If you use scales in your business, whether it be in a retail or manufacturing environment, you have probably heard that you should calibrate your scales regularly. Maybe you are thinking: why should I do that? They don’t go out of calibration, do they? If you are using weight as part of your business processes, you should care about the quality of the weighing results. The only way to ensure that a scale maintains its accuracy is to check it periodically. Inaccurate weighing results could cost you money. It also could potentially put you in trouble with local W&M authorities or federal regulators.
Why do scales need to be calibrated?
Most scales in use today are electronic, utilizing a weighing sensor. Even though the sensor is electronic, the weighing sensor still has mechanical components that can introduce errors in the weighing result. Something as simple as an unlevel scale can influence the weighing result. User weighing errors, like side loading, or overloading, can eventually change the scale calibration. Other factors that influence weighing results are:
- Vibrations – such as from a fan, air conditioning, or machinery
- Temperature – large temperature variations will affect weighing results. Ideally, scales should be located away from drafts or windows where incoming sunlight might affect the scale.
- User Errors – errors like overloading a scale can eventually deform the load cell, and cause the scale weight to drift. Dropping items on the scale, called shock loading, can also damage the load cell, and affect performance.
Even if all the mechanical and human error could be eliminated, there is still a margin of error built into every scale, called measurement uncertainty. High-level calibrations, like an ISO 17025 calibration, can help the end user understand and minimize the effect.
No scale is ever 100% accurate. Every scale has a built-in error called measurement uncertainty. Even test weights used to calibrate scales have a small margin of allowable error. It might sound surprising that there is error built into a scale, but we can account for that error using a formula that will calculate the smallest amount of weight you can put on a scale and still get a weight within an operating tolerance.
For example, most scales have the highest error rate at the low end of a scale’s capacity. If you know how much error your processes will allow and the maximum weight to be weighed on a scale, you can use the measurement uncertainty to calculate the “safe” weighing range of a scale. This will tell you the smallest weight you can put on a scale and be confident of its accuracy. Certain calibration tests, like an ISO17025 calibration test, include the measurement uncertainty test.
Types of scale calibration tests and reports
There are several types of scale calibration tests and reports. Depending on how critical the weighing information is to your operation you may opt for a variety of calibration tests.
In its most basic form, a scale calibration tests a scale’s ability to display the weight of a known test weight. In the United States, following NIST’s Handbook 44 test procedures, the technician will do a corner/eccentricity/shift test, which tests the scale’s ability to repeat a weight when the load is placed off center, and a single point load test. Most scale calibration companies will also inspect the critical weighing components and provide a calibration sticker noting when the next test is due. The test report does not indicate if the scale needed any adjustments to bring it to an acceptable weight.
An ISO 9001 and ISO 9001 compliant test builds on the work performed in the basic test. Using NIST Handbook 44 test procedures, the scale is tested using a 5-point increasing load test. This will test the scale at a variety of capacities. For small capacity scales, the 5-points are normally 0%, 25%, 50%, 75%, and 100%. For larger capacity scales, the amounts will be a percentage of the overall capacity, as it is sometimes not reasonable or possible to load a scale to full capacity using certified test weights.
The reporting for the ISO 9001 test adds additional information to the report, such as an As Found/As Left test result, scale information like manufacturer, model, serial number, location, Pass/Fail results, and a record of the test weights used for traceability and audit purposes.
The ISO 17025 test includes all the tests performed on the ISO 9001 test and adds additional tests. The number of test points for the increasing load test is increased to 9. A repeatability test is added. The repeatability test measures the scale’s ability to accurately repeat a weight.
The reporting for ISO 17025 includes all the items reported in the ISO 9001 report plus a determination of measurement uncertainty. Some scale calibration companies can also provide a test to calculate the minimum weight the scale can weigh accurately.
How often to calibrate?
We can reasonably see the reasons and advantages of calibrating a scale. The question of how often to calibrate, however, depends on several factors:
Legal for trade applications: Most states require an annual calibration for scales used in legal for trade applications. These calibrations are performed by the state and sometimes with the assistance of a scale calibration company. For example, in New York state, a truck scale is calibrated/sealed by the state but requires a local calibration company to provide the certified test weights.
ISO 9001 or ISO 17025: Companies with a quality system under ISO 9001 or ISO 17025 usually have the scale calibration frequency written into their standard operating procedures. Depending on the sensitivity of the manufacturing tolerance, scales may be required to be calibrated as frequently as each month.
Other applications: In cases where quality systems or legal requirements don’t dictate the calibration frequency, it is suggested that you work with a calibration provider to determine a reasonable calibration frequency. For example, you initially start calibrating your scales every quarter. After several test cycles, you find that your scales are holding the calibration well and don’t need to be adjusted. Working with the provider, you might determine that you want to move to a semi-annual calibration. Always remember that a long period between calibration checks opens you to the possibility that the scale has been out of calibration for a while, potentially losing you money with over and under-weighing.
Why hire a scale calibration company?
Certified test weights
Test weights are tested and certified by a testing agency against a national standard. Test weights are recertified based on the type of weight and class. Certified test weights are traceable to a state, and then a USA-certified standard. The USA standards are traceable to the international standard, located in France. A calibration company will have a variety of weights in multiple sizes to allow it to calibrate a wide range of scale classes and capacities.
Most scale companies are ISO 9001 or ISO 17025 certified. These quality certifications require that scale calibrations are performed to the standards set out by these systems. Calibrations will meet audit requirements needed by manufacturers with similar quality systems.
Your local scale company will have years of experience servicing and calibrating scales. Most companies, even if they specialize in one scale line, will work with a variety of scale manufacturers. They are very familiar with the local W&M regulations and Handbook 44. They can provide traceability for test weights and prior calibrations if they service your account regularly. Using a scale calibration company can also save you money vs doing the work in-house. You free up a maintenance resource and can eliminate the cost of maintaining your test weights.
Brady Systems calibration services
Brady Systems provides a variety of calibration services in Upstate New York and selected counties in Pennsylvania. We have 65+ years of experience servicing and performing maintenance services on a variety of scale makes and models. We calibrate static scales from lab balances to large-capacity tank systems and vehicle scales. Our test weights are certified regularly with accredited calibration labs. We also provide ISO 17025 calibration services as an authorized signatory for METTLER TOLEDO.
Brady utilizes METTLER TOLEDO’s scale calibration software, called MiraCal+. This software provides ISO 9001 and ISO 1075 calibrations with options for measurement uncertainty and additional annexes for Handbook 44. These certificates meet EURAMET standards. We can also provide additional services for recommending scale calibration frequencies and verification of a scale’s suitability for your process using GWP software. MiraCal+ and GWP provide a standardized and structured method for providing you with data regarding scales in your facility and their performance. This gives you solid proof that your scales are accurate and fit for purpose.
Local service backed by a worldwide reputation
Backed by the worldwide reputation of METTLER TOLEDO, Brady Systems is your local provider. Working in most of New York state and Northern Pennsylvania, we offer all the knowledge and authority of a global company in your backyard. Our employees live and work in the same neighborhoods you do. We strive to provide you with friendly, fair, and honest service. But we also offer you all the expertise that comes from representing the largest-scale provider in the world.