MOISTURE ANALYSIS NON DESTRUCTIVE METHODS FOR HYGROSCOPIC MATERIALS
In order for a capacitance based moisture analyzer to provide accurate moisture information for a given commodity, a series of capacitance readings, over a relevant moisture range for the commodity first have to be taken. For example, if a particular grain generally arrived with between 12% & 18% moisture, capacitance values for a series of samples with different moisture levels, ideally between 11% and 19%, at .5% intervals, would first need to be gathered. The moisture content of each of the samples, needs to be determined by an existing method, generally an oven test.
Proprietary Software then carries out the task of correlating the moisture levels to the capacitance values, in the form of a moisture curve. The next time a like sample is placed in the capacitance analyzer, it again takes a capacitance reading, only this time, it knows the relative moisture percentage.
Although loss on drying and Karl Fischer are two of the most popular test methods for determining moisture, they both have severe drawbacks when it comes to routine moisture checking in a quality control setting, or wherever frequent or routine testing is carried out.
Oven tests can take many hours to complete for each sample, making it probably the least productive method for determining moisture levels. Moisture balances or laboratory dryers can reduce the time to minutes instead of hours, but 15 minutes can turn in to hours, with enough samples.
Karl Fischer Moisture analysis, another preferred direct method, requires a high degree of operator training, as well as sophisticated and expensive instrumentation.
Indirect methods like capacitance, and NIR, provide usable accuracy in seconds, require very little operator training, and are non destructive to the sample.
Methods of determining moisture, are generally chosen for suitability based on the physical characteristics of the sample. For example if one was to want to obtain the amount of water in a sample of oil, it would be easier to add a liquid chemical to the oil, that would react relative to the amount of moisture, than to heat the oil, and try to drive off the water that way, so Karl Fischer is what is used in petrochemical moisture determination. For a sample of coffee beans, it is easier to heat them up to drive off moisture, than to try to arrange for a chemical reaction, where the sample would first need to be homogenized, to help ensure the chemical reaches the areas where the moisture is.
Capacitance based analyzers need to be able to determine the density of the sample, and therefore need a measurement of the samples mass.
This can be done with a balance, a fixed volume container, or by making assumptions about the volume of the sample, as in the case of the moisture probe.
Here the signal transmits to about a ten inch circumference, so the calculations, are based on a soccer ball sized sample.
FROM LEFT TO RIGHT
(1) AP-6060 uses an oscillating balance to measure a samples mass.
(2) Sinar SP Moisture Probe is designed for insertion in to bulk samples, where it assumes the samples volume. There must be enough sample
(Soccer Ball Size) surrounding the sensor, in order to get an accurate reading.