Moisture occurs in nearly all substances. In our last column, we discussed ways in which inappropriate moisture levels can negatively affect the properties of the host material (e.g. the substrate) as well as impact cost and product quality. There are many techniques for the measurement of moisture. Because these have largely grown up in discrete industries, there is a good deal of confusion and ambiguity concerning techniques and measurement units, calibration problems, and moisture interaction with host materials.

Techniques and Measurement Units

Depending upon the technique, different units are used to measure and define the moisture level. These techniques can be broken down into three major areas:

Standard gravimetric methods employ a known weight sample which is heated to drive off the free water by evaporation after which the remaining dry weight is measured. The difference in the sample weight before and after heating is a measure of the moisture content. Weight is the unit of measure and moisture content is specified as percentage of the bulk weight.

Electrical impedance techniques make use of the huge difference in the dielectric constant of water compared to most common host materials. By applying an electrical potential to the sample, measurements of the current flow, resistance to flow, or the charge capable of being put in to the field of view, a reading of the relative conductance can be made and the moisture level gauged. This technique uses electrical measure as units to define the level of moisture.

Spectroscopic methods generally use the near infrared part of the spectrum, more exactly one of the three vibration energy levels of the hydrogen/oxygen bands. For measurement of moisture in solids a surface reflectance technique is used. The substrate, in the absence of water, has a characteristic spectroscopic fingerprint. With careful calibration, changes in the spectroscopic fingerprint when water is present can be used to indicate moisture level. This technique can be used either in gases and transparent solids, or, by surface reflection, for non-transparent solids. As with gravimetric methods, this technique expresses the moisture content as a ratio.