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USP General Chapter <41>:
Determining of the Operating Range of a Balance 


USP General Chapter <41> was revised on December 1, 2013 and then again for a verbiage change on July 1, 2014. A global team of experts drafted the new version, which is now binding. Here's a synopsis of the changes:
 Renamed from "Weights and Balances" to "Balances", which now indicates its major scope.
 USP Chapter 41 does not reference minimum sample weight any longer, rather it defines Repeatability and Accuracy, which is of crucial importance to determine the Operating Range of a balance.
 USP Chapter 41 now defines that weighing shall be performed using a balance that is calibrated over the operating range and meets the requirements defined for repeatability and accuracy.
 A minimum accuracy of 0.10% for important measurements is now dictated.
 USP states it is not necessary to use a small test weight to assess the repeatability because it's virtually independent of the sample mass within the balance's capacity.
 USP Chapter 41 now applies to all balances that are used for material that must be accurately weighed.



Key Benefits of the Current USP General Chapter <41> Requirements:
 The main goal of this requirement is very simple... to ensure accurate weighing!
 It gives the user a better approach for ensuring a balance's compliance by calculating the measurement uncertainty and will also determine if the desired smallest net weight will meet the USP requirement.



USP Chapter 41: Repeatability
According to the current USP Chapter 41, the "Repeatability" part of the test defines the starting point of a balance's operating range, which is limited to the maximum capacity of the balance and begins at the point at which the balance's repeatability is less than or equal to 0.10%. This demand specifies the usable weighing range of the balance and taking measurements below the starting point of that operating range is not permitted. 

The current USP Chapter 41 requires that the repeatability of a balance be determined based on at least 10 measurements using one test weight. For this test, since it is not necessary to use a small, often hard to handle test weight, one manufacturer recommends to use a test weight at half load in order to combine the repeatability and accuracy tests (the accuracy test is described here). For instance: testing a 220 gram analytical balance, use a 100 gram test weight. 

Repeatability Requirements/Calculations: 
 Repeatability is satisfactory if 2 times the standard deviation (SD) divided by the desired smallest net weight (N) doesn't exceed 0.10%. The "desired smallest net weight" is the smallest weight the user plans to use on that balance (the user dictates this value). This test will determine if that "desired smallest net weight" chosen is satisfactory or not.






 Calculation to determine the actual starting point of the balance's operating range (ORmin)






 If the SD < 0.41d, it must be replaced by 0.41d, where d is the readability of the balance. This special condition was put into place because a balance always has a measuring uncertainty error and the 0.41d accounts for the rounding error.





What does this mean?
The absolute minimum is now defined as 820d, which means that the maximum operating range for a balance could go from 820d to max capacity. However, depending on the installation location and environmental conditions, which can have an effect on the repeatability, the optimal starting point of the operating range may be higher. 





† optimal operating range if the SD from 10 measurements is less than 0.41d
†† actual results may vary depending on the installation location/environmental conditions 


NOTE: Since the test dictates "desired smallest net weight", users must always tare the vessel prior to weighing samples. For all practical applications, the weight of the sample and vessel cannot be combined to be above the operating range starting point. 


USP Chapter 41: Accuracy
According to the current USP Chapter 41, the "Accuracy" part of the test describes the quality of the weight to be used. One measurement is taken with a single test weight, which is required to have a mass between 5% and 100% of the balance's capacity. The deviation of the measured value should be within 0.10% of the test weight value and the measurement uncertainty of the test weight shall not be more than onethird of 0.10%. 


What does this mean?
This means you must use a calibrated weight to prove the uncertainty of that weight. However, you do not need to use a higher class weight, an ASTM Class 2 or OIML F1 or F2 traceable weight with certificate would be sufficient.
For example, if using a 100g test weight, the displayed weight must be between 99.90000 g and 100.10000 g. The calibration uncertainty of the test weight must not be more than 0.03 g. If these requirements are met, then the balance fulfills the accuracy part of this test. 


How to Perform the USP Chapter 41 Test: 

Repeatability Test:
Be sure the balance is warmed up, leveled and calibrated before starting.
 Set the display of the balance to zero by pressing the "Tare" button.
 Position the appropriate test weight on the center of the weigh pan.
 When the display has stabilized, document the weight.
 Repeat steps 1 through 3, nine times.
 Once the 10 test results have been taken, calculate the SD (see formula below).





Accuracy Test:
 Position the appropriate test weight on the center of the weigh pan.
 When the display has stabilized, document the weight to complete.



Overview of the Changes from Past vs. Current USP Chapter 41 



Past USP Chapter 41 Version 
Current USP Chapter 41 Version 
Chapter Title 
Weights and Balances 
Balances 
Area of application 
Valid for material to be "accurately weighed" for Assay 
Valid for all material that needs to be "accurately weighed" 
Determination of the minimum sample weight 
Determination of the balance's operating range 
REPEATABILITY 


Repeatability tolerance 
0.1(49)% 
0.10% 
Expansion factor, K 
3 
2 
Acceptance criteria 
3 x SD/N ≤ 0.1(49)% 
2 x SD/N ≤ 0.10% 
Smallest possible sample weight/lowest starting point 
1,000 d 
820 d (in optimal conditions) 
Operating Range 
n/a 
(2 x SD x 1000) to max capacity
or
(2 x 0.41d x 1000) to max capacity (if SD < 0.41d) 
ACCURACY 


Tolerance 
n/a 
Satisfactory if its weighing value is within 0.10% of the test weight value 
Test weight 
n/a 
Between 5% and 100% of the
balance's capacity 
Calibration uncertainty of the weight 
n/a 
Must be ≤ 1/3 of 0.10% 




Below is the Past USP Chapter 41 Version 
Pharmaceutical laboratories compliant with the
USP (United States Pharmacopoeia) it's the "Measurement
Uncertainty" or minimum sample quantity is very important
during an FDA audit. This is the USP guideline which determines
the minimum sample weight you can weigh on a particular balance.
Measurement uncertainty is defined by the USP (section <41>
Weights and Balances) as three times the standard deviation
divided by the amount weighed. This should not exceed
0.001 (0.1%). This should be determined experimentally
using the appropriate class weights as defined in the USP and
performing at least 10 replicates of that weight.
The standard deviation is used to evaluate a balance with regard
to its reproducibility. For a confidence interval of ±
3 œ, 99.7% of the values measured lie within these limits
around the mean. (Pertch,T. GIT Lab Journal Volume 7, 2003
pp 9092) 


Calculating
minimum sample weight that you can weigh on balance 

1.
Take 10 weightings on balance with the appropriate class weight
as defined by USP. There is an entirely separate paragraph
within the USP that clearly defines what class of weight is
used with balances of different readabilities. The USP
clearly defines when Class
1, Class 2 and Class 3 weights should be used. 

2.
Determine the standard deviation from the 10 readings recorded. 

3.
Calculate the min sample weight by using the following formula:
[Standard deviation x 3] x 1000 = minimum sample weight 
The
in house calibration technician would perform the about test
and the minimum sample weight that is calculated would be
enter into the Sartorius
Genius Balance via the keypad. If someone attempted
to weigh something below the "minimum sample weight"
that was calculated the Sartorius Genius
Balance would warn the operator by being prompted via
the display.
The above example for calculating the minimum sample weight
is for reference and should not be considered the present
requirement for pharmaceutical guidelines. Please review
the current specifications called out with the United States
Pharmacopeia (USP) before performing any weighing in a production
applications.
Calibration technicians required to comply USP specifications
should obtain a copy of the current USP 28 NF23 and read
the sections regarding the use of balances in full.
Regulations can change and this web page should not be consider
the current guidelines. You can review an example of
a calibration procedure
that includes SQmin here. 

From the technical specifications for Sartorius
balances, we can estimate what the minimum sample quantity
will be for each balance.
( Minimum Sample Weight is based on manufacturer's specified
data.)
Model 
Capacity 
Readability 
Repeatability (s) 
Measurement
Uncertainty (3s) 
Minimum Sample Weight** 
SECURA6131S 
610 g 
0.001 g 
0.001 g 
0.003 g 
1.5 g 
SECURA5131S 
510 g 
0.001 g 
0.001 g 
0.003 g 
1.5 g 
SECURA3131S 
310 g 
0.001 g 
0.001 g 
0.003 g 
1.5 g 
SECURA2131S 
210 g 
0.001 g 
0.001 g 
0.003 g 
1.5 g 
SECURA61021S 
6100 g 
0.01 g 
0.01 g 
0.03 g 
12 g 
SECURA51021S 
5100 g 
0.01 g 
0.01 g 
0.03 g 
12 g 
SECURA31021S 
3100 g 
0.01 g 
0.01 g 
0.03 g 
12 g 
SECURA21021S 
2100 g 
0.01 g 
0.01 g 
0.03 g 
12 g 
SECURA11021S 
1100 g 
0.01 g 
0.01 g 
0.03 g 
12 g 
SECURA6121S 
610 g 
0.01 g 
0.01 g 
0.03 g 
12 g 
SE2 
2.1 g 
0.0001 mg 
0.00025 mg 
0.00075 mg 
0.75 mg 
ME5 
5.1 g 
0.001 mg 
0.001 mg 
0.003 mg 
3 mg 
CPA2P 
500 mg 
0.001 mg 
0.001 mg 
0.003 mg 
3 mg 
MC21S 
21 g 
0.001 mg 
0.002 mg 
0.006 mg 
6 mg 
CPA2PF 
500 mg 
0.001 mg 
0.002 mg 
0.006 mg 
6 mg 
ME235P 
60 g 
0.01 mg 
0.015 mg 
0.045 mg 
45 mg 
CPA225D 
100 g 
0.01 mg 
0.02 mg 
0.06 mg 
60 mg 
ME235S 
230 g 
0.01 mg 
0.025 mg 
0.075 mg 
75 mg 
ME254S 
250 g 
0.1 mg 
0.07 mg 
0.021 mg 
21 mg 
LA230S 
250 g 
0.1 mg 
0.1 mg 
0.3 mg 
300 mg 
CPA423S 
420 g 
0.001 g 
0.001 g 
0.003 g 
3 g 
CPA4202S 
4200 g 
0.01 g 
0.01 g 
0.03 g 
30 g 
CPA8201 
8200 g 
0.1 g 
0.1 g 
0.3 g 
300 g 
LA2200 
2200 g 
0.1 g 
0.05 g 
0.15 g 
150 g 
CPA34000 
34000 g 
1 g 
0.5 g 
1.5 g 
1500 g 
** These numbers are the minimum
sample weight according to manufacturers specs.
To obtain the true minimum sample weight, this must
be calculated at the location where the balance is being
used. 

To assist our customers in determining the minimum sample
quantity for their balances, Sartorius has incorporated this
calculation on the Genius
Series and some of the CP and LA
Series of balances. This new feature, called SQmin,
will soon be available on many Sartorius semimicro, micro
and ultramicro balances. For more information, contact Sean
Carey ext. 8807 or Rich Stressler ext. 8330 at Sartorius Corporation
(16312544249).
The USP<41> test and the activation of the SQmin program
on Sartorius balances is performed by Authorized Sartorius
Service Personnel as follows:
 NIST traceable weights of the appropriate weight and class
are used to perform the replicate weightings (at least 10x)
 All weight traceability data and data from the replicate
weighings are entered onto a USP<41> Test Certificate
 Calculations are performed in strict accordance to USP<41>
 SQmin is activated by an Authorized Service person through
the Service Menu
 The experimentally determined value for Sqmin is entered
into the balance
 A signed test certificate for testing according to USP<41>
is provided by the Authorized Service person to accompany
the balance calibration and maintenance documents.
Please note setting SQmin can not be done by end users.
Setting the SQmin is achieve using propriety software connect
through the RS232 interface of the balance. At this
time Sartorius will NOT release this software to the end user
enabling the end user to set the SQMin feature. 

Below is an excerpt of the Chapter 41 

Interpretation In the new Chapter 41, the minimum sample weight is no longer mentioned. Rather, it is necessary to determine the operating range of a balance. This range is limited above and below by the maximum capacity of the balance and begins at the point at which the balance's repeatability is less than or equal to 0.10%. 

41 Balances This chapter states the requirements for balances used for materials that must be accurately weighed (see General Notices, 8.20). Unless otherwise specified, when substances must be "accurately weighed", the weighing shall be performed using a balance that is calibrated over the operating range and meets the requirements defined for repeatability and accuracy. For balances used for other applications, the balance repeatability and accuracy should be commensurate with the requirements for its use. For discussion of the theoretical basis of these requirements, see general information chapter Weighing on an Analytical Balance 1251, which may be a helpful – but not mandatory – resource. 

Repeatability Repeatability is assessed by weighing one test weight NLT 10 times. [Note – The test weight must be within the balance's capacity, but the weight need not be calibrated. Because repeatability is virtually independent of sample mass within the balance's capacity, use of a small test weight, which may be difficult to handle, is not required.] Repeatability is satisfactory if two times the standard deviation of the weighed value, divided by the nominal value of the weight used, does not exceed 0.10%. If the standard deviation obtained is less than 0.41d, where d is the scale interval, replace this standard deviation with 0.41d. In this case, repeatability is satisfactory if two times 0.41d, divided by the nominal value of the weight used, does not exceed 0.10%. 

Accuracy The accuracy of a balance is satisfactory if its weighing value, when tested with suitable weights, is within 0.10% of the test weight value. A test weight is suitable if it has a mass between 5% and 100% of the balance's capacity. Its maximum permissible error (mpe), or alternatively its calibration uncertainty, shall be NMT onethird of the applied test limit of the accuracy test. [Note – Applicable standards are the following: ASTM E617 (available from http://www.astm.org) and OIML R 111 (available from http://www.oiml.org).] 

Repeatability The new USP requires that the repeatability of a balance be determined based on at least 10 comparable weighed values. For this test, Sartorius recommends using one weight close to approximately half of the maximum capacity of the particular balance (in other words, for a 200 g analytical balance, you need to use a weight of 100 g). The USP states that it is not necessary to use a small test weight to assess the repeatability. Sartorius recommends performing a repeatability test at approximately half of a balance's maximum capacity in order to combine the repeatability test and the accuracy test (the accuracy test is described later in this document).
Determine the standard deviation from 10 comparable weighed values and multiply these by an expansion factor of 2 (in other words, 2 x the std. deviation). If this value is multiplied by 1,000, this will yield the starting point of the operating range for a particular balance.
If a standard deviation less than 0.41 digit is obtained (in other words, 0.041 mg for an analytical balance with a readability of 0.1 mg), this must be replaced by 0.41 d so that the smallest possible starting point of an operating range of up to 820 d (2 * 0.41 * 1,000) is obtained. For an analytical balance with a readability of 0.1 mg, this means the starting point yielded is 82 mg. 
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