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Laboratory Electronics


  • what is a material moisture?
  • control of temperature indication reliability
  • choosing a weighting dryer
  • drying parameters
  • preparing samples for drying
  • example drying parameters settings

Materials’ moisture

Materials’ moisture is one of criteria deciding about substance quality. In many cases determining moisture has to be performed quickly and in the most reliable way. It concerns all laboratories and research centres connected directly with production. Using in such cases a traditional method which is based on weighing, drying for few hours in a stove and again weighing is impossible. Time needed for determining moisture using such method is too long. An ideal device allowing on taking a quick and accurate moisture measurement is then a weighting dryer. In the weighting dryers just like in a traditional method a thermogravimetry effect is used.

Thermogravimetry - is a process which determines mass loss the appears during substance heating. During such process a sample is weighted before and after heating and then a difference is calculated from those two specified weights. The name is derives from a Latin three part name:

thermo = heat, gravi = weight, meters = method

What is material moisture


Material moisture is a set of all components that appear in a sample that evaporate during a process of its heating. It leads to mass decline. The majority of users do not notice a difference between moisture and water content in a sample. In the thermogravimetry methods (also a weighting dryer) all changing components evaporate during sample heating process. So moisture content is not only water evaporation but also fats, oils, alcohol, organic solvents, spices, and other elements that can be created as a result of a thermal break down (burning products). Total loss of all elements results in effect in mass decline. The difference in masses before and after drying allows on determining moisture. There is no possibility of selecting only pure water loss from loss of other elements.

Methods of defining moisture

There are many methods of determining moisture content in materials. Generally it is possible to divide those methods in two categories: absolute moisture defining and deductive moisture defining. Applying the absolute methods the moisture contents is marked directly i.e. as a proportion of mass before drying to mass after drying. In case of deduction methods the moisture content is marked directly. The physical properties are measured that are associated with humidity in a studied substance (i. e. electromagnetic rays absorption or electrical conductivity).

A weighting dryer as a device for moisture measurement

A weighting dryer consists of a precise laboratory scale and a drying chamber connected to it, which assures stable drying temperature during the measurement. Such construction causes for a relative humidity measurement to differ from a method acknowledged as a traditional one.

A weighting dryer principle of working comes down to:

  1. weighing an examined sample: before, during the process of drying till its total drying without the need for taking out from a stove,
  2. automatic measurement finishing when total moisture evaporation from the sample takes place (drying to solids) or when set drying time passes,
  3. counting the drying results according to a formula for an adopted way of work,
  4. transferring measurement data to a printer or to a computer where there is need for preparing measurement documentation.

Control of temperature indication reliability

A new weighting dryers’ element is an outside thermometer PT 101 which fulfils a control function. It is a perfect solution for those users that have to exactly control drying temperature’s course.

The thermometer’s measurement element is a resistor temperature sensor PT 1000 of 12 cm in length that which has a stainless steel casing of 3 mm in diameter.

The sensor is connected with a reading head using a cable of about 1m in length of  average mechanical resistance.

An additional function of this thermometer is a possibility of drying material temperature measurement. For substances dark in colors during drying appears an effect of heat consumption. It leads to unintentional temperature rise of a dried sample.

The moisture result then may not necessarily reflect the actual state. Basing on a difference between a set temperature and a real temperature (measured with a control thermometer) can determine a drying temperature correction coefficient.

Correction coefficient = (T measured - T set) / T set


  • T measured - temperature read from a thermometer,
  • T set - temperature set on a weighting dryer.

For every produced weighting dryer at present there can be a thermometer attached which is an additional equipment.

Choosing a weighting dryer

A weighting dryer being a universal work tool connects in itself two functions:

  • allows on accurate mass measurement with 1 mg accuracy (realizing a scale function),
  • it defines humidity in samples (realizing a drying function).

When deciding to purchase a certain type of a weighting dryer it is essential to know scope of work that are going to be preformed using the device.

If the weighting dryer is going to realize only drying process it is sufficient usually to have small maximum load scale i. e. 30g for WPS 30S. When stating moisture content usually small amount of applied load is used. It is possible then to not use a calibration weight since for humidity calculations proportion is important: in case of material drying need in very high temperature there is a special version of a weighting dryer that posses such possibility (drying up to 250°C), in case of an additional weighting dryer usage as a scale it is necessary to have a calibration weight. It is used for scale calibrating, eliminating gravitation acceleration influence. The calibration should be performed if the outside conditions changed considerably i. e. temperature of about 5 -10°C or a weighting dryer was moved to another work place.

Scale calibration- it is a set of operations that set relations between a displayed value by a scale and a standard mass (calibration weight) that provide scale load and performing display corrections if such need occurs. A weighting dryer’s scope of weighting designed simultaneously for stating moisture and sample weighting should be big i.e. 110g  for WPS 110S or 210g for WPS 210S.

A weighting dyer’s legalization

One should definitely distinguish between a weighing and a drying function. The Measuring Office’s procedure concerning the weighting dryers’ legalization is limited to specifying scale errors of meteorological parameters.

  • Max 30/110/210g
  • d = 1 mg,
  • e = with 10 mg,
  • class of accuracy II

All produced weighting dryers have the certifications of type issued by the President of GUM 887/98 (The Central Measuring Office) decision number 65/95 receiving marks RPT 9553. The legalization does not concern the errors in determining moisture that can be an effect of an incorrect drying method (sample preparation, not properly selected drying temperature etc.).

Selection of drying parameters

Selection of a drying method

During a process of drying in order to obtain constant mass the process of determining moisture is finished when the 3 following results of dried mass sample will be the same. During drying in a provided time the process of assuming moisture ends when the set drying time passes.

Choosing calculation algorithm

The weighting dryer allows on stating moisture offering 7 different possibilities. Depending on the requirements one should before beginning drying choose one of them.

A. Drying for obtaining solid mass

                       Mass of evaporated water
      1. in[%] ----------------------------- x 100 %
                           Initial sample mass 

                        Current sample mass
      2. in[%] ----------------------------- x 100 %
                          Initial sample mass

                     Mass of evaporated water
      3. in[%] ----------------------------- x 100 %
                          Final sample mass

B. Drying in an assumed time or earlier to obtaining constant mass

                     Mass of evaporated water
      4. in[%] ----------------------------- x 100 %
                          Initial sample mass

                         Current sample mass
      5. in[%] ----------------------------- x 100 %
                          Initial sample mass

                     Mass of evaporated water
      6. in[%] ----------------------------- x 100 %
                          Final sample mass

C. Drying in a set time

                     Mass of evaporated water
      7. in[%] ----------------------------- x 100 %
                          Initial sample mass

Selection of parameters for weight drying work

Drying temperature

A value depends on the type of material. Too low temperature results in partial water evaporating (sample not dry enough). Too high temperature results in material burning effect (sample overheating) or a possibility of other chemical reactions occurring. The drying temperature is provided in trade or in factory norms. If there are no norms they should be picked experimentally. The temperature often used is 104°C. It is a temperature which favors giving back free water contained in the sample.

Weighting dryer graph 1

The drying temperature using a traditional method “stove” given by the trade or factory norms. It is often too low for drying in the weighting dryer and its optimal value should be settled experimentally. The temperature value is set from a weighting dryer’s keyboard. It is automatically stabilized through a regulation set. The set scaling takes place in a factory in comparison to a point that lies approximately 1 mm above a middle of an empty scale pan. Because of the temperatures distribution in a drying chamber and the physical features of a studied material (sample layer thickness, sample color) this temperature may slightly differ from the real temperatures of a dried material.

Sampling time 

It is a time interval given in seconds between the following dried sample mass measurements. It is a criterion based on which the weighting dryer assumes for the sample to be dried. The too short sampling time results in premature measurement finishing. The too long sampling time will result in oscillating around a final result. One should remember that the measurement ends when 3 following humidity measurements are the same. It practice it means that with a set sampling time to 2 sec. drying will be finished if for 6 sec. the same mass result is going to appear. If the sampling time is set to 20 sec. then through 1 minute the mass result must be unchangeable.

Weighting dryer
Correct choice of drying parameters

Weighting dryer
Too short sampling time, too low drying
temperature, too big sample mass

Weighting dryer
Disturbances in a drying process, too
long sampling time

Sample properties

A sample mass (depends on a material type) should be chosen in such way so a thin layer covers the scale pan. In case of materials that do not give back water easily the sample mass has a very big influence on a measurement result. Since humidity is an average measurement (recounted accordingly to mass indications) one should not use weights that are too small below 1 g. Weighing and at the same time drying of such small masses can be burdened with exaggerated relative error.

Weighting dryer sample properties

Black coloring – it allows on consumption of more heat. Sample overheating is possible since with a drying temperature set to 120°C the sample consumes heat reaching temperature of approximately 135°C. In such cases a drying temperature correction coefficient should be determined. From big amount of heat provided the material consumes considerable amount of it (picture A). For the materials of light coloring the effect of heat consumption does not apply and has little effect on the results concerning materials humidity (picture B).

Weighting dryer black coloring

Granularity– is recommended for drying fragmented samples. The results received  are characterized by lower result scattering and shorter time of measurement endurance.

Material consistency
– is a specific characteristic of a certain material. One should remember that materials of compact consistency (clay, molding masses) loose moisture a lot harder.
Time of measurement lasting

In order to obtain constant mass in case of drying the measurement endurance time depends on choosing the drying parameters such as sampling time, drying temperature and the sample size. In case of drying in a set time of the measurement endurance is specified by a user.

Preparing the samples for drying

When preparing a sample for humidity measurement one should pay attention to few facts. If not paying enough attention to this phase of preparation work it is possible to at the beginning of the drying process to have a sample of false moisture.
If the sample is to be reduced (i. e. ground) before drying it should be prepared in such way so it does not generate heat. It should be remembered that the heat generated in a process of preliminary processing causes partial moisture loss before an appropriate drying.

Sample taking 

A source from which a sample will be extracted and prepared plays a very important role in and credibility repetitiveness of the results. After sample extraction it should represent the analyzed product as a whole.

In the majority of cases the product can be homogenized by mixing or stirring before sample taking. It is also common to take few samples from different product parts or from defined intervals and calculating an average from the obtained values. A different way to be sure that the sample is reflects the product is mixing few samples extracted from different areas of the product. Which one of the mentioned above is most effective for individual usage depends on the requirements of the analysis performed.

When the sample is being extracted and later on prepared it is important that it does not loose moisture as a results of the outside conditions’ influence. Therefore the sample should be extracted and prepared as quickly as it is possible. It is recommended that only one sample is prepared at a time. 

Extracting few samples simultaneously requires preparing hermetically sealed containers (desiccators). The samples that are before drying will stay in them. However one should make sure that the containment conditions do not interfere with sample status or conditions. Hot substances loose moisture very fast. If they are being kept in a wrapping, moisture can condense on a container’s walls. Before analyzing such a sample the condensed moisture must be stirred back into the sample. If the condensed moisture is not stirred back into the sample a value measured by a weighting dryer will be too low. The wrapping’s overall dimensions have to be adequate for the number of samples. If the wrapping is too big and not completely filled then the sample can change its moisture.

Material division

All of the materials that we specify humidity for can be divided to 3 groups:

  • solids (i.e. paper, blotting-paper, wood),
  • loose materials (i.e. cereal, sands, moulding masses, tobacco, powders),
  • semi-fluid materials (i.e. butter, margarine, yogurt etc.).

Each one of the mentioned above materials is characterized by different work method. It consists of few factors such as: 

  • sample size selection (....),
  • sample placement on a scale pan,
  • drying using additional elements type: quartz sands, blotting-papers etc.


Depending on a structure of a solid body (compact, loose) a process of defining humidity runs quicker or slower. The surface value of a solid body decides about the speed of drying and measurement accuracy. The surface of the solid body should be as big as possible. Because solids give back moisture by external surface sample thickness plays an important role.

Weighting dryer solids

Loose bodies

Dried are in natural form in which they occur or after grinding down. Grinding down the sample favors obtaining smaller dispersions between the following measurements. The sample’s mass cannot be too big. The sample should cover in thin layer the whole surface of a scale pan. The dark coloring materials absorb heat during the process of drying. The coefficients correcting the drying process should be determined for them.

Semi-fluid bodies

The semi-fluid substances are dried in such form in which they exist. Big amounts of fat existing in some substances make the process of defining humidity much harder. In such cases additional elements should be used that increase sample’s active surface through which moisture is given away. Such elements include quartz sand, blotting paper. Before specific drying additional elements should be dried so that their humidity is close to zero.

Weighting dryer semi-fluid bodies

Scale pans in a drying process

Standard element existing in a drying process is a scale pan. Made out of aluminum foil has 92 mm in diameter what provides a lot of space for free sample arranging.

Using the same scale pans for numerous dryings may result in leaving pieces of a previously examined sample. If the scale pan has not been completely cleaned or if some of the cleaning agents stayed on it, it may result in receiving false results (uniqueness).

Sample arrangement on a scale pan

In order to obtain repeatable results one should use a sample thinly spread on the scale pan. The sample should be adjusted to the scale pan quickly enough so it does not loose its moisture before analysis. If the sample is placed randomly, insufficiently, unevenly then in result heat conducting to the dried substance is going to be uneven. As a result the sample will not be dried completely or the drying will extend in time unnecessarily.

Weighting dryer sample arrangement 1

If the sample is built unevenly on the scale pan then the upper layers receive more heat than the lower ones. The sample can burn or its surface can form a crust (impervious layer) which makes giving away moisture from the lowers parts impossible. 

Weighting dryer sample arrangement 2

If we apply a very thin sample layer or a crust form on the sample’s surface then it would make it hard or even impossible to give away moisture.

Weighting dryer sample arrangement 3

Preparing a weighting dryer for work

A weighting dryer as every precise electronic scale one should put in a place where there is no vibrations and shielded from blasts of air. The temperature changes in a room where the weighting dryer works cannot be too big.
When taking a series of measurements it is recommended to reject 1 measurement because of the possibility of making an additional error resulted from not reaching stable temperature of the drying chamber.

Preparing a weighting dryer for work

The weighting dryers WPS series determine a sample’s initial weight, which is necessary for judging the results only once with a cover lowered. All later calculations performed during the analysis are performed using this original value.
If sample placing on the scale pan takes too much time then part of the moisture evaporates from the sample before its analysis. As a result we receive a result informing about too low moisture content. This can raise doubts about correctness of the weighting dryer’s work. If the sample consists of highly volatile matter then it is recommended to place the sample on the scale pan outside of the chamber.

Sample drying settings for different materials

Dried grain 5-7 110 10 Grind a sample
Semolina 2 - 2,5 130 10 Grind a sample
Wheat grain 2 - 2,5 130  10 Grind a sample
Barley grain 2 - 2,5 130 10  
Rock powder  8 -10  160  15  
Coffee 2 150 10  
Instant coffee 5 10 150 Grind a sample
Whole bean coffee 3,5 - 4  120 15 Grind a sample
Ground coffee  2 - 3  106  5  
Cocoa powder 2,5 105 10  
Cocoa grain 4 -5 130 10 Grind to fine powder
Limestone 12 -14 160 10  
Powdered coal 4 160 10  
Loess 10 -15 160 15  

Low-fat powdered milk
4 90 15  
Melted and hard cheeses 4 - 5 130 5 Mix with sand
Whey concentrate 2 -3 90 10  
Whole milk 5 110 15 Mix a sample
Multivitamin  3 -3,4 115  10 Crush into a coarse-grained powder
Orange juice concentrate 2 - 3 115 20
Mix a sample
Quartz sand 10 -14 160 10  
Oily seeds 3 - 4  90 15 Grind a sample
Rice (boiled) 3,5  105  10 Grind a sample
Camay soap, white 3 120  15 Peel off the pieces
Sesame seeds  130  20  
Soya flour approx. 4,6  95  10  
Sunflower seeds 3-3,5  100  15 Grind a sample
Rape 5 - 8  130  40 Without breaking up
Starch glue 1,5  100  20 Intermix
Active carbon 10  80  10  
Cotton 3 - 4  110  10 Grind a sample
Pea 3,5  135  10 Grind a sample
Tobacco 1,5  100  15 Tear into small pieces
Textile material 0,8 -1,2  85  10 Withdraw the fibres
Dry yeast 150-160  80 15 Crush a sample
Dry apple pomace  5 - 8  100  10 Sample break up
Wet apple pomace 5 - 8  100   10 Sample break up
Butter 2 - 5  138  15  
Mustard 2 - 3  130  20  
Oat flakes 2 - 4  120  15  
Yoghurt 2 - 3  110  15  
Margarine 3 - 4  138  20  
Walnuts 2,8  100  20 Crush into a coarse-grained powder
Washing detergents 160  15  
Wheat flour 130  10  
Bran wheat 4,0  150  10 Grind a sample
Cellulose pulp 2,5 130 20  
Beet sugar 2 110  20