Principles of two different measurement techniques for moisture determination
Before describing the various moisture measurements, it is important to define moisture content. Moisture content is usually expressed as a percentage by weight of total product (wet basis) or dry product (dry basis).
Wet basis moisture content: M = 100 x (wet weight - dry weight) / wet weight
Moisture content on dry basis: M = 100 x (wet weight - dry weight) / dry weight
According to the above formula, the moisture content of the wet base cannot exceed 100 per cent. Dry basis moisture may exceed 100 per cent and is a non-linear function. Moisture content can be determined by a number of techniques. These can be divided into two main categories, primary and secondary metrics.
I. Radio Frequency Dielectric Techniques
This method relies on a high dielectric constant of water relative to most solids.
A number of techniques have been developed to determine the dielectric, including RF, microwave and time-domain reflectometry. In order to measure the relative dielectric constant of a material, the material needs to be electrically coupled to a sensing circuit. This can be done by placing the material between two parallel electrodes, but this does not facilitate on-line applications. If the sensing circuit operates at RF, it is easy to propagate RF energy through the material and thus couple to the product without physical contact. Planar fringe field electrodes provide a unilateral measurement structure with less impact on the process.
The electrical analogue of a solid product is a capacitor in parallel with the leakage conductivity. These components are affected by moisture, but the dielectric constant is very predictable while the loss factor is not. The combined components represent a complex impedance that can be easily measured, but it can be affected by variables other than moisture.
True dielectric moisture meters are rare because most low-cost instruments make no attempt to separate the dielectric and loss components. The lowest cost instruments make little or no attempt to measure the combined impedance with any long-term stability and repeatability.
It is a penetrating measurement and can measure non-homogeneous products.
It has a large measurement area that provides a more representative overall average moisture for the product.
It is relatively inexpensive compared to other in-line technologies.
It is very reliable and robust with no moving parts to wear out or break.
A variety of mechanical sensors are designed to suit a wide range of process conditions and can be used in high temperature environments.
II. Infrared Technology
Near Infrared Reflectance (NIR or IR) technology is a widely used technology for on-line moisture testing. Much of its popularity is due to its ease of use.
A light source (usually a quartz halogen bulb) is collimated and filtered to a specific wavelength. A filter mounted on a rotating wheel slices the light into a series of pulses of a specific wavelength. The filtered light beam is directed onto the surface of the product to be measured. A portion of the light is reflected back to the detector (usually lead sulphide). A specific wavelength of light is absorbed by the water. If the filter is selected so that one wavelength will be absorbed by the water (the sample beam) and one wavelength will be unaffected by the water (the reference beam), the amplitude ratio of the two reflected wavelengths will be proportional to the amount of water in the water.
Easy to apply. Typically mounted 6 to 10 inches above the product. Moderate product height variations have little or no effect on the measurement.
A small spot measurement area combined with a scanning frame provides a product profile.
Specific wavelengths can be selected to measure variables other than humidity.
