Information on Weight and Grammage Measurement

Content Overview

Coating Weight Measurement by Eddy Current Testing | Terminology | Unit | Techniques | Deposition

Coating Weight Measurement by Eddy Current

The weight of conductive coatings is often measured by non-contact eddy current thin film sensors. This technique induces eddy current into conductive coating, which respond with a electromagnet field (EMF) which is measured by the EMF detectors or directly by eddy current sensor. The obtained signal relates to the amount of material, that again correlates with coating weight. This method is an alternative to the "cut-out-method" since it is non-destructive and delivers results on the weight of conductive material without preparation of the specimen. Benefits include: 

• Contact free and contact mode
• High speed measurement for industry productivity
• High repeatability and accuracy
• Sensor to substrate distance up to 45 mm
• Portable testing options
• No wearing

Terminology for Weight Based Film and Material Description

Different terms are used in different industries for describing weight per area depending on the deposition process and substrate. The terms include:

• Grammage (syn. areal weight, area weight): Commonly used in paper, fiber or fabric industry grammage is the mass per unit of area.
• Basis weight or fiber areal weight (also known as FAW): Commonly used in technical textile industry to describe the amount of carbon or glass fiber material on woven and non-woven materials
• Coat weight (syn. coating weight, deposition weight): In wet and wet chemical deposition processes coating weight refers to the amount of coating applied to a substrate for a given surface area.

Weight Units

Even though the terms for grammage are different, they all describe the amount of added material in weight per area. This type of description has a long history since weight scales have been around for many thousands of years. It is a physical quantity value that is giving in [mass] [length]^-2. Most common units include g/m² (gram per square meter), kg/ m² (kilogram per square meter), oz/ ft² (ounces per square feet) or oz/ yd² (ounces per square yard).

Weight Measurement Techniques

The destructive "cut-out-method" is applied across several industries. Here cutouts e.g. of 100 mm² size are taken from one or several position of the material of interest. In some cases, where weight should only be determined of a certain part of the material, then a separation by chemical process or washing processes is done with respective pre- and post-measurements of the weight.

Non-destructive methods use thickness-to-weight correlation by absolute thickness measurement e.g. by laser beam triangulation or confocal microscopy or NIR absorption. Another technique includes beta rays transmission-based equipment that require large capital expenditure (CAPEX) and a certain degree on safety measures. The most appropriate solution depends on the material properties (electrical, optical and x-ray absorption). The selection of technologies is discussed in our thickness measurement section.

Deposition Processes and Measurement Positions

Coatings are being applied by vacuum, atmospheric plasma and wet processes depending on the application and its requirements. Especially wet processes, such as slot die, doctoral blade or doctoral knife, slurry or spin coating require monitoring of area weight or coating weight, finally to achieve electric, ionic or mechanical performance. Processes run Roll to Roll (R2R) or Sheet to Sheet (S2S). Substrates include paper, woven and non-woven, membranes, polymers or plastics, glass and metal sheets. Measurement of area weight is done after or in-between of critical manufacturing steps. Processes may include coating, drying or calendaring.