Ensuring Accuracy & Precision in Determining Water Content

Accuracy and precision are used to characterize each measurement or series of measurements. The moisture analyzer is used to obtain the water content determination result. This is then compared to a true value to measure accuracy in the form of a rate of compliance (ISO 5725-1:2002). As such, information on the real value of the product water content is required, which can be obtained through validation of the testing method.

δ = xi - xo

δ – error of water content determination
xi – result of water content determination
xo real value of product water content

The water content determination results are obtained using one and the same device, for one and the same product, carried out in specified conditions by one and the same operator. Precision is then measured in terms of the rate of compliance between these. Precision of the measurements does not refer to the real value or other specified value. It depends on the distribution of random errors (ISO 5725-1:2002). Product heterogeneity can cause the values obtained within one measurement series to vastly differ. Measuring the distribution of results of water content determination provides an estimate of standard deviation.

s –  standard deviation
xi –  successive results of water content determination
–  arithmetic average value of a measurement series of water content determination
n –  series measurement quantity

Moisture Analyzer Operation Test

Drying Temperature Control

Concerns moisture analyzers of MA R, MA X2.A, MA X2.IC.A, MA 3Y series

To check whether the moisture analyzer operation is correct there are two methods available. The first is to ensure that the set analysis temperature, for example, 105 oC, is reached by the heating system of the moisture analyzer. This is done by comparing indications of the control thermometer resting at the location of a disposable pan with the predetermined value.

δT = Ti - To

δT  –  error of measurement of the drying temperature
Ti  –  value of the temperature measured using the GT105k-12/Z control thermometer
To –  value of the predetermined temperature (the drying temperature)

It is necessary to assume a maximum difference of ± 3 oC between the predetermined temperature value and the value measured using the control thermometer. The product analysis will not necessarily be accurate and precise with a positive result of the drying temperature control. Optimization of the drying parameters in reference to the specific product features (structure type, color, susceptibility to radiation absorption) guarantees accuracy and precision.

Procedure

The user manual of each moisture analyzer should be referred to in order to understand the procedure for verification of the drying temperature. The control thermometer during the measurement is presented in the following drawing.

Ensuring Accuracy & Precision in Determining Water Content

Image Credit: Radwag Balances & Scales

Sodium Chloride

As a Reference Material in Water Content Determination

Concerns moisture analyzers of MA R, MA X2.A, MA X2.IC.A, MA 3Y, PMV series

Drying of the substance of known moisture / dry mass content is the second method used for verifying moisture analyzer operation. One such substance is sodium chloride (NaCl) of concentration of 10% ± 0.5%, which is a colorless, clear liquid with a density of 1.07 g/cm3.

Drying  Accuracy

The assumption is that it should be possible to provide dry mass content in accordance with the certified value if the heating component of a moisture analyzer operates correctly as a result of drying of the reference substance (NaCl).

δMC = MCi - MCREF

δDM =  DMi - DMREF

δMC – moisture content measurement error
δDM  – dry mass content measurement error
MCi  –  moisture content result obtained during drying of the reference substance (moisture analyzer)
MCREF  certified moisture content of the reference substance
DMi  – the result of dry mas content of the reference substance (moisture analyzer)
DMREF  certified value of dry mass content of the reference substance

Measurement Precision

Precision of measurement carried out using the moisture analyzer can also be assessed using the reference substance (NaCl). Measurement precision is the difference between the results of multiple dry mass content carried out at the same time and in the same conditions. It can be expressed in terms of standard deviation or maximum range between the measurements.

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Last updated: Nov 29, 2019 at 9:52 AM

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