Water is essential to life; it plays a critical role in the physical and chemical functions of our bodies, the food we eat, and the materials that surround us. In many industries it is important—if not critical—to measure the water content of substances in order to assess quality, adjust manufacturing processes, and ensure that products meet regulations and guidelines.
The amount of available water dictates the shelf life and stability of many systems; for example, the presence of water in food greatly impacts its susceptibility to chemical, enzymatic, and microbial activity. Water content is also important for the processing and handling of:
• Personal care products
• Pulp and paper products
• Speciality chemicals
Measuring the amount of water contained in certain materials can be very difficult due to the complexity of the water molecule and its strong intermolecular bonding capabilities. In most cases, measurement of water is better defined as the measurement of moisture content, defined as the mass of water per unit mass of dry material.
The MB Series of moisture analyzers (or moisture balances) from OHAUS measure moisture thermo-gravimetrically. Thermo-gravimetric moisture analysis defines moisture as the loss of mass observed when the sample is heated and is based, in theory, on the vaporization of water during the drying process; this measurement does not distinguish weight loss of water from loss of volatile components or sample decomposition.For this reason, moisture content as measured by thermo-gravimetric techniques includes all substances which vaporize when heating a sample and are measured as weight loss during the heating process. Therefore, we use the term “moisture content” rather than the water content when using a thermo-gravimetric device.
What Is Thermo-gravimetric Moisture Analysis?
Moisture content influences the weight, density, viscosity, refractive index, and electrical conductivity of a material. Methods for testing moisture content tend to exploit one or more of these physical or chemical properties. Direct measurements address the presence of water itself, either through its removal or through chemical interaction. The use of a thermo-gravimetric moisture analyzer is a way to directly measure the moisture content of a sample by using the loss on drying (LOD) technique. LOD measures the weight of a sample before and after a drying procedure and uses the weight delta to determine the percentage of moisture as the weight removed by the drying process in comparison to the initial weight of the sample. Typically this process is done in a drying oven with a balance to determine the initial and final weight of the sample and using a simple mathematical calculation to determine the moisture content ([initial weight – end weight]/initial weight). This process typically takes several hours to complete and is vulnerable to user error. A moisture analyzer works on the same principle, but is an automated system that employs a microprocessor-controlled heating element and an analyzer all in one device; by using such a device, the moisture content of a sample can be measured in minutes rather than hours.
Halogen vs. Metal Heating
Thermo-gravimetric moisture analyzers efficiently dry a sample by transferring energy by both radiation (the transmission of energy in the form of waves or particles through a medium—in this case, the sample) and convection (heat transfer by mass motion). In comparison, a conventional drying oven uses mostly convection to dry a sample. Both metal and halogen heating elements radiate energy in the infrared spectrum. (Both methods are employed in the OHAUS MB Series.) Infrared (IR) radiation is part of the electromagnetic spectrum, falling between microwave energy and visible light. Infrared waves include thermal radiation and have the wavelength frequency range from 0.75 micrometers (long wavelength limit of visible red light) to 1.5 micrometers (borders on microwaves). Infrared energy is not visible to the human eye. The red light often associated with infrared heating is actually reflected red light from the visible spectrum. Some moisture analyzers utilize a metal heating element, which is simply a low-resistance piece of metal which converts electricity into heat. Such heaters are ideal for an environment (such as food processing) where the presence of glass components are prohibited due to regulatory or safety concerns. Metal heaters are not ideal as they have a very large thermal mass and take significantly longer to heat up than halogen heaters, and are thereby harder to control and do not provide optimum reproducability in a moisture analyzer. Halogen radiators have a tungsten heating element contained in a compact glass tube filled with halogen gas to preserve the tungsten element. The halogen radiator emits infrared radiation in the short wavelength range of 0.75–1.5 micrometers. The compact nature of the halogen radiator improves the heating/cooling response time, shortening the time for the heating power and ultimately shortening time requirements to complete sample drying. It also allows finer control during the heating process.
Courtesy of Ohaus Europe Gmbh
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