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A Guide to Calibration Weights

By Aimee O'Driscoll, 19 August 2019

Calibration weights are mainstays in most labs as they help ensure that lab balances are giving you accurate readings. However, they are not all made the same. Calibration weights are assigned a class which is based on the accuracy or tolerance of the weight. There are different sets of classes of weights, including ASTM, OIML, and NIST. In addition, there are certifications granted to different weights.

In this guide, we explain the different classes of calibration weights and the certificates that can be purchased alongside them. We also look at other factors to consider when purchasing weights such as style, material, and construction.

Classes of Weights

There are three main calibration weight class systems that you’ll come across:

NIST: National Institute of Standards and Technology (NIST) Class F weights are often used in industrial settings to verify Class III, Class IIII, and non-designated scales. They may also be used in warehouses and manufacturing settings to calibrate scales used for the shipping or production of large products. NIST Class F weights aren’t typically used in laboratories as they are not accurate enough to verify the scales used for most laboratory applications.

ASTM: ASTM International (formerly American Society for Testing and Materials) is a nonprofit non-governmental organization that develops voluntary consensus standards. It has developed 10 calibration weight classes as guided by document ASTM E 617: ASTM Class 000 thru ASTM Class 7. The higher the class number, the higher the level of tolerance (and less accurate) the weight will be. Most laboratory applications require ASTM weights of Class 4 or below. ASTM class weights are the most common type of calibration weights used in US laboratories.

OIML: The Organisation Internationale de Métrologie Légale or International Organization of Legal Metrology (OIML) is an intergovernmental organization that provides standards and systems with the goal of harmonizing legal metrology procedures. From lowest (most accurate) to highest tolerance, the OIML classes are E1, E2, F1, F2, M1, M2, and M3. Most laboratory applications require OIML weights of Class F2 or below. OIML class weights are more commonly used outside the US.

Some manufacturers of calibration weights create their own class systems. For example, Troemner’s UltraClass weights are classed as UltraClass, UltraClass Gold, and UltraClass Platinum. However, Troemner provides a reference to explain what these classes equate to in terms of ASTM and OIML.

Tolerance Levels

The following tables shows the tolerance levels for the different classes of weights, starting with OIML.

OIML:

E1
E2
F1
F2
M1
M2
M3
mg
mg
mg
mg
mg
mg
mg
5000 kg
25000
80000
250000
800000
2500000
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

ASTM:

 

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

...

Certifications of Weights

While browsing calibration weights, you might come across mentions of certifications. Indeed, depending on your application, you may require that your calibration weights come with a certificate confirming that the lab testing the weights is accredited. Here are the terms you’ll commonly see and what they’re referring to.

  • ISO/IEC 17025: The International Organization for Standardization (ISO) is an independent non-governmental organization comprising a membership of 164 national standards bodies. The International Electrochemical Commission (IEC) is an international standards organization for electrotechnical fields. Together with other liaison organizations, they created the ISO/IEC 17025 standards. This is the international reference to be used by testing and calibration laboratories that want to display their ability to provide reliable results.
  • NIST/NVLAP: NVLAP refers to a calibration program developed by the National Voluntary Laboratory Accreditation Program (NVLAP) at the (NIST). It provides a means to assess calibration labs for competency, offering accreditation in various fields. It follows the international standard ISO/IEC 17025 (discussed above) when accrediting calibration laboratories.

For example, many Ohaus weight sets come with the option of an NVLAP certificate. Certificates do come at a cost, however. For example, these Ohaus Class 4 weight sets without a certificate range in price from $345–$740, whereas adding a certificate bumps the price range to $825–$1,642. Certified weights often have longer lead time as well, as the weights may need to be sent out for certification.

 

Ohaus Class 4 weights.

Ohaus Class 4 weight sets.

Similarly, Troemner offers an optional NVLAP+ Accredited Certificate which indicates the weights meets several standards, including  NVLAP Handbook 150-2, ISO/IEC 9000, and ISO/IEC 17025, among others. For the OIML Precision Weights Class E1, for example, these certificates incur an additional cost of $160–$241.

One question we’ve been asked before is whether or not NIST (NVLAP) calibration certifications expire. A certification confirms that a specific standard has been met at the time of testing. As such, there is no expiration date, but rather a general guideline that items should be tested on a regular basis. However, NIST does not specify recommended re-calibration intervals.

How often weights should be tested will depend on their frequency of use and the necessary accuracy of the application. Weights that are improperly handled (without gloves or forceps) should also be tested more frequently, or may need to be re-tested immediately depending on the sensitivity and / or legal requirements of the application.

Other Factors to Consider

Aside from class and certification, you’ll come across a few more options when browsing calibration weights. Here are the main factors to consider:

Style

There are various styles of calibration weights suitable for different applications. Here are the most common ones found in a lab setting:


Cylindrical calibration weight.

Cylindrical

Many weights have a simple cylindrical shape. These often have a ridge around the top to make them easier to handle.

Grip handle calibration weight.

Grip handle

A grip handle on a cylindrical weight can make it easier to deal with larger weights.

Pipe handle calibration weight.

Pipe handle

Similarly a pipe handle can help with transporting large weights. 

Knob calibration weight.

Knob

Some weights come with a small tapered section or a knob at the top, making them easier to pick up by hand or with a forceps.

Hook calibration weight.

Hook

Hooked weights are often used for measuring torque, pressure, or tensile strength.

Slotted calibration weight.

Slotted

Disk-shaped slotted weights are useful when you want to adjust a load or force. They can be used alone or slotted onto a stand or hanger. 

Leaf calibration weight.

Leaf

Leaf weights are thin metal tabs used for very low denominations. A fold in the tab makes it easy to pick up the weight with a forceps.

Material

The materials used to create calibration weights vary depending on the weight and use. Most weights are cast from stainless steel or aluminum, with the latter often being used for smaller weights. For example, in Ohaus ASTM Class 1 weight sets, 5mg to 30kg weights are made from stainless steel and weights from 1mg to 2mg are constructed from aluminum.

Some heavier and less accurate weights are made from cast iron, for example, these ASTM Class F pipe handle weights:

 

Class F pipe handle weights.

 

Cast iron test weights may be painted gold or silver depending on the units their weight is measured in. In this case, gold represents metric units and silver indicates avoirdupois units.

Construction (Single or Two-Piece)

Most calibration weights comprise a single piece of cast metal. However, certain weights, such as Troemner’s UltraClass weights (1g and above), have a two-piece construction. A two-piece weight consists of a knob that screws into the body of the weight. There is a cavity below the thread, containing adjusting material, which is usually the same as the material used to make the weight.

 

Troemner UltraClass weight sets.

Troemner UltraClass weight sets.

While these may have slightly larger uncertainties, they are adjustable so don’t need to be replaced as often.