How to Use a Moisture Analyzer for Dry Bulk Powders

September 8th, 2010

The moisture content of virtually any product on the market has a bearing on the product’s quality, shelf life and a host of other considerations.   Ideal moisture content varies, of course, with the product under consideration.  Ideal moisture content is determined by and in some cases governed by industry associations and regulatory authorities.  Proof of compliance is eased when a moisture analyzer is employed on a regular basis as batches are prepared for packaging, whatever the product, and records are kept.

Moisture analyzers such as the Kern models offered by Tovatech operate on the thermogravimetric principle using halogen heaters in combination with an analytical balance to precisely determine the percentage of moisture in a sample during a programmed testing cycle.  Simply stated the sample is weighed before moisture analysis begins, and after the cycle is completed. The loss in weight is the moisture content.

Let’s Take Cement as an Example

Hygroscopic material such as cement used in construction is a good example of where excess moisture content can quickly change a bag to a block if the bag is left open for a period of time.  Cement quickly absorbs moisture from the surrounding atmosphere. The recommended moisture content for cement is 0.8% and this must be checked on a regular basis as each new shipment of bulk cement is packaged in bags for delivery.  While bulk dryers can do this job, that process takes much more time than is required for a moisture balance.

As with all dry bulk compounds, the constituents of the cement must be thoroughly mixed before batch testing is done. Because moisture content may be lower at the surface than inside the batch, a representative sample should be extracted for testing.

As recommended by Kern place 8 – 12 grams of cement on a clean sample plate after performing the tare function to cancel out the weight of the sample plate.

  • Center the sample and level it to the extent possible.
  • For repeatability sample sizes should be exactly the same from test to test.
  • Individual samples should not be taken in advance and set aside.
  • Larger sample sizes mean that a larger volume must be processed, which can decrease test performance and accuracy.  Samples that are too small also impact repeatability.

Set the moisture analyzer temperature to 138˚C (280˚F) and the timer for 5 minutes.  Carefully place the sample pan into the halogen chamber, close the lid and start the process.  The unit’s halogen lamps remove moisture until the pre-set drying time is reached.  The moisture analyzer then computes and displays the sample’s moisture content.  For record keeping the moisture analyzer can be connected to a PC and statistics printer via an RS 232 interface.

Maintaining Precision

Before using a moisture balance it must be calibrated to accommodate the effect of gravity at its specific location.  This exercise can be completed by retaining an outside calibration service.  Recalibration is also recommended on a regular basis as spelled out in the company’s operations manual.  Environmental changes can affect the accuracy of the analytical balance, meaning that changes in the environment also call for recalibration.  Records must be maintained.

In addition to calibrating for weighing accuracy, calibration is required to assure temperature accuracy.  Some moisture balances offer companion temperature calibration sets that can be operated by in-house personnel.  Using the Kern MLB-N moisture analyzer as an example, allow it to cool at least 3 hours then insert the measuring probe as instructed by the manual. Push the measuring probe as closely to the thermal sensor of the moisture analyzer as possible. The temperature is measured at two points and by following the instruction manual can be corrected at both temperature points.

In summary, moisture content is a quality control issue across a broad range of laboratory and production operations. Procedures for conducting moisture analysis and record keeping must be a part of an organization’s standard operating procedures.  A compact moisture analyzer is a valuable tool for any organization requiring moisture documentation.

Where do you use a moisture analyzer in your operations?  What procedures are in place for record keeping and ongoing recalibration of the moisture balance?


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