EVALUATION OF MEASUREMENT UNCERTAINTY FOR THE MOISTURE EVALUATION OF MEASUREMENT UNCERTAINTY FOR THE MOISTURE CONTENT

T. Sinhaneti, P. Phuauntharo

Humidity Laboratory, Thermometry Metrology DepartmentHumidity Laboratory, Thermometry Metrology DepartmentNational Institute of Metrology (Thailand) National Institute of Metrology (Thailand)

ABSTRACT

The moisture content is normally estimated from a measurement of mass of

ABSTRACT

The moisture content is normally estimated from a measurement of mass of wet sample, mass of dry sample and mass of container. A well-known method is the loss on drying method based on the weight loss of substance after a is the loss on drying method based on the weight loss of substance after a drying process. Therefore an uncertainty of the measurement is depended on an accuracy of weighting of the three parameters as well as a method precision. In this work, the moisture content of a sample is measured based on two loss on drying methods i.e. oven method and moisture analyzer method. Then the measurements uncertainty is evaluated for the moisture content both Then the measurements uncertainty is evaluated for the moisture content both on dry basis and wet basis. For dry basis, the document “NATA Guide to measurement uncertainty” is approached, while an analytical method based on measurement uncertainty” is approached, while an analytical method based on the document “Guide to the Expression of Uncertainty in Measurement (GUM)” is applied for wet mass basis. Based on facilities of National Institute (GUM)” is applied for wet mass basis. Based on facilities of National Institute of Metrology (Thailand), all sources of uncertainty especially accuracy and weighting precision of the weighting instrument are analyzed. The overall uncertainty for moisture content of the sample is estimated to be within 0.07 % d.b. and 0.2 % w.b. with the confidence level of the measurement of 95%.

PRINCIPLE

Moisture content through loss-on-drying methods can be defined as a percentage of its wet mass “wet basis” or dry mass “dry basis” percentage of its wet mass “wet basis” or dry mass “dry basis”

(1)%100%100.)basis”(w.b“wet content Moisture w×

+=×=

w

mm

m

m

m

(2)

%100%100.)basis”(w.b“wet content Moisturem

×+

=×=

dw mmm

%100%100(d.b.) basis”“dry content Moisture w×=×=

− dm mmm(2)%100%100(d.b.) basis”“dry content Moisture

d

w×=×=

−

d

dm

m

mm

m

m

Molecule of sample mw

HeatingMolecule of water

m mmm md

Figure 1: Moisture content measurement by loss on dying method.

MEASUREMENT METHODS

Classical oven method:Classical oven method:• Widely usedas reference method• Using a high accuracybalancefor• Using a high accuracybalanceformass measurement of sample• Drying a samplein a separateoven• Drying a samplein a separateovenat temperature above 100°C untilreachinga constant final mass.• Typical uncertainty: 3 % of reading• Typical uncertainty: 3 % of reading

Figure 3: Weighting the sampleFigure 3: Weighting the sample

Moisture analyzer method : • Combines an infrared heating element and a top pan balance in a single unit.• Real time display of moisture content• Real time display of moisture content•Typical uncertainty: 9 % of reading

Figure 2: Moisture AnalyzersSource: SartoriusSource: Sartorius

EVALUATION OF MEASUREMENT UNCERTAINTY FOR THE MOISTURE EVALUATION OF MEASUREMENT UNCERTAINTY FOR THE MOISTURE CONTENT

Phuauntharo, U. Norranim

Humidity Laboratory, Thermometry Metrology DepartmentHumidity Laboratory, Thermometry Metrology DepartmentNational Institute of Metrology (Thailand) National Institute of Metrology (Thailand)

EVALUATION OF UNCERTAINTYEVALUATION OF UNCERTAINTY

A sample was used to measure its moisture content by means of the two methods. Using a moisture analyzer, the moisture content of the sample is 4.85 % d.b. and 4.62 % w.b. For oven method, the results of 5.02% d.b. and 4.78% w.b are found.The uncertainty of measurement can be evaluated in two different methods which

• For dry basis (according to NATA guideline)

The uncertainty of measurement can be evaluated in two different methods which are given below :

• For dry basis (according to NATA guideline)

Source of uncertaintyStandard uncertainty

Oven methodStandard uncertainty

Moisture analyzer Source of uncertainty Oven method+ ( % d.b.)

Moisture analyzer + ( %d.b.)

Accuracy balance of mass of container (ma)(0.00012 g for balance and a0.002 g for analyzer balance)

0.00001 0.00024(0.00012 g for balance and a0.002 g for analyzer balance)

0.00001 0.00024

Repeatability of balance reading at ma

(0.0001 g for balance and 0.0006 g for analyzer balance)0.00001 0.00007

(0.0001 g for balance and 0.0006 g for analyzer balance)Accuracy balance of mass of container with wet sample (mb)

0.0003 0.0050

Repeatability of balance reading at mb 0.0002 0.0015Repeatability of balance reading at mb 0.0002 0.0015

Accuracy balance of mass of container with dry sample (mc) 0.0003 0.0052

Convection current (0.01% of dry material) 0.0006 0.0006Convection current (0.01% of dry material) 0.0006 0.0006

Absorption of moisture during cooling (0.04% of md) 0.0024 0.0025

Achieving constant mass 0.0318 0.0324Achieving constant mass 0.0318 0.0324

Weighing resolution (0.0001 g for balance and 0.001 g for analyzer balance)

0.0002 0.0026

Combined standard uncertainty 0.0319 0.0334Combined standard uncertainty 0.0319 0.0334

Expanded uncertainty 0.064 0.067

Table 1: Uncertainty budget of moisture content in dry basis using the oven method and the analyzer method (∼1 % of reading)

• For wet basis (According to GUM)

Standard uncertainty Standard uncertaintySource of uncertainty

Standard uncertaintyOven method

+ (% w.b.)

Standard uncertaintyMoisture analyzer

+ (% w.b.)

Repeatability of balance reading at ma 0.001 0.023Repeatability of balance reading at ma

(0.0001 g for balance and 0.0006 g for analyzer balance)0.001 0.023

Weighing resolution (0.0001 g for balance and 0.001 g for analyzer balance)

0.002 0.023(0.0001 g for balance and 0.001 g for analyzer balance)Accuracy balance of mass of container with wet sample(mb)(0.00012 g for balance and a0.002 g for analyzer balance)

0.003 0.045

Weighing resolution 0.002 0.024Weighing resolution 0.002 0.024

Accuracy balance of mass of container with dry sample (mc) 0.003 0.048

Weighing resolution 0.000 0.001

Accuracy balance of mass of container (ma) 0.000 0.002

Method precision 0.028 0.027

Combined standard uncertainty 0.028 0.082

Expanded uncertainty 0.06 0.16

Table 2: Uncertainty budget of moisture content in wet basis using the oven method (∼1 % of reading) and analyzer method (∼3 % of reading)(∼1 % of reading) and analyzer method (∼3 % of reading)

CONCLUSION

• For dry basis, the both method yield the same level of uncertainty (∼1 % ofreading)dueto thecomponentof achievingmasscomponent.

CONCLUSION

reading)dueto thecomponentof achievingmasscomponent.• For wet basis, the oven method yields about 3 times better uncertainty comparing to

the analyzer method (∼3 % of reading) due to higher weighting accuracy.

• A comparisonto indirect methodswill be performedwith purposeof providing

FUTURE WORK• A comparisonto indirect methodswill be performedwith purposeof providing

traceability to physical method of moisture content measurement.• Improvingthemeasurementfacility to providemeasurementservicefor customers.• Improvingthemeasurementfacility to providemeasurementservicefor customers.• Studying the reproducibility of moisture content for agriculture products.

REFERENCESREFERENCES

1. GUM Guide to the Expression of Uncertainty in Measurement2. NATA Guide to Measurement Uncertainty for Construction Materials Testing2. NATA Guide to Measurement Uncertainty for Construction Materials Testing3. Filomena C. Mouro, Sandra C. Calisto, Maria A. Trancoso , XIX IMEKO World Congress 2009; 2630-2633