IPFNA Pattern Selection

In the world of precision measurement, ensuring that an instrument operates within its specified tolerances is essential for complying with quality standards, industry regulations, and standards such as those established by the OIML (International Organization of Legal Metrology). Our calculator simplifies one of the most critical steps in this process, using OIML R-111.

 

What is Maximum Allowable Error (MAP) and why is it so important?

 

The Maximum Permissible Error (MPE) is the maximum difference allowed, by regulation or specification, between the nominal value of a standard weight and its actual conventional mass value. In simpler terms, it is the small error that is "tolerated" for a high-precision weight.

The key to good calibration lies in the following fundamental principle: the instrument you use to calibrate must be significantly more accurate than the instrument you are calibrating.

 

The Golden Rule in Calibration: The Third Condition (EMP/3)

 

To ensure the validity of a calibration, metrology establishes a crucial rule: the combined uncertainty of your standards (in this case, the sum of the EMP of the weights you will use) must be at least three times better than the resolution (or smallest division) of the instrument to be calibrated.

Condition to be met: Resolution ≥ (Sum of EMP / 3)

Our calculator automates this check for you. It determines the optimal combination of weights to reach the maximum capacity of your scale and then verifies whether this essential condition is met.

 

Interpreting the Results

 

• It can be calibrated: Excellent! The result indicates that the uncertainty of your standard weights is low enough to perform a reliable calibration of your instrument, complying with the rule of thirds.

• Calibration not possible: This result means that the sum of the errors in your weights is too large compared to the resolution of your scale. To proceed, you would need to use a set of weights of a higher precision class (for example, going from F1 to E2).

 

Our tool gives you the confidence to perform accurate calibrations that meet the highest quality standards. Start calculating now and ensure the reliability of your measurements!

​

How the tool picks the weights

​

To load the instrument up to its maximum capacity, the calculator takes the weights of the selected class from largest to smallest, skipping any that would exceed the capacity. It then adds up the MPE of the weights used (OIML R111 values) and checks the rule of thirds:

Resolution (d or e) ≥ Σ MPE of the weights / 3

​

Worked example: 200 g analytical balance

​

Balance with a capacity of 200 g and d = 0.0001 g (0.1 mg). A single 200 g weight reaches the capacity:

Class F1: MPE = 1.0 mg → 1.0 / 3 = 0.333 mg > 0.1 mg → cannot be calibrated

Class E2: MPE = 0.30 mg → 0.30 / 3 = 0.100 mg ≤ 0.1 mg → can be calibrated (right at the limit)

Class E1: MPE = 0.10 mg → 0.10 / 3 = 0.033 mg ≤ 0.1 mg → can be calibrated with margin

Second case: 30 kg platform scale with d = 1 g and class M1 weights. The tool uses 20 kg + 10 kg: Σ MPE = 1000 mg + 500 mg = 1500 mg → 1500 / 3 = 500 mg = 0.5 g ≤ 1 g → it can be calibrated.

​

MPE of common weights (OIML R111)

​

1 g: E2 0.030 mg · F1 0.10 mg · F2 0.30 mg · M1 1.0 mg

100 g: E2 0.16 mg · F1 0.50 mg · F2 1.6 mg · M1 5.0 mg

1 kg: E2 1.6 mg · F1 5.0 mg · F2 16 mg · M1 50 mg

20 kg: E2 30 mg · F1 100 mg · F2 300 mg · M1 1000 mg

​

Frequently asked questions

​

What does IPFNA mean?

It is the Spanish acronym for non-automatic weighing instrument (NAWI): scales and balances that need an operator during weighing. Their metrological requirements are set by OIML R76 (in Mexico, NOM-010-SCFI).

Should I enter d or e?

d is the actual scale interval and e is the verification scale interval, both shown on the data plate. On approved instruments e ≥ d; for a legal verification use e, and if the instrument has no e, use d.

Why are the MPEs of the weights added up?

Because reaching full capacity usually takes several weights, and in the worst case their errors add in the same direction.

What if my weights do not pass?

Use a more accurate class (for example, going from F1 to E2), or have your weights calibrated and use their conventional mass value and its uncertainty instead of the class MPE.