Moisture Vapor Transmission Rate Calculator

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Moisture Vapor Transmission Rate (MVTR) Calculator

Calculate the rate at which water vapor passes through a material.

cm² m²
Hours Days
function calculateMVTR() { // 1. Get Input Values var initialWeightStr = document.getElementById('mvtr-initial-weight').value; var finalWeightStr = document.getElementById('mvtr-final-weight').value; var areaStr = document.getElementById('mvtr-area').value; var timeStr = document.getElementById('mvtr-time').value; var areaUnit = document.getElementById('mvtr-area-unit').value; var timeUnit = document.getElementById('mvtr-time-unit').value; // 2. Parse inputs to numbers var initialWeight = parseFloat(initialWeightStr); var finalWeight = parseFloat(finalWeightStr); var areaValue = parseFloat(areaStr); var timeValue = parseFloat(timeStr); var resultDiv = document.getElementById('mvtr-result'); resultDiv.style.display = 'block'; // 3. Validate Inputs if (isNaN(initialWeight) || isNaN(finalWeight) || isNaN(areaValue) || isNaN(timeValue)) { resultDiv.innerHTML = 'Error: Please enter valid numeric values in all fields.'; return; } if (areaValue <= 0 || timeValue <= 0) { resultDiv.innerHTML = 'Error: Area and Time must be greater than zero.'; return; } // 4. Perform Calculations // Calculate absolute weight change (works for both desiccant gain or water loss methods) var weightChangeGrams = Math.abs(finalWeight – initialWeight); // Normalize Area to Square Meters (m²) var areaInMetersSquared = areaValue; if (areaUnit === 'cm2') { areaInMetersSquared = areaValue / 10000; } // Normalize Time to Days var timeInDays = timeValue; if (timeUnit === 'hours') { timeInDays = timeValue / 24; } // Main Formula: MVTR = Weight Change / (Area * Time) var mvtrResult = weightChangeGrams / (areaInMetersSquared * timeInDays); // Secondary Calculation: Convert to g/100in²/day for reference (1 m² ≈ 1550 in²) var mvtrImperial = mvtrResult / 15.5; // 5. Display Results resultDiv.innerHTML = '

Calculated MVTR Result

' + " + mvtrResult.toFixed(2) + ' g/m²/day' + '(Approx. ' + mvtrImperial.toFixed(2) + ' g/100in²/day)' + '
' + 'Test Details:' + 'Weight Change: ' + weightChangeGrams.toFixed(3) + ' g' + 'Standardized Area: ' + areaInMetersSquared.toFixed(4) + ' m²' + 'Standardized Duration: ' + timeInDays.toFixed(2) + ' days' + '
'; }

Understanding Moisture Vapor Transmission Rate (MVTR)

Moisture Vapor Transmission Rate (MVTR), often used interchangeably with Water Vapor Transmission Rate (WVTR), is a critical measurement in materials science. It quantifies the rate at which water vapor passes through a substance, such as a plastic film, packaging material, or breathable fabric, under specific conditions of temperature and humidity.

Why is MVTR Important?

Knowing the MVTR of a material is essential in various industries:

  • Food Packaging: Controlling MVTR prevents food spoilage. Chips need a low MVTR barrier to stay crisp (keeping moisture out), while fresh produce might need a higher MVTR to allow respiration (letting moisture escape).
  • Pharmaceuticals: Many drugs degrade quickly when exposed to moisture. Blister packs and pill bottles must have very low MVTR characteristics to ensure shelf life.
  • Apparel and Textiles: In activewear or outdoor gear, "breathability" is essentially a measure of MVTR. A high MVTR allows sweat vapor to escape, keeping the wearer dry, while still blocking liquid water from the outside.
  • Construction Materials: Vapor retarders used in walls and foundations are rated by their MVTR (often referred to as "perms") to control moisture migration and prevent mold growth within building envelopes.

How the MVTR Calculator Works

This calculator uses data derived from gravimetric test methods (like ASTM E96). In these tests, a material sample is sealed over a cup containing either a desiccant (which absorbs moisture) or water. The assembly is placed in a controlled environment, and the change in weight over time indicates how much moisture vapor has passed through the material.

The fundamental formula used in this calculator is:

MVTR = ΔW / (A × T)

Where:

  • ΔW (Weight Change): The absolute difference between the final weight and the initial weight of the test assembly (in grams).
  • A (Area): The surface area of the material sample being tested, converted into square meters (m²).
  • T (Time): The duration of the test, converted into days.

Units of Measurement

The standard international unit displayed by this calculator is grams per square meter per day (g/m²/day). However, some industries, particularly in North America, use grams per 100 square inches per day (g/100in²/day). The calculator provides an approximate conversion for convenience.

Factors Influencing MVTR

It is important to note that MVTR is not a static property of a material; it is highly dependent on environmental conditions. The primary factors influencing the rate include:

  • Thickness: Generally, the thicker the material, the lower the MVTR.
  • Temperature: Higher temperatures usually increase the kinetic energy of water molecules, increasing the transmission rate.
  • Humidity Gradient: The difference in relative humidity (%RH) across the barrier drives the transmission. A larger difference results in a higher rate.

Therefore, when comparing MVTR values on spec sheets, always ensure they were tested under the same temperature and humidity conditions.

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