Types of moisture analysis equipmentlication.

Some of the more commonly used methods to determine the presence of water in raw materials, and finished goods, include: Loss on Drying (Ovens,Thermogravimetrics / Infrared, Microwave) Karl Fischer, Capacitance, (Includes RF Loss of Signal) ,X-ray, and NIR.

It should be noted that industries often adopt methods not only based on their ability to provide accurate results, but also on many other factors that may be particular to their specific application, including; ease of implementation, speed, cost, and portability.

LOSS ON DRYING- Classic laboratory direct method moisture test for solids and semi-solids, where materials are heated so that moisture present in the material evaporates. The ending weight is subtracted from the beginning weight, and expressed as percentage. Although this method is widely used as the standard in many industrial applications, care has to be taken to prevent chemical changes like carbonization, to the sample. Further complications arise from the inability to completely dehydrate samples, as well as the unintentional loss of volatiles. However, many of the shortcomings may be accounted for, with a basic understanding of sample drying characteristics.

KARL FISCHER (TITRATION) -Direct Laboratory Electro-Chemical Method of Moisture Determination. Where very small levels of moisture need to be determined, or where volatile substances are present, loss on drying may not be suitable (Plastics, Transformer Oil, Lyophilized Products)

Coulametric or Volumetric Karl Fisher Titration can be used to measure trace amounts of water, depending on whether the sample is a solid or liquid, and on the percentage of moisture being measured. Homoginizers, or newer, 'vaporizer ovens' may be needed to help release the moisture from solid samples that can not be dissolved. Applications include testing for moisture in transformer oil, or for trace amounts in plastics.

CAPACITANCE Results accurate to as close as .1% +/-in seconds.Free flowing granular sample types, corn, coffee, crumbled cookies, dried fruit. Drying curves may need to be established for each commodity, and programmed by end user or factory. These instruments make up the majority of moisture sensing equipment in the agricultural sector, because of their excellent compatibility with samples like seeds, grains, and feeds. They are fast , accurate, portable and durable, although because they are an indirect method, it is recommended to have a back up reference direct method available as an occasional calibration check.

The benefit to an inline system, is that it removes some of the labor, and subsequent human error. Also the moisture percentage readings can generally be converted in to a 4-20 ma electronic signal, that can then be directed in to the equipment controls, to help automate the process. For example, the moisture content of a sample moving along a conveyer belt system, through a dryer, can be converted in to a signal that can alter the speed of the conveyer belt, thereby changing the rate, or perhaps even the temperature, at which the product is dried.

SENSORS FOR DIFFERENT SAMPLE TYPES AND ENVIRONMENTS

NIR (Near Infrared) Sensors are popular because the sensor doesn't touch the sample or cause sample degradation. They are used in 0-60% moisture determination applications, in materials where the moisture on the surface is representative of, or at least can be correlated to the moisture throughout the rest of the sample. Multiple sensors can also be used to gather moisture data, at different locations. Environmental temperatures have little effect with applications.

A sample specific calibration is required

Capacitance Based Sensors actually make contact with the sample, and give readings in the 0-80% moisture range with RF Sensing technology. The Sensor projects an RF field into the sample and measures the loss or change in RF dielectric constant as affected by moisture. Thus, the system response is a measure of the total water in the RF field, which is directly related to the moisture content of the material.

They are often employed in agricultural applications. Grains, and seeds work well, as well as feed products, although limited applications are found in powders and sand as well. As long as the chemical make up of the product, density, and temperature are consistent, they work well, with the added benefit of reading through the sample area, which can be as large as a one foot sphere, or as small as a two inch sphere, depending on the manufacturer.

Sample specific calibrations are required.

Microwave Sensors read moisture through the entire sample, and is appropriate for most baled moisture-content applications, including cotton, tobacco, and recycled paper, to cite just a few

In particular, different heat sources work differently, some having less of an effect on materials prone to volatiles loss and carbonization, than others. The SHIMADZU MOC 120H (THERMOGRAVIMETRICS) Moisture Analyzer utilizes a mid infra-red heat source that helps prevent carbonization and reduces the effects of sample color on heat absorption. Glass fiber pads may also be utilized. Altering the temperature and time parameters of a test will also facilitate accurate testing. At 48 c, degradation reactions are considered to be absent for the majority of known food products. The below files show some different sample types, and the effects of temperature & time changes on their drying characteristics.

For most quality control applications, actual oven tests are not very practical. Results from forced air ovens can differ from vacuum oven test results, and microwave ovens can alter the chemical structure of some products, limiting it' s use. Direct Method Loss on Drying Tests, can be quickly and effectively conducted with the use of moisture balances, commonly referred to as thermogravimetrics, or Infra-Red. These instruments reduce the drying time dramatically, and because they weigh and dry the sample simultaneously, reduce many of the environmental factors that can result in inaccurate results. Care must be taken however, to utilize optimal sample preparation techniques, and optimal time and temperature settings. Many come with software or programs in order to help achieve this.