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A Guide to Balance and Scale Calibration

By Aimee O'Driscoll, 11 December 2018

Calibration is crucial to most applications and is especially important when it comes to balances and scales. Without regular calibration, you just never know how inaccurate your measurements are, and you might as well be guessing the readings.

In this guide, we’ll explain why calibration is necessary, how often it needs to carried out and how balances and scales can be calibrated.

Why Calibration is Necessary

Accuracy, albeit at varying levels, is clearly important for virtually any application carried out in the lab. So why would the accuracy of a balance or scale be different from one day to the next? There are various possible reasons for this, but here are a few:

  • Movement: Transporting a unit from one location to another, such as during shipping, has the potential to cause the greatest inaccuracy, but even shifting the unit a few millimetres on the benchtop can result in slight changes in the balance mechanisms, thus requiring recalibration.
  • Different gravitational force: This is another consideration when moving the balance to a distinctly different location. Gravitational pull varies depending where you are on Earth, and that difference in force can affect balance readings.
  • Vibrations: If the balance has been situated close to other lab equipment that vibrates, similar to moving the balance itself, this can cause changes in the mechanisms of the balance.

Even if a balance hasn’t been moved and you think that there have been no changes in the environment, you may still get unstable readings over time, a phenomenon referred to as “drift.” Things like static charge (often created in dry environments) and even slight temperature fluctuations can have a long-term effect on readings.

How Often Is Calibration Carried Out?

Typically, a balance or scale will need to be calibrated before initial use. After that, a calibration schedule can be set up depending on factors such as how often the unit is used and the  degree of accuracy required for the application. For example, calibration may need to be carried out before each use, or once a month may suffice. Whatever the schedule, calibration should be considered part of the regular maintenance of the unit, and not just performed on an as-needed basis.

Aside from the scheduled calibrations, the unit will likely need to be calibrated in certain other cases, such as if it has been moved, leveled, or overloaded, or hasn’t been used for a long period of time.

How Is Calibration Carried Out?

Calibration can be performed by the user, a technician (from a calibration service), or by the unit itself (if it has an internal calibration feature). Many labs will hire a calibration service to come and calibrate all units at the same time, for example, annually or quarterly, and then perform their own calibrations as needed in between.

If you do use a calibration service, it’s important that the company is accredited according to required guidelines for your industry, for example, by the ANSI-ASQ National Accreditation Board (ANAB) in North America.

Calibration is typically carried out with the unit in the same location where it will be used and away from sources of air flow and vibrations. The balance should be switched on at least 30 minutes prior to testing as the electronics need time to warm up before they’ll deliver accurate readings.

There are two main types of calibration: internal and external.

Internal Calibration

Some units come with an internal calibration feature whereby the machine can automatically perform its own calibration. Depending on the unit, you may be able to set it to calibrate at certain times of day, at set time intervals, or whenever you press a calibration button. This feature typically involves built-in motor-driven calibration weights.

 

Examples of balances.

Left to right: Explorer® Precision Balances and Entris® Analytical Balances come with the option of internal or external calibration, and Explorer® Precision High Capacity Balances always come with internal calibration.

To give you an idea of the price you’ll pay for internal calibration, a 200g capacity Entris® Analytical Balance is $1,648 with external calibration and $2,201 with internal calibration, a difference of $553.

Internal calibration pros:

  • The process doesn’t take time away from lab staff.
  • You can usually still calibrate externally if you wish.

Internal calibration cons:

  • Units with this feature tend to be more expensive.
  • Periodic external calibration is still required.
  • The process typically doesn’t calibrate the unit at full capacity.

External Calibration

While some units come with the option of internal calibration, others don’t, and you may decide it’s not worth it to pay the extra anyway.

Thankfully, external calibration is fairly straightforward and can be carried out using calibration weights. Balances come with a calibration function that you can use to perform calibration using the appropriate weights. Some balances ship with a set of weights included, but you can purchase them separately.

Weights should always be held with forceps or gloves to prevent oil or residue from your hands sticking to the weight and altering its mass.

 

Calibration weights with forceps.

Troemner OIML Precision Weights and Analytical Precision Weights come with a forceps for handling.

Weights should always be stored such that they are protected from dust and debris. Some weight sets come with their own handy storage box to do just that.

 

Calibration weights in boxes.

Troemner Cal-Paks™ and OIML Precision Weight Sets come with a neat case to keep them protected from dust and debris.

External calibration pros:

  • Units are less expensive than those with built-in calibration.
  • You can calibrate using the full capacity range of the balance or scale.

External calibration cons:

  • The process can be time-consuming if regular calibration is required.

Tests that Are Carried Out During Calibration

The tests that are carried out during calibration can vary depending on the application, industry, and any regulations that need to be followed. For example, some labs will follow general guidelines provided in ISO 9001, while others may need to adhere to more specific procedures, such as those found in the EURAMET Calibration Guidelines, the NIST Handbook 44, or Organisation Internationale de Métrologie Légale (OIML) recommendations.

Plus, the calibration method will vary depending on your unit and the manufacturer’s instructions should be followed closely. Some units have the option to customize your calibration process.

One of the simplest calibration methods is span calibration, which is performed using the weight that corresponds to the maximum capacity of the balance. The Ohaus video below shows an example of span calibration.

 

 

Linearity calibration is similar, but you also use a midpoint weight as an extra reference point.

 

 

Here are some other common tests that may be performed during calibration:

  • Eccentricity test: The same weight is placed on different (pre-determined) positions on the load receptor, to check that the same reading is delivered no matter where the load is placed. This can also be used to determine the errors of the indication.
  • Repeatability test: The same weight is placed on the same spot on the load receptor and weighed multiple times.
  • Weighing test: The accuracy of the balance is tested using a range of weights that cover most of the balance’s range. The load is increased gradually then decreased using five to ten weights. The balance may be calibrated with increasing or decreasing loads.
  • Minimum weight test: This is to find the minimum load required to deliver acceptably accurate results (since relative error increases with smaller loads). This test is less common than the ones above but is necessary for some applications, for example, in pharmaceutical labs.

For most tests, depending on the lab and intended application, there will be acceptable range in measurements. This is referred to as “tolerance.”

Conclusion

While calibration often seems like a complex topic, it remains a crucial practice in any lab and should be top of mind when dealing with balances and scales. Following the manufacturer's guidelines is always a good place to start. But it’s also important to consider any additional requirements for your industry and specific application.