Water is one of the key components of life, but understanding it's presence and effect in manufacturing and processing, is tantamount to the characteristics of the finished product. If one were to bake a cake, or mix some concrete for a quick home repair, they would find specific instructions on how much water to add, and realize specific consequences if those instructions were not followed. The same holds true for everything from raw materials to finished products.
The problem becomes how to measure the amount of moisture at any particular stage in a product or materials development. Depending on the characteristics of the sample being tested, the amount and type of moisture being measured, and the budgetary and time constraints on the facility doing the testing, there are a number of options available.
Moisture Balances(LOSS ON DRYING / GRAVIMETRICS)(Most versatile relative to sample types. 2 through15 minute time depending on sample. Wet & Dry Samples, concrete, tortillas, etc..)
Because oven tests can take many hours to complete, other , less time consuming methods of moisture analysis needed to be developed.Moisture balances and fluid bed dryers, are two types of loss on dryingmoisture analyzers in use today.
Moisture balances use a heat lamp (generally infrared) to produce the heat required to remove residual moisture, in far less time. Average tests take between 1 and 15 minutes. They are the most versatile type of moisture analyzer because any material that can be tested in an oven, can as a rule be tested on a moisture balance. Digital moisture balances detect the endpoint by noting the loss of weight change.
Another loss on drying analyzer is a fluid bed dryer, which blows hot air up through the sample, and fluidizes it, drying the sample more evenly. By sensing the change in relative humidity, the fluid bed dryer can actually separate surface moisture from bound moisture, and develop a representative moisture curve.
Not all processes allow for the inclusion of an online continuous moisture measurement system. The sample type, size, and environment are factors, as well as budgetary constraints and processing protocal. It may be neccessary to measure density and temperature as well as moisture, so a sample may be better analyzed in a laboratory setting. Many facilities use table top moisture meters as a back up to their inline system.
METHODS FOR DETERMINING MOISTURE IN DIFFERENT SAMPLE TYPES
(Most Versatile, Often used as the standard. Long test times to 16 hrs.)
Moisture analysis is often determined by the use of a laboratory oven. While there are many different makes of laboratory oven, they generally fall in to one of three categories: natural convection, forced air, and vacuum ovens. Assuming all calibration and environmental parameters are followed, the results are accurate enough for many applications.
Environmental factors include:
Sample homogeneity- May be improved with a grinder if needed.
Vibration-Can cause fluctuations with analytical balances, especially when measuring below .1% moisture
Power Fluctuations-Dedicated Lines and power surge protectors, along with proper grounding and equipment placement can help reduce the effects of both direct surge and static electricity.
Ambient temperature and humidty-Many samples gain or lose significant moisture just from improper storage. Exposure to ambient factors should be kept to a minimum.
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)
Where very small levels of moisture need to be determined, or where volatile substances are present, loss on drying may not be suitable. Coulametric or Volumetric Karl Fisher Titration can then be used to measure trace amounts of water, depending on whether the sample is a solid or liquid, and the percentage of moisture being measured. Vaporizer ovens or homoginizers may be needed to help release the moisture from difficult samples. Applications include testing for moisture in transformer oil, or for trace amounts in plastics.
Capacitance (RADIO FREQUENCYPOWER LOSS)(Results accurate to as close as .2% +/-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)
Agricultural commodities, cereals, cookies, and coffee are examples of materials where the moisture levels can be determined by taking advantage of the electrical properties of water, either with resistance or capacitance.
This Sinar Moisture Spear is used for spot checks of agricultural commodoties. The probe is inserted in to a sack, and gives a reading in about 6 seconds..
Resistance ( Long temperature adjustment time 5 minutes, Low Cost, Agricultural Products Corn / Coffee may be suitable
as well as soil, sand, and compost.
Low cost DC Resistance Based Moisture Anlayzers offer relatively accurate results for many applications. As with anything else, lower cost generally means lower functionality.
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. 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.
A sample specific calibration is 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
NIR Inline Continuous Moisture System
Laboratory Moisture Balance
RF Capacitance based Inline Continuous Moisture Sensor
Control panel for Inline Continuous Microwave Based Moisture Analyzer
