There is a general misconception that any E1 class weight can be used as a reference standard to calibrate another E1 class of weights. But we should be aware that OIML R-111-1 does not explicitly state anywhere that class E1 can be used to calibrate another E1 class of weights. Hence the conclusion is people are confused and have misinterpreted things, therefore this needs clarification.
These are the extracts from OIML R-111-1.
OIML R-111-1 CLAUSE 3.1. (Page number 4). Class E1:
Weights are intended to ensure traceability between national mass standards with values derived from the International Prototype of the Kilogram and weights of class E2 and lower.
Comment: As per this it is only E2 class. It doesn’t mention that E1 class can be used to calibrate another E1 class of weights it talks only regarding the calibration of E1 class.
OIML R-111-1 CLAUSE 2.3 Reference weights.
The reference weight should generally be of a higher class of accuracy than the weight to be calibrated. In the calibration of weights of class E1, the reference weight (it doesn’t say that it is E1 class as misunderstood by many) should have similar or better metrological characteristics (magnetic properties, surface roughness) than the weight that is to be calibrated.
Comment 1: It does not say it is E1 class of weight, it only says reference weights should be of higher accuracy weights than E1.
Comment 2: Metrological characters reference only for magnetic properties and surface roughness, it should not be interpreted as similar or better for error and Measurement uncertainty of E1 class.
We all know that the IPK weight (Measurement Uncertainty) is kept at INTERNATIONAL BUREAU OF WEIGHTS AND MEASURES in France. Our national IPK is a copy of that IPK at France. This is used as PRIMARY National standard and will have a Measurement Uncertainty of 20-30 micrograms. Using this PRIMARY National standard NMIs can create their own reference standards that will have a Measurement Uncertainty of 30-40 micrograms. This is used for the calibration of E1 class of weights to achieve error including Measurement Uncertainty as specified in OIML-R-111-1 for E1 class weights.
Comment 1: These reference standard weights does not come under OIML R-111-1 specifications or classification.
Comment 2: Reference standard weights used for calibration of class E1 should be directly traceable to IPK through any NATIONAL METROLOGY INSTITUTE (NMI).
Further in spite of the above reference weight class E1 having maximum permissible error (including expanded uncertainty) is used to compare and calibrate another test weight of class E1 requiring similar maximum permissible error (including expanded uncertainty) it is practically not possible as the following additional uncertainty components will get added to the test weight and expanded uncertainty of the test will be more than 250µg for 1kg and then goes to class E2.
- Uncertainty of reference weight.
- Uncertainty due to drift of reference weight.
- Uncertainty due to buoyancy correction.
- Uncertainty due to display resolution.
- Uncertainty due to calibration process type A depending on weight, design and weighing cycles,
Then deviation of test weight value from the reference value including the uncertainty will be more than the maximum permissible error for class E1 as per OIML-R-111-1.
Hence, reference weight class E1 cannot calibrate test weight class E1.
However Reference standard weights that are having measurement uncertainty better than class E1 weights which are suitable for calibration of E1 class of weights are already available from OIML class weights manufacturers like Sartorius, Mettler and others.
Hence it is commendable that Reference standard weights having measurement uncertainty better than class E1 weights suitable for calibration of E1 class of weights are available with OIML class weights manufacturers like Sartorius, Mettler and others.
MINIMUM REQUIREMENTS FOR CALIBRATION OF E1 class of weights as per OIML-R-111-1
- Calibration certificate of reference weight be directly traceable to IPK through any NATIONAL METROLOGY INSTITUTE (NMI).
- Calibration certificate should contain true mass and conventional mass values along with their respective measurement uncertainties.
- Calibration certificate should include type of material, its real density along with measurement uncertainty.
- Measurement uncertainty including error of reference standard weight should preferably be 13 better than E1 to be calibrated.
- Buoyancy correction- air density must be determined as per OIML-R-111-1 Annexure-E either equation E.1-1 or E.3-2.
Article no: RSM 1-2022 Page 2 of 3
- Ambient conditions during calibration. a) Temperature change during calibration should be ± 0.3° centigrade per hour with a maximum of ± 0.5° per 12 hours. b) Range of humidity (RH) of the air should be in between 40% to 60% with a maximum of ± 5% per hours. C.2.1.1. For E1 and E2 class weights, the temperature should be within 18°C to 27°C. The environmental conditions should be within the specifications laid down for the weighing instrument.
- C.2.1.2 – If the air density deviates from 1.2 kg/m3 by more than 10%, mass values (true mass) should be used in calculations and the conventional mass should be calculated from the mass.
- C.2.2 – weighing instrument. The metrological characteristics of the weighing instrument used should be known from earlier measurements and its resolution, linearity, repeatability, and eccentricity should be such that the required Measurement Uncertainty can be reached.
- C.2.3 – Reference weights. Reference weights should generally be of a higher class of accuracy than the weight to be calibrated. In the calibration of weights of class E1, the reference weight should have similar or better metrological characteristics (magnetic properties, surface roughness) than the weights to be calibrated.
- C.3.2 – Weighing design. For E1 class of weights subdivision method is mainly used to calibrate sets of E1 class weights when the highest accuracy is required, However these methods require more advanced mathematics. For typical weighing design techniques published documents references are given in OIML-R-111-1 (last page).
- C.4 – Weighing cycles. Only cycles ABBA and ABA are useful in subdivision weighing.
- C.4.1 – Comparison of the test weight with one reference weight subdivision design and ABBA cycle should be utilized to eliminate linear drift.
Better understanding and clarity regarding difference between true mass and conventional mass, importance of actual density measurement, its measurement uncertainty and effect of air density during calibration is required.
For more information OIML D28 conventional value of the result of weight in air can be referred.