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How a Moisture Analyzer Works

 

A Quick Introduction to Moisture Analysis

A moisture balance is an essential instrument for R&D labs, quality control departments and any other application where moisture content of a product is critical when developing new, or meeting established manufacturing criteria. Correct moisture content is specified by regulatory authorities and graphic-user-interface-em.jpgindustry trade associations. It covers an almost endless range of products. You’ll find examples in our moisture balance blog posts.

What a Moisture Balance Does

A moisture analyzer provides accurate and precise determination of the moisture content of samples. Two specifications to check are (1) readout, also called resolution or readability. This is the smallest unit that can be displayed on the control panel and is presented in grams and percent moisture.  Repeatability (2) is the ability of a moisture balance to display the same result when the same sample is tested repeatedly. It is expressed as a standard deviation. Other important selection criteria are listed below.

How it Works

Laboratory moisture analyzers available from Tovatech operate on the gravimetric principle, or loss of weight on drying (LOD). The concept is simple: the sample is weighed, it’s heated to evaporate all moisture, and when the drying process is complete the moisture content of the original sample is calculated based on the weight loss. Depending on the model moisture analyzers use halogen, infrared or glass-free metal heaters positioned above a precision balance. Here’s a simplified sequence: 

  1. The balance is programmed to perform a specific analysis (see our posts for examples). 
  2. A sample is placed on a tared sample dish. 
  3. The lid is closed to start the drying process

heating-flexibility.jpgAs the analysis progresses the moisture balance displays the current drying status and keeps the operator updated on each stage of the process. The analysis automatically terminates (a) when drying is complete and the dry weight is stable, or (b) after a fixed time specified by the operator. The final result is shown on the moisture balance digital display.

At the end of the calculation all relevant data concerning initial mass, residual mass, test parameters and results can be transferred to a USB device or to a PC via an RS-232 interface. Data acquisition software allows the operator to produce a graph plotting results against time, providing information on speed of drying and confirmation of dry weight stability. Advanced models allow the operator to view the drying curve (weight vs. time) in real time either on the display or on an attached PC. Test results can be stored in the computer memory for subsequent comparison. All models meet GLP/ISO record keeping requirements.

Selecting a Heating Profile

Different substances react differently to heat in a moisture analyzer. While some models have a fixed heating rate other models allow the operator to select heating profiles that determine how heat is applied to the sample. Heating profiles may be established by trial and error but once established can be stored in the instrument memory and recalled for conducting analyses. Here are four profiles that may have slightly different designations by moisture analyzer manufacturers.flexibility-in-heater-choice.jpg

  • Standard drying heats rapidly to the set temperature and is a widely used process because it is suitable for most substances.
  • Soft or gentle drying is intended for substances that do not tolerate fast temperature increases or that tend to form skin or crust on their surface, inhibiting drying.  The operator can adjust the rate at which the set temperature is reached.
  • Fast heating can be used to accelerate testing of samples with a moisture content between 5% and 15%. The temperature is increased very quickly and briefly exceeds the set drying temperature. Then the temperature is regulated down to the set temperature.
  • Step drying can be used for substances such as gypsum that release free moisture at one temperature and bound moisture at a higher temperature. The analyzer heats to the first set temperature, holds at that temperature for a defined period, then heats to the second set temperature. The individual increments regarding duration or temperature increase can be freely selected. The samples should contain at least 15% moisture.

Calibrating Moisture Analyzers

programmable-methods.jpgAccuracy is a paramount requirement for a moisture analyzer. Accuracy is set and maintained by initial calibration and periodic recalibration, the latter of which is performed based on schedules contained in a company’s operations manual. Two calibrations should be performed, one for weighing and one for temperature. Procedures are spelled out in user manuals.

Externally calibrated moisture analyzers use a separate weight equal to the balance weighing capacity. An example is the Kern MLS 50-3D. The 50 g weight is placed on the tared sample dish and the calibration program activated. The balance goes through the calibration program and will correct itself if necessary with no user intervention. Internally calibrated moisture balances such as the EM 120-HR perform the exercise automatically.

Temperature calibration is internal on the EM 120-HR. Optional temperature calibration kits are available for other models. 

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Selecting Your Moisture Analyzer 

You should know the moisture range that you plan to measure and make sure that the instrument has adequate sensitivity to deliver accurate results. For example, low moisture molding plastics require an instrument with a readout of 0.0001 g or 0.001% moisture whereas foods can be tested with lower resolution instruments. The equipment you select should accommodate these requirements. 

real-time-view.jpgWe mentioned readout (readability, resolution), and repeatability above. Repeatability data improve as sample size increases. The MS-70, for example, has repeatability of 0.05% for a 1 g sample and 0.01% for samples greater than 5 g.  

Drying temperature can usually be set in 1⁰C increments. Lower and upper temperatures vary based on manufacturers. As examples the IL-50.001 moisture balance has a temperature range of 50 to 180⁰C; the EM 120-HR and XM 60-HR ranges are 30 to 230⁰C. 

The memory capability of a moisture balance – that is how many drying programs it can store for easy recall – is a great time saving feature if analyses are performed on a variety of products.

Note the maximum capacity that can be handled by a moisture balance. Your sample weight including the sample pan should not exceed this. 

Please review our selection of moisture analyzers for their technical specifications, standard and optional features. We invite you to download our moisture analyzer white paper for a more detailed look at these precision instruments. And please contact us for answers to your questions or help in deciding which moisture balance provides the best cost/performance ratio for your operations.