Generally, WVTR is inversely proportional to the thickness of the material.
  • Temperature & Humidity: Higher temperatures and humidity gradients usually increase the transmission rate.
  • Material Structure: Crystallinity, orientation, and chemical composition of the polymer affect permeability.

Typical WVTR Values

Different materials have vastly different barrier properties:

  • Aluminum Foil: ~0 g/(m²·day) (Near perfect barrier)
  • HDPE (High-Density Polyethylene): ~5-10 g/(m²·day)
  • LDPE (Low-Density Polyethylene): ~10-20 g/(m²·day)
  • Breathable Fabrics: > 1000 g/(m²·day)
// Function to toggle between Circular and Square input fields function toggleDimensions() { var shape = document.getElementById('sampleShape').value; var circleDiv = document.getElementById('circleInput'); var squareDiv = document.getElementById('squareInput'); if (shape === 'circle') { circleDiv.style.display = 'block'; squareDiv.style.display = 'none'; } else { circleDiv.style.display = 'none'; squareDiv.style.display = 'block'; } } function calculateWVTR() { // 1. Get Values var m1 = parseFloat(document.getElementById('initialMass').value); var m2 = parseFloat(document.getElementById('finalMass').value); var hours = parseFloat(document.getElementById('duration').value); var shape = document.getElementById('sampleShape').value; // Validation flags var isValid = true; var areaM2 = 0; // Check Mass and Time if (isNaN(m1) || isNaN(m2) || isNaN(hours) || hours <= 0) { isValid = false; } // Calculate Area based on shape if (shape === 'circle') { var diamMM = parseFloat(document.getElementById('diameter').value); if (isNaN(diamMM) || diamMM <= 0) { isValid = false; } else { // Area of circle = pi * r^2 // Radius in meters = (diamMM / 2) / 1000 var rM = (diamMM / 2) / 1000; areaM2 = Math.PI * rM * rM; } } else { var wMM = parseFloat(document.getElementById('widthMM').value); var lMM = parseFloat(document.getElementById('lengthMM').value); if (isNaN(wMM) || wMM <= 0 || isNaN(lMM) || lMM <= 0) { isValid = false; } else { // Area = w * l (convert mm to m) areaM2 = (wMM / 1000) * (lMM / 1000); } } // 2. Logic Execution or Error Handling var errorDiv = document.getElementById('error'); var resultDiv = document.getElementById('result'); if (!isValid) { errorDiv.style.display = 'block'; resultDiv.style.display = 'none'; return; } errorDiv.style.display = 'none'; resultDiv.style.display = 'block'; // 3. Perform Calculations var deltaMass = Math.abs(m2 – m1); // Grams var timeDays = hours / 24.0; // Convert hours to days // WVTR = Mass Change (g) / (Area (m2) * Time (days)) var wvtr = deltaMass / (areaM2 * timeDays); // 4. Output Results // Formatting to appropriate decimal places document.getElementById('wvtrResult').innerText = wvtr.toFixed(2) + " g/(m²·day)"; document.getElementById('massChange').innerText = deltaMass.toFixed(4); // Scientific notation for area if it's very small, otherwise fixed if (areaM2 < 0.001) { document.getElementById('areaResult').innerText = areaM2.toExponential(4); } else { document.getElementById('areaResult').innerText = areaM2.toFixed(5); } }

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