Calibration Weight Classes Explained: What They Mean & Why They Matter

OIML Class E1 Reference Weights

These are also used as primary references for calibrating high tolerance reference standards and calibration weights. These can also be used to calibrate analytical balances and Class I and Class II laboratory balances that you normally find in calibration laboratories, metrology departments, or other similar controlled environments. Balances or scales are used extensively in industries, laboratories, and schools for taking accurate measurements. However, how do you ensure these scales give you accurate readings every single time? This is where calibration weight classes come into the picture.

Calibration weight classes basically give you information regarding the accuracy or tolerance of the weights that you use for the calibration of weighing scales. Every calibration weight is assigned a class based on its accuracy and tolerance range.

The classes are mainly defined by three main organizations i.e., ASTM (Formerly the American Society for Testing and Materials), OIML (International Organization of Legal Metrology), and NIST (National Institute of Standards and Technology).

This blog details the major calibration weight classes, highlighting their meaning and importance. So, keep on reading till the end.

• Classes and Accuracy
• Calibration Weight Classes
• NIST Calibration Weight Classes
• NIST Class F
• ASTM Calibration Weight Classes
• ASTM Class 7
• ASTM Classes 4-6
• ASTM Class 3
• ASTM Class 2
• ASTM Class 1
• ASTM Classes 0,00, and 000
• OIML Calibration Weight Classes
• OIML Class E0 Reference Weights
• OIML Class E1 Reference Weights
• OIML Class E2 Reference Weights
• OIML Class F1 Calibration Weights
• OIML Class F2 Calibration Weights
• OIML Class M1, M2, M3 Scale Weights
• Scale Class vs. Calibration Weight Class Comparison Chart
• Tolerance Levels 
• For OIML Calibration Weight Classes
• For ASTM Calibration Weight Classes
• Obsolete Classes

Classes and Accuracy

There are two main classes each having a certain level of accuracy and uses.

1. Higher Classes

These include classes like E1, E2, and F1 and the weights in these classes possess a lower degree of tolerance. This essentially means the values of these weights are extremely close to their nominal masses which makes them a perfect choice for high-precision analytical balances and microbalances. 

2. Lower Classes

These include weight classes like F2, M1, and M2 that possess wider tolerances. These are perfect for applications where less precision is needed.

Calibration Weight Classes

Following are some major calibration weight classes and their types:

NIST Calibration Weight Classes

NIST or National Institute of Standards and Technology classes are used for applications involving high-precision measurements and they are the most widely used mass standard in most parts of the world. These are used to test commercial precision balances used in manufacturing plants or warehouse settings to ensure higher efficiency and shipping accuracy.

NIST Class F

The class F weights are often used in a wide range of industries for the verification of Class III, Class IIII, and non-designated scales.

ASTM Calibration Weight Classes

The ASTM weight classes or American Society for Testing and Materials weight classes are available in a wide range of very high to very low-precision classes. These are used in industry and research sectors at a large scale.

ASTM International is a non-profit organization that provides a forum for the development of international voluntary consensus standards for various materials, products, systems, etc. This agency has set forth about 10 classes for calibration weights where higher numbers suggest an increase in tolerance and a decrease in accuracy.  

ASTM Class 7

These are used in applications where you need to take rough measurements or conduct simple weight calibrations. The class 7 calibration weights have the highest tolerance. They are used to calibrate Class IIII, non-designated balances, and Class III scales. Some examples include floor scales, shipping scales, etc.

ASTM Classes 4-6

ASTM Class 4 weights, ASTM Class 5 weights, and ASTM Class 6 weights are used in student laboratories and some research centers. They allow accurate calibration of precision balances, Class I and Class II, Class III, IIIL, and unmarked scales. These scales typically have a readability range of about 0.5g to 1g.  

ASTM Class 3

These are used to calibrate moderate precision balances that possess readabilities of 0.1g to 0.01g. These are extensively used in R&D, analytical testing, and pharma research settings. The calibration of Class I, II, laboratory balances, and ASTM Class 4, 5, and 6 calibration weights is done using the ASTM class 3.

ASTM Class 2

Most high precision balances including Class I and Class II laboratory balances and ASTM Class 4, 5, and 6 weights are calibrated with this class.

The scales calibrated with these weights have a readability range of 0.01g to 0.001g. These are used for balances that find diverse uses in chemical compounding and pharmaceutical industries.

ASTM Class 1

astm class 1 calibration weights are used for Analytical and semi-micro balances with readabilities of 0.1mg to 0.01mg. These are used to calibrate high tolerance weights like ASTM Class 4, 5, and 6 weights. The analytical balances, Class I, and Class II laboratory balances, and similar devices can be calibrated with this weight class.

These calibration scales are very sensitive and that is why it is recommended that you handle them with care by wearing gloves and using tweezers. This prevents oil or dirt from affecting their tolerance.

These are used for scales that find applications in research and development, analytical testing, chemical manufacturing, and pharma quality control settings.

ASTM Classes 0,00, and 000

These calibration weight classes are used almost exclusively in laboratories dedicated to metrology where careful handling and the stability of the environment are guaranteed. These three weight classes provide a reference for calibrating weights with higher tolerances.

These weight classes are used for calibrating digital weighing machines like microbalances, analytical balances, Class I and Class II laboratory balances, and ASTM Class 4, 5, and 6 weights that are used in secondary calibration labs, state laboratories, and for quality control and in manufacturing. 

OIML Calibration Weight Classes

The OIML weight classes are provided by The Organisation Internationale de Métrologie Légale or International Organization of Legal Metrology (OIML). This intergovernmental organization works to harmonize the legal metrology processes on a global level. These weight classes are available in lowest to highest tolerance ranges including E1, E2, F1, F2, M1, M2, and M3. The F2 weight class of this is used for most laboratory calibration applications.

Following is an overview of some key OIML weight classes :

OIML Class E0 Reference Weights

This is used mainly for calibrating other reference standards and scales like microbalances, analytical balances, and Class I and Class II laboratory balances. These are ideal for metrology uses where careful handling and a stable environment are provided.

OIML Class E2 Reference Weights

These can be employed to calibrate other calibration weights. They can also be used for high-precision analytical weighing scales and class II, I balances. These are ideal for balances with 0.1mg to 0.01 mg readability.

OIML Class F1 Calibration Weights

These are used to calibrate high-precision top-loading balances, Class I and Class II laboratory balances, and digital scales having readabilities in the range of 0.01g to 0.001g.   

OIML Class F2 Calibration Weights

These are used for the calibration of semi-analytical balances you normally see in schools. These are ideal for applications where simple weight calibrations are enough. This weight class is used to calibrate Class III, IIIL, and IIII scales.

OIML Class M1, M2, M3 Scale Weights

These weight classes are perfect for general laboratory, industry, technical, and educational research settings. These cast iron or steel calibration weights are ideal for calibrating Class III, IIIL, or IIII digital scales used in classrooms and similar less demanding applications that require simple weight calibrations.

Scale Class vs. Calibration Weight Class Comparison Chart

The following calibration weight class chart gives a comparison between scale classes and calibration weight classes to help you identify which standard to use for your calibration purposes.

Classification of Weight Devices Tested	ASTM Accuracy Classes	OIML Accuracy Classes	NIST Accuracy Classes
Class I	1	F1
Class II	1,2	F1, F2
Class III	3,4,5,6	M1, M2, M1-2, M2-3	F
Class III L	3,4,5,6	M1, M2, , M1-2, M2-3	F
Class IIII	3,4,5,6	M3	F

Tolerance Levels 

The calibration tolerance refers to the permissible deviation from a specified value. It is the difference between a known value standard and the reading on a calibrated device.

Tolerance levels depend on scale classes where higher classes like class I have tighter or higher tolerances making them useful for high-precision measurements. The next section covers tolerance levels for the widely used OIML and ASTM weight classes. 

For OIML Calibration Weight Classes

	E1	E2	F1	F2	M1	M2	M3
	mg	mg	mg	mg	mg	mg	mg
5000 kg		25000	80000	250000	800000	2500000	25000
2000 kg			10000	30000	100000	300000	1000000
1000 kg		1600	5000	16000	50000	160000	500000
500 kg		800	2500	8000	25000	80000	250000
200 kg		300	1000	3000	10000	30000	100000
100 kg		160	500	1600	5000	16000	50000
50 kg	25	80	250	800	2500	8000	25000
20 kg	10	30	100	300	1000	3000	10000
10 kg	5	16	50	160	500	1600	5000
5 kg	2.5	8.0	25	80	250	800	2500
2 kg	1	3	10	30	100	300	1000
1 kg	0.5	1.6	5	16	50	160	500
500 g	0.25	0.8	2.5	8.0	25	80	250
200 g	0.1	0.3	1.0	3	10	30	100
100 g	0.05	0.16	0.5	1.6	5.0	16	50
50 g	0.03	0.10	0.3	1.0	3.0	10	30
20 g	0.025	0.08	0.25	0.8	2.5	8.0	25
10 g	0.020	0.06	0.20	0.6	2.0	6.0	20
5 g	0.016	0.05	0.16	0.5	1.6	5.0	16
2 g	0.012	0.04	0.12	0.4	1.2	4.0	12
1 g	0.010	0.03	0.10	0.3	1.0	3.0	10
500 mg	0.008	0.025	0.08	0.25	0.8	2.5
200 mg	0.006	0.020	0.06	0.20	0.6	2.0
100 mg	0.005	0.016	0.05	0.16	0.5	1.6
50 mg	0.004	0.012	0.04	0.12	0.4
20 mg	0.003	0.010	0.030	0.10	0.30
10 mg	0.003	0.008	0.025	0.08	0.25
5 mg	0.003	0.006	0.020	0.06	0.20
2 mg	0.003	0.006	0.020	0.06	0.20
1 mg	0.003	0.006	0.020	0.06	0.20

For ASTM Calibration Weight Classes

	0	1	2	3	4	5	6	7
	mg	mg	mg	mg	g/mg	g/mg	g/mg	g/mg
5000 kg					100 g	250 g	500 g	750 g
3000 kg					60 g	150 g	300 g	450 g
2000 kg					40 g	100 g	200 g	300 g
1000 kg					20 g	50 g	100 g	150 g
500 kg					10 g	25 g	50 g	75 g
300 kg					6 g	15 g	30 g	45 g
200 kg					4 g	10 g	20 g	30 g
100 kg					2 g	5 g	10 g	15 g
50 kg	63	125	250	500	1 g	2.5 g	5 g	7.5 g
30 kg	38	75	150	300	600 mg	1.5 g	3 g	4.5 g
25 kg	31	62	125	250	500	1.2 g	2.5 g	4.5 g
20 kg	25	50	100	200	400	1.0 g	2 g	3.8 g
10 kg	13	25	50	100	200	500 mg	1 g	2.2 g
5 kg	6	12	25	50	100	250	500 mg	1.4 g
3 kg	3.8	7.5	15	30	60	150	300	1.0 g
2 kg	2.5	5	10	20	40	100	200	750 mg
1 kg	1.3	2.5	5	10	20	50	100	470
500 g	0.6	1.2	2.5	5	10	30	50	300
300 g	0.38	0.75	1.5	3	6	20	30	210
200 g	0.25	0.5	1	2	4	15	20	160
100 g	0.13	0.25	0.5	1	2	9	10	100
50 g	0.06	0.12	0.25	0.6	1.2	5.6	7	…
30 g	0.037	0.074	0.15	0.45	0.9	4	5	44
20 g	0.037	0.074	0.1	0.35	0.7	3	3	33
10 g	0.025	0.05	0.074	0.25	0.5	2	2	21
5 g	0.017	0.034	0.054	0.18	0.36	1.3	2	13
3 g	0.017	0.034	0.054	0.15	0.3	0.95	2.0	9.4
2 g	0.017	0.034	0.054	0.13	0.26	0.75	2.0	7
1 g	0.017	0.034	0.054	0.1	0.2	0.5	2.0	4.5
500 mg	0.005	0.01	0.025	0.08	0.16	0.38	1	3
300 mg	0.005	0.01	0.025	0.07	0.14	0.3	1	2.2
200 mg	0.005	0.01	0.025	0.06	0.12	0.26	1	1.8
100 mg	0.005	0.01	0.025	0.05	0.1	0.2	1	1.2
50 mg	0.005	0.01	0.014	0.042	0.085	0.16	0.5	0.88
30 mg	0.005	0.01	0.014	0.038	0.075	0.14	0.5	0.68
20 mg	0.005	0.01	0.014	0.035	0.07	0.12	0.5	0.56
10 mg	0.005	0.01	0.014	0.03	0.06	0.1	0.5	0.4
5 mg	0.005	0.01	0.014	0.028	0.055	0.08	0.2	…
3 mg	0.005	0.01	0.014	0.026	0.052	0.07	0.2	…
2 mg	0.005	0.01	0.014	0.025	0.05	0.06	0.2	…
1 mg	0.005	0.01	0.014	0.025	0.05	0.05	0.1	…

Obsolete Classes

Following conversion chart can help you equate obsolete weight classes with current standards. 

Current Classes	Obsolete Classes
ASTM Class 6,7	Class C
ASTM Class 6	Class F
ASTM Class 0, OIML Class E2	Class M
ASTM Class 4, OIML Class F2	Class P
ASTM Class 5	Class Q
ASTM Class 1,2	Class S
ASTM Class 3, OIML Class F1	Class S1
ASTM Class 7	Class T

Conclusion

Weighing scales are an indispensable part of almost every industry you can think of. Their correct calibration is what ensures better quality control, efficiency, and enhanced customer trust.

Proper scale calibration is only possible when you use the correct weight class after considering the type of your scale, balance or mass standards, and the intended applications. Using the correct calibration weight class is the only way to keep your weighing balances in the best shape so that they always generate accurate results.

You can visit us at Accurate Meezan today to buy our top-quality weighing scales and test calibration scales at very reasonable prices.