How to Interpret Moisture Meter Readings on Different Scales

Posted by PAUL LAURENZI on Jan 29, 2016 3:28:00 PM

This post was originally posted on July 16, 2013 and was updated on March 15, 2015.

Moisture meters are indispensable tools for restoration contractors, home inspectors and flooring professionals, but moisture meter scales can create some confusion.  An understanding of the different moisture meter scales, exactly what they indicate and how they compare will assist building inspectors to make the most accurate and efficient use of these tools.

meterThe most common moisture meter scales are the wood scales and what’s commonly referred to as a reference scale. However, even within these categories, scales may differ somewhat from manufacturer to manufacturer and even from one type of meter to the next. 

The key difference between these scales is that the wood scale provides actual moisture content of the material with a percentage reading, whereas the reference scale provides only relative readings. Wood, for example, is generally considered “safe”  with a moisture content of between 8% and 11% in most parts of the U.S., while drywall  must be below 1% moisture content to be considered structurally sound and free from mold or decay.

The Wood Scale 

Most wood moisture meters are calibrated to cover a range of 6% to 40% moisture content, though this may vary slightly by model and manufacturer. Different species of wood may measure differently based on their electrical characteristics, but the basic wood scale is generally reliable for any construction-grade lumber. Some meters also have species correction charts or built-in species correction capabilities to allow you to correct readings for a variety of wood species.

The differences that may require a correction when testing different species of wood become even more significant when entirely different materials are involved. Thus, a wood-scale meter should not be used to measure concrete, drywall or other building materials. 

The Reference Scale 

Often, a moisture meter’s reference scale will range from 0-100, but that reading should not be confused with a percentage. Some meters use a 0-300 reference scale, so readings taken on a reference scale do not correlate with one another. The reference scale does not offer exact measurements in terms of %MC, but instead serves two purposes. First, a reference reading gives you a general indication on whether the material is high or low in moisture. Second, you can make a more specific assessment of a particular building material by testing a piece of the same material that is known to be dry and comparing the readings. 

The reference scale is generally used to measure non-wood materials.

The Drywall Scale 

Speak to a Specialist

With most moisture meters, drywall is measured using the reference scale and comparisons to dry materials. However, Delmhorst offers several moisture meters that are calibrated specifically for drywall

Hay Scale

Hay moisture meters work in the same way that wood moisture meters do, by calculating a percentage moisture content. The meters work best when measuring a hay stack or bale and the moisture percentages range from 6% to 40%.
For farmers, distributors or rangers, measuring moisture content goes a long way in protecting the integrity of the hay and the safety of the person storing it. Just a few percentage points can be the only difference between a superior-quality resource and spontaneous combustion due to bales of hay being too moist.

Paper Scale

For those manufacturers who supply the wood for the millions of tons of paper produced every year, a healthy moisture content is crucial. Before it becomes its final product, paper is first wood and then sliced to make fiber. And during each of these transitional phases, knowing the content of water in the material is necessary to ensure a top-quality paper product.

These meters are programmed for a paper scale and there are also specialized electrodes that are used to test the paper’s content. These electrodes roll with the movement of the pulp web and are able to take continuous measurements without compromising the integrity of the fabric.

Whatever the material you’re working with, moisture meters offer an efficient, cost-effective way to ensure that the materials are dry enough to work with and to identify trouble spots.  However, understanding the different scales and the need to use the right meter and the appropriate scale is critical to effective use.

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