Moisture Analysers: Getting Started With The Adam Equipment PMB
What is a moisture analyser?
A moisture analyser is a laboratory device, consisting of a weighing and heating element that is used to determine the moisture content of a sample through heating and drying. Modern moisture analysers often use LOD (Loss-on-drying) testing methods over traditional oven testing. While the analysers are called moisture analysers, they not only examine water that is evaporated by the heat, and may include oils, alcohol, flavourings or other components that are changed when heating.
Adam Equipment's PMB moisture analyser
The Adam Equipment PMB Moisture analyser effectively drys samples using heat from halogen lamps mounted above the sample. The user can set parameters for drying the sample that will affect the results. The parameters used for drying the sample are described in detail in the operator manual.
The initial parameters are concerned with the temperature profile; that is if the heat is slowly ramped up to a target temperature, or allowed to change more quickly to reach the target. Or, if the STEP function is used, a first temperature is used for a target followed by a different temperature after a period of time.
The STEP function could allow the temperature to increase to a value greater than the final target temperature initially then settle at the final target temperature as the test progresses.
The user can set the interval time; that is the time interval the results are checked, printed (or downloaded) and stored in moisture analyser's internal memory. If the analyser is set to automatically end the test, the test will stop when the results measured at the selected interval time no longer change more than a preset amount.
If automatic stop is not selected then the test will run a preset time or until the user stops the test.
During any test the results can be output to the RS-232 and USB interface and are also stored in the internal memory at the interval time. The results from the memory can be transferred to a USB memory stick for downloading to a PC.
Sample preparation is the single most important consideration to obtaining accurate results with a moisture analyser. The sample must be prepared so the whole of the sample is heated uniformly and the moisture is near the surface of the material.
For powders, no further preparation is necessary. For most other materials the sample should be ground, chopped, grated or otherwise made as fine as possible. Liquids or paste may need to be spread onto glass fibre pads to distribute them evenly and thinly. Some materials that may splatter as they are heated (oils, butter) may need to be spread on the glass fibre pads or mixed with dry sand. Trial and error is often the only way to find the best method for the material.
When the sample is placed on the weighing pan it should be as thin as possible. However, enough material should be used to ensure good repeatability of the results. Typically samples of 3 to 10 grams are used.
Some materials will require special preparation - for example, when testing cheese. If the cheese is very soft it may be necessary to use dry sand as a buffer material so it does not splatter or form a skin. To do this it would be necessary to prepare the sand by drying it in an oven until you were certain no moisture remains. Put the sand on the sample pan and, and press tare to remove the sand's weight reading from the display. Mix the sand with the cheese so you have smooth mixture, spread this mixture on the sample pan and then do the test. An alternative to this preparation is to spread the cheese on dry spun glass gauze to assist in spreading the heat and preventing splatter or skins forming. Remember to tare the analyser with the gauze in the weighing pan for accurate results.
However some materials can hold moisture within them in a way that is not suitable for this type of test. For example sugar will bind water to the sugar molecules so strongly that heat will not remove the water before the sugar is burned. Some plastics may also not be suitable for this type of method due to the very low percent moisture and the heat required to extract this moisture.
When initially setting the parameters for a material, it is often a matter of trial and error to find the most suitable settings. The PMB uses a default setting for drying test that is suitable for many applications. It is suggested this default setting is used for the first test then the user can adjust the settings as required to try alternate test programs.
If you have some experience with an oven method or other moisture analysers the temperature should be set to these same values. However if you do not have any experience then the initial temperature setting is a guess at first. Try using 110ºC initially and then increase it if there are no signs that the material is burned, or decrease it if the material burns or goes excessively brown. Some material need to be dried at lower temperatures if they form a skin over the surface during drying.
It may be helpful to have the analyser connected to a PC for data collection or to a printer during these tests to help determine the best settings for test.
If you set the parameters for a test to run a maximum time instead of an automatic stop you can observe the results during the test. Begin the test and watch the output from the moisture analyser. After a few minutes the moisture should increase. After some time the moisture will become stable for a long period of time. Terminate the test at this time.
Repeat the test at other temperatures. Generally, try to use the highest temperature possible that does not damage the material. After you are getting consistent results with 5-10g samples you can try reducing the size of the samples. Often good results are obtained using samples of 2-5g.
By observing the results the time and temperatures needed to finish test it is possible to determine the typical time of a test, the typical moisture and the interval time that will be needed to determine the automatic stopping point.
Know the material you want to analyse
As described above, some materials are not well suited for this method of moisture determination. It is important to know enough about the materials so that you do not endanger or inconvenience others. For example some solvents have very low flash points that are not suitable for drying with heat sources that can reach 100ºC. Other materials will cause odour or fumes that can be dangerous or objectionable. Care must be taken to ensure that heating of the sample is not dangerous or likely to cause damage to the environment.
Within PMB moisture analysers, the temperature of the weighing chamber is measured by a sensor just above the samples. As the temperature approaches the final value the lamp will be cycled on/off to maintain the temperature at the set point.
When set for Single or Step heating the initial increase in temperature will happen quickly, it will usually take about 1 minute to gain the first 90% of the final temperature and then another 30 seconds to reach the final temperature. This method helps avoid the temperature overshooting the set point and thus avoids damaging the sample.
If the Rapid function is enabled the heat in the chamber is allowed to overshoot the target for a short period of time then will settle at the target value. This allows quicker heating of the sample.
RS-232 data from the PMB analyser
The PMB moisture Analyser will output the current results using the interval time set for the test. These results can be used to plot the progress of a test or to keep records of a test. The data can be collected via the RS-232 and displayed using AdamDU, Adam Equipment's own data collection software.
Effect of calibration on moisture determination
The PMB moisture analyser can be used as a laboratory balance if it is calibrated with a mass standard of OIML Class F2 or better. The results of a moisture determination test are not dependent upon the accuracy of the weight values as the results are a ratio of 2 weights. The temperature accuracy of the drying chamber can be calibrated using the built in temperature calibration program and an external thermometer designed for the PMB.