Performance Verification of Analytical Balances according to ISO/IEC 17025

 

Verification of Analytical Balances according to ISO/IEC 17025 Standard

 

Accurate analytical measurements are fundamental to the functioning of modern society. Inaccurate results can have an enormous social and economic impact (Prichard et al. “Quality Assurance in Analytical Measurement”, 2007) such as:

“In trade, it could lead to the supply of sub-standard goods and the high cost of replacement with subsequent loss of customers”
“In environmental monitoring, mistakes could lead to hazards being undetected or to the identification of unreal hazards”
“In supply of drinking water, it could lead to harmful contaminants being undetected”
“In healthcare, the incorrect medication or the incorrect content of an active ingredient in a tablet can be catastrophic for the patient”.

 

Calibration and verification are the most important actions to ensure the correct indication of measuring instruments (K. Sommer et al. “Calibration and Verification: Two Procedures

Having Comparable Objectives and Results”; OIML BULLETIN; vol. XLII, number 1, 2001).

They are considered major requirements for the competence of testing and calibration laboratories according to the International Standard  ISO/IEC 17025:

“Equipment used for testing, calibration and sampling shall comply with specifictions…” (5.5.1)  and

“Before being placed into service, equipment (including that used for sampling) shall be calibrated or checked to establish that it meets the laboratory’s specification requirements and complies with relevant standard specifications. It shall be checked and/or calibrated before use”. (5.5.2)

ISO/IEC Guide 2 defines verification as: “Confirmation by examination and provision of evidence that specified requirements have been met”.

Verification is the confirmation, based on evidence (test results) that a certain number of specified requirements have been fulfilled. For example, the verification of an analytical balance  will prove that the performance of the balance is still in agreement with the calibration certificate.

The acceptance / rejection criteria used for the verification of analytical balances are:

i) Trueness (% relative error), comparing to the weights of certified standard weights. The average weight xaverage of ten consecutive measurements of a certified standard weight is measured using the analytical balance. The absolute error is calculated and then the % relative error according to:

 

Absolute error  = x_average – x_t

 

% relative error  = [ (x_average – x_t )/ x_t]  x 100

Where x_t  the true value of  the certified weight and xaverage  the average weight of the certified weight determined experimentally ( ten consecutive measurements of the certified weight using the analytical balance of interest).

ii) Absolute and % relative error are a measure of the accuracy (trueness) of the analytical balance.

The definition of trueness according to VIM 2010 (International Vocabulary of Terms in Legal Metrology)  is:  closeness of agreement between the average of an infinite number of replicate measured quantity values and a reference quantity value.

 

iii) Repeatability  (% coefficient of variance, %RSD), comparing to the weighs of certified standard weights.

The standard deviation from the mean value of ten consecutive measurements of a certified weight is determined. Then the % relative standard deviation (%RSD) is calculated.

Standard deviation = s = [Σ(x_i-x_average)² / (n – 1)]^1/2

Where x_average the average value of the ten consecutive measurements and x each individual measurement and n the number of measurements

% relative standard deviation (%RSD) =  s / x_average* 100

Repeatability  (%RSD) is a measure of the precision of the analytical balance.

The definition of precision according to VIM 2010 (International Vocabulary of Terms in Legal Metrology)  is: closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions.

 

iv) Drift, refers to weight readings that do not stabilize, or unstable readings with no weight applied. All analytical balances show some uncertainty. The clearest indication of a malfunction is a change in the amount of instability.

 

The acceptance / rejection criteria (tolerance) for analytical balances are given in the table below:

 

Acceptance / Rejection Criteria for Analytical Balances
Trueness, % Relative Error	0.01
Repeatability, % RSD	0.0001
Drift, g	0.001

The above procedure was followed for the verification of a KERN 770-13 precision balance. The results are given in the table below:

 

Table 1: Balance Verification Table – Trueness and Repeatability Assays

Balance I.D.		Standard Weight I.D.
Brand	KERN	Nominal Value	99,9996
Model	770-13

Measurement #	Value (g)	Drift (g)
1	99.9998	t = 0     1.0000
2	99.9997	t = 5     1.0001
3	99.9996	t = 10   1.0001
4	99.9997
5	99.9998
6	99.9998
7	99.9997
8	99.9997
9	99.9997
10	99.9997
Average (g)	99.9997
Absolute Error (g)	0.0001
Relative Error (%)	0.0001
Accepted /not Accepted  Trueness	Accepted
Std. Deviation (g)	0.000063
%RSD	0.000063
Accepted /not Accepted  Repeatability	Accepted
Drift (g)	0.0001
Accepted /not Accepted  Drift	Accepted