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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.
USP General Chapter 41 Requirements
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.
   
USP Chapter 41 Repeatability Calculation
   
  • Calculation to determine the actual starting point of the balance's operating range (ORmin)
   
lab balance balance operating weighing range
   
  • 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.
   
Measuring balance uncertainty 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.
   
Balance Readability (d) Operating Range Starting Point Example
Optimal † Typical ††
0.1 µg 0.0001 mg 0.0000001 g 0.082 mg 1 mg MSA2.7S0TRDM
1 µg 0.001 mg 0.000001 g 0.82 mg 2 mg MSE6.6S000DM
10 µg 0.01 mg 0.00001 g 8.2 mg 20 mg Secura225D-1S
100 µg 0.1 mg 0.0001 g 82 mg 100 mg Secura124-1S
1000 µg 1 mg 0.001 g 820 mg 900 mg Secura613-1S
  10 mg 0.01 g 8.2 g 8.2 g Secura6102-1S
  100 mg 0.1 g 82 g 82 g Secura3101-1S
  1000 mg 1 g 820 g 820 g Quintix6100-1S
   
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 one-third 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.

  1. Set the display of the balance to zero by pressing the "Tare" button.
  2. Position the appropriate test weight on the center of the weigh pan.
  3. When the display has stabilized, document the weight.
  4. Repeat steps 1 through 3, nine times.
  5. Once the 10 test results have been taken, calculate the SD (see formula below).
 
US Pharmacopoeia balance tests
 

Accuracy Test:

  1. Position the appropriate test weight on the center of the weigh pan.
  2. 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 90-92)
 
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 NF-23 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**

SECURA613-1S

610 g

0.001 g

0.001 g

0.003 g

1.5 g

SECURA513-1S

510 g

0.001 g

0.001 g

0.003 g

1.5 g

SECURA313-1S

310 g

0.001 g

0.001 g

0.003 g

1.5 g

SECURA213-1S

210 g

0.001 g

0.001 g

0.003 g

1.5 g

SECURA6102-1S

6100 g

0.01 g

0.01 g

0.03 g

12 g

SECURA5102-1S

5100 g

0.01 g

0.01 g

0.03 g

12 g

SECURA3102-1S

3100 g

0.01 g

0.01 g

0.03 g

12 g

SECURA2102-1S

2100 g

0.01 g

0.01 g

0.03 g

12 g

SECURA1102-1S

1100 g

0.01 g

0.01 g

0.03 g

12 g

SECURA612-1S

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

CPA2P-F

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 semi-micro, micro and ultra-micro balances. For more information, contact Sean Carey ext. 8807 or Rich Stressler ext. 8330 at Sartorius Corporation (1-631-254-4249).

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 RS-232 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 one-third 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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