How a Moisture Analyzer Works
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Precise measurement of moisture content - and documenting the findings - is a quality control step often mandated by government agencies such as the FDA and by trade and professional associations. Precisely determining moisture content is accomplished byanalyzing small samples, usually measured in grams, in an instrument called a moisture analyzer. Here we present an explanation of how a moisture analyzer works.
For those curious as to why moisture content is important, we've provided some reasons at the later in this post. But first let's start with what we are talking about when we determine moisture content.
Moisture has weight. While this is obvious when picking up a full bucket of water or contemplating water pressure at the 12,500-foot depth of the Titanic (5,542 psi if you're interested) in this instance we're talking, again, about moisture analysis in small samples.
But moisture is not always as obvious as it may seem. Indeed, there are several types of moisture that together indicate the moisture content of the sample. The laboratory moisture balance must be capable of identifying and measuring these types of moisture. Briefly they can be described as:
- Surface Moisture. The most obvious although you may not see it, surface moisture is attached to the surface of the sample and its presence affects the weight of the substance. Surface moisture is most easily removed in a moisture analyzer.
- Absorbed Moisture. This is moisture lurking in pores or capillaries of products. This means it is more difficult to measure. Absorbed moisture can be inside the substance particle (think wet soil) or in tiny capillaries or crevices of roughly-shaped substances. Determining absorbed moisture content takes more time than required for determining surface moisture.
- Chemically Bonded Moisture. Also called water of crystallization these are water molecules that are chemically bonded to a material at the molecular level. An example is gypsum. Measuring moisture content involving water of crystallization is more complex than surface and absorbed moisture because the substance may also have surface and absorbed moisture as well. It encompasses three steps - accounting for surface moisture, absorbed moisture and water of crystallization.
The Moisture Analysis Process
From the above discussion you may be led to the thought that removing moisture from a sample will change its weight, thereby enabling researchers and QC personnel to determine the moisture content of the sample.
And you'd be right!
Just as laundry coming out of the washing machine is heavier than after being pulled from the dryer, so too is the weight of the sample being analyzed. Although, again, we're taking in grams and milligrams vs. pounds of towels and sheets.
Moisture analyzers such as the models offered by Tovatech operate on what is called the thermogravimetric method, a fancy word meaning loss of weight on (heated) drying, abbreviated as LOD. Simply stated, they calculate the before and after weight with the difference denoting the sample's moisture content.
These precision weighing units consist of two critical components. The first is a precision analytical balance, the second is a source of heat usually supplied by an accompanying halogen or infrared heater.
Moisture analyzers can be programmed to conduct analysis according to accepted standards, and can also be programmed to include new procedures developed in the QC or product development labs.
Moisture analyzer programming parameters include recommended sample weight, drying time, drying temperatures and how the heat is applied, called the drying profile. This latter point is often if not always determined based on the type of sample as mentioned above discussing types of moisture. It also is governed by the particular characteristics of the sample.
Moisture Analyzer Drying Profiles
Moisture analyzers offered by Tovatech allow users to set drying profiles to accommodate the types of moisture in substances being analyzed. We go into more detail on this in our post on setting up a moisture analyzer but here’s a summary of four basic drying profiles:
- The most commonly used mode and suitable for most substances is what is called standard drying.
- Samples that do not tolerate the full power of the halogen heater should be analyzed using the soft or gentle drying mode. This helps protect sensitive samples from decomposing or forming a protective “skin.” In the soft drying mode samples are heated evenly from the outside to the inside by heat convection.
- For samples with moisture content between 5% and 15% fast drying should be selected. This setting quickly raises the temperature to approximately 40% above the set temperature (called preheat) to rapidly drive off the bulk of the moisture and then returns it to the set temperature to approach the end point slowly.
- Samples with more than 15% moisture call for step drying. One example of step drying is for measuring two different types of moisture in the same sample - see chemically bonded moisture above. The first step is a low temperature to remove surface moisture then it ramps to a high temperature to remove water of crystallization. Some moisture analyzers display both moisture content counts.
The Importance of Correct Moisture Content
Few industries are exempt from delivering products with the correct moisture content. In addition to regulatory compliance reasons why include shelf life, quality, pricing, weight and the ability of a product to be processed.
At a practical day-to-day level consumers don't want to pay for water when buying a product by the pound. That's why, for example, labeling on a package of chicken thighs at the supermarket may have the notation "less than 7% retained water."
Excess moisture can speed the development of mold in baked goods, can cause salt and sugar to harden in the container, and can quickly turn a bag of Portland cement into a block.
Too little can mean the manufacturer is delivering more product than being paid for.
More critical, perhaps, is acceptable moisture content in pharmaceuticals and other tightly regulated products.
Periodically checking moisture content of samples prior to packaging by using a precision moisture analyzer allows manufacturers to quickly modify their procedures along the production line. Precision moisture analyzers are also used at the receiving dock to determine the acceptability of materials before they enter the process lines.
For an example of how a moisture balance is put to practical use see our post on the use of these instruments for quality control during injection molding operations.
How to Maintain a Laboratory Moisture Balance
Step 1: Calibrating a Moisture Balance
Moisture balances are delicate instrument capable of weighing in milligrams or calculating to 0.001% moisture. Because of this it's not ju
st a matter of taking them out of the box and plugging them in. Without going into detail in this post, the effect of gravity varies throughout the world and because of this the moisture analyzer must be calibrated before it is put into use, and recalibrated periodically. This interesting subject is covered more thoroughly in our post how to calibrate a moisture analyzer.
Step 2: Cleaning a Moisture Analyzer Weighing Mechanism
Cleaning the unit is critical. While steps are recommended in user manuals cleaning involves removing parts including sample dishes, dish retainers and holders, draft shield and the associated base plate. These can be individually washed using soft cloth dipped into water containing a mild liquid dishwashing soap and wrung out. Solvents or aggressive cleaning agents should not be used.
After cleaning wipe the components with a cloth dampened with fresh water then dry them with another soft cloth.
The same cautions apply to the body of the moisture balance. If you spot loose crumbs or residue remove them with a soft brush or vacuum cleaner before wiping them down with the detergent solution. Use extreme caution to ensure no liquid penetrates the device.
Step 3: Cleaning a Moisture Analyzer Temperature Sensor
The moisture balance temperature sensor is not only crucial to conducting a moisture analysis it is an extremely delicate component of your equipment. Along with the halogen or infrared lamp, it is located in the lid. Cleaning is done in the same fashion as for the base of the unit. The halogen lamp itself should not be touched or damaged.
Moisture analysis is an exacting process requiring precision equipment operated and maintained by trained personnel. A major advantage of laboratory moisture analyzers is that once the analysis method is programmed into the instrument, the technician needs minimal training and expertise to test samples and obtain relatively reproducible results.
Tovatech offers a range of laboratory moisture analyzers with halogen and infrared heaters, and is prepared to provide assistance in selecting and using them in your laboratory or quality control department. Contact our scientists for unbiased advice.