Scale Calibration Guide
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If you rely on a scale for any part of your business — you need to make sure it’s calibrated. Unfortunately, even the highest quality scale won’t be accurate forever. Normal wear and tear occurs and over time, accuracy slips. It can degrade even quicker if the scale is exposed to electric or mechanical shock or is in a manufacturing environment with moisture, oils, metal chips, etc.
Calibration is the process of testing the scale, to ensure the level of accuracy you require. In a laboratory setting, where results are dependent upon exact weights, scale calibration is of particular importance. An inaccurate scale could significantly hurt your business. In processing and manufacturing, incorrect measurements could result in product quality issues, scrapped batches, or even a product recall.
How Does the Scale Calibration Procedure Work?
A known standard or certified mass is placed on your scale. That weight reading is recorded. If the weight readings match the standards applied or fall within the calibration tolerance (more about that below), the scale does not need any adjustment. If the weight readings do not match or fall within the tolerance, service will likely be needed to restore the scale to the acceptable state of accuracy.
While an instrument may be capable of measuring a particular range of weight, known as the instrument range, it may or may not be calibrated for that entire range. Instead, there may be a calibration range that’s chosen, which ranges from zero to a maximum weight of your choosing (also known as the span). Once the calibration range is defined, the calibration is done at several points from zero to span to ensure high accuracy.
Once you realize that scale calibration is essential to the success of your business, you’re sure to have some questions:
- What is factory calibration?
- How often should you calibrate your scales?
- What is legal-for-trade calibration?
This guide was designed to give you some insight to scale calibration so you can create a plan of action and ensure that you don’t have any problems associated with having an inaccurate scale.
Types of Scale Calibration: What Is Factory Calibration?
When you purchase a scale, it often comes with “factory calibration” – meaning that it was calibrated at the facility before you got the scale. In the few days that have passed since it left the factory, you might assume that your scale is delivered calibrated and ready for use, right? Wrong. The location of the factory and your location are two different places – with two different elevations and therefore, the on-site calibration is the only way to ensure your scale is accurate where you intend to use it.
If you do decide you want to have your scale calibrated once it arrives, where do you go? Third-party calibration services are often the solution, but not all third-party calibration services are created equal. Your ultimate goal is to restore your scale to its original, optimal accuracy state, so the details matter. Some third-party calibration services, like Precision Solutions, will not only calibrate the scale, but also make adjustments and perform repairs as needed to get the deviation from the mass standard back within tolerance.
How Often Should You Calibrate Your Scales?
How often you calibrate your scales depends on a few different factors — manufacturer’s recommendations, how often you use the scales, the environment they’re in, and how essential an exact weight is to your business. Some are calibrated once per month, others are only calibrated once per year, while some are even spot checked daily for accuracy.
Usually, after considering these factors, it’s determined that somewhere in the middle —certified calibration once per quarter with a weekly user spot check — is ideal to ensure quality control of scales that are used fairly often. However, your need may vary based on the information below.
Always consult the manufacturer’s recommendations first. Manufacturers should have a recommendation for the frequency of calibration, and since all scales are made differently, it’s usually safe to assume the manufacturer knows best.
If your scales are used multiple times throughout the day, every day of the week, normal wear and tear will occur faster than in scales that are used a few times per week. Therefore, if you use these scales more frequently, they should be calibrated more frequently — perhaps monthly.
The surrounding environment also plays a role. For example, if your scale is in an area that contains dust, fluids, or other substances, your scale could have a buildup of these substances that interfere with the performance of the scale. Or, if you have the scale located in a place where there are vibrations, static electricity or mechanical shock, scale accuracy can suffer.
In general, you should also consider a more frequent calibration to ensure these substances and other work environment factors are not getting in the way of getting a precise weight.
Finally, consider how important an accurate weight is to your business. If your company can’t afford to have even the slightest inaccuracy in weight, it’s likely that more frequent calibrations will be necessary. It may be easy to understand the importance of accuracy when blending pharmaceutical ingredients where a mistake in weighing a single batch could easily exceed $100,000 in cost. But what about rocks and stones from a quarry where a truckload might only be worth a few hundred dollars. Would a small error of 1% make much of a difference? Maybe not for that one load, but when you take into account that the same scale also weighed 100 other trucks each day, you could be looking at a loss of over $75,000 if you wait a whole year to calibrate that scale.
Calibrations are done at a variety of frequencies, depending on a variety of factors. It’s important to note that if your equipment is frequently calibrated and almost always needs adjustment or repairs, it could be a sign of a bigger issue.
While the frequency of calibrations depends upon use, the rapid deterioration of accuracy warrants additional troubleshooting.
Once you’ve determined the appropriate frequency for your scales, you should look into signing a service plan with a calibration company. Many companies, like Precision Solutions, will work with you to learn more about your process and then create a maintenance plan tailored to you and your equipment
What Is Calibration Tolerance and How Do I Calculate It?
Calibration tolerance is defined by the International Society of Automation (ISA) as “permissible deviation from a specified value; may be expressed in measurement units, percent of span, or percent of reading.“ When it comes to scale calibration, tolerance is the amount the weight reading on your scale can differ from the nominal value of the mass standard that has optimal accuracy. Of course, ideally, everything would match up perfectly. Since that’s not the case, tolerance guides ensure that your scale is measuring weights within a range that will not negatively affect your business.
While the ISA specifically states that tolerance can be in measurement units, percent of span or percent of reading, it’s ideal to calculate the measurement units. Eliminating the need for any percentage calculations is ideal, as those additional calculations only leave more room for error.
The manufacturer will specify accuracy and tolerance for your particular scale, but you should not use this as your only source to determine the calibration tolerance you will use. Rather, in addition to the manufacturer’s specified tolerance, you should consider:
- Regulatory accuracy and maintenance requirements
- Your process requirements
- Consistency with similar instruments at your facility
Let’s say, for example, your process requires ±5 grams, test equipment is capable of ±0.25 grams, and the manufacturer states accuracy for your scale is ±0.25 grams. Your specified calibration tolerance would need to be between the process requirement of ±5 grams and the manufacturer’s tolerance of ±0.25 grams. To narrow it down even further, the calibration tolerance should be consistent with other, similar instruments at your facility. You should also use an accuracy ratio of 4:1 to decrease the chance compromising the calibrations. So, in this example, the accuracy of the scale should be ±1.25 grams or finer(5 grams divided by 4 from the 4:1 ratio). Furthermore, to properly calibrate the scale in this example, the calibration technician should be using a mass standard with an accuracy tolerance of at least ±0.3125 grams or finer(1.25 grams divided by 4 from the 4:1 ratio).
What Is Legal-for-Trade Calibration?
Commercial scales require legal-for-trade calibration. That means the scale needs to be calibrated according to the standards found in “Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices” — also known as “Handbook 44.” This comprehensive list of requirements is mandatory for all scales used for commercial purposes. A new version is published every year by the National Institute of Standards and Technology (NIST).
Do I Need Legal-for-Trade Calibration?
If you use your scale for commercial purposes, you need legal-for-trade calibration. For example, at the grocery store, your lunch meat and cheese is weighed on a scale and is sold to you at a price determined by that weight. That grocery store scale is an example of a commercial scale.
Commercial scales require legal-for-trade calibration. A state weights and measures inspector will follow up to ensure that your commercial scale has proper calibration. A device found to be out of tolerance by a state inspector is generally tagged and taken out of service. To avoid the problems associated with a device being found out of calibration by a state inspector, owners of commercial scales should have them calibrated and adjusted periodically to keep them in proper operating order.
Do I Need to Register My Weighing Device?
Yes. Many states, like Pennsylvania and New Jersey, require that you register commercial scales with the state. Also, you must make sure that the state you plan to use the scale in approves of the purchase. Most states accept scales that are National Type Evaluation Program (NTEP) approved. NTEP-approved scales get a Certificate of Conformance (CC or CoC), showing its approval. This document is usually in the form of a metal plate or tag on the scale with the make, model and capacity information.
The state will inspect and test the scale then review the results. As long as your scale has the Certificate of Conformance, and the scale performs to all of the relevant standards and tolerances in Handbook 44, you will pass inspection. If your device is out of tolerance, the state weights and measures official will not adjust it to bring it back within tolerance. Rather, you will be required to repair and adjust the scale. Only a licensed and professional scale calibration and repair company like Precision Solutions can do this.
How Do I Register My Weighing Device?
A professional calibration service company, like Precision Solutions, can help you with this process and, in some cases, can register your measuring device for you. Or, if you’d prefer to do it on your own, you can contact your state office of weights and measures to request a registration application. There is a registration fee that varies by state. Contact information for both Pennsylvania and New Jersey are listed below.
Pennsylvania Weights and Measures Inspection
2301 North Cameron Street
Harrisburg, PA 17110
717-787-9089
New Jersey Office of Weights and Measures
1261 Route 1&9 South
Avenal, NJ 07001
732-815-4840
Consistent scale calibration is essential for businesses that rely on accurate measurements of weight to do business. Over time, the performance of scales will begin to decrease — having a calibration and preventive maintenance plan in place that takes into consideration your environment, frequency of use, etc., will ensure performance does not suffer and restores your scale to higher accuracy. If you’re using a commercial scale, having a calibration plan in place will provide legal-for-trade calibration and give you the peace of mind that comes with knowing you’re meeting the demands of Handbook 44 and state requirements.
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How Do I Choose a Company to Calibrate My Scale?
All of this information won’t mean anything if it’s not documented and “traceable” to a primary standard. What does this mean, exactly?
Documentation is pretty straightforward. You must have a calibration certificate to show calibration results. If any part of the documentation isn’t there, the calibration is not traceable, and therefore, is like it never happened.
More importantly, it needs to be traceable to a primary standard. What is a primary standard? It’s the standard set by NIST. This does not mean you need to have your instruments calibrated by NIST, but it does mean that the company you use needs to have test standards that are calibrated by an unbroken chain of calibrations, traceable to NIST. Using a calibration company that is accredited to the ISO 17025 standard provides this assurance of calibration traceability as well as proficiency in performing the calibrations.
Given the importance of this information, you can see why it’s advantageous to select a company that specializes in calibration — and why it’s so important to choose that company carefully.
There are two types of calibration companies — those that are accredited and those that are not. Why does it matter? The ones that are accredited are traceable.
That means you can be confident that the laboratory documents calibration results, has a system and procedure that is in working order and has employees that are educated and skilled in everything from the calibration process to the documentation of results. It also means that to keep their accreditation, they have to be audited on a regular basis and pass so that you can be confident in their technical proficiency and documentation abilities.
In addition to looking for an accredited calibration company, you want to make sure the team is educated and experienced in scale calibration and understands your process needs. Using Certified Weighing Technicians (CWT), certified by the International Society of Weights and Measures, provides you with the assurance that the technicians have the experience and knowledge necessary to perform expert calibrations. The CWT program requirements include years of experience in scale calibration, formal NIST H-44 training and passing a required written exam, and, to remain current, the technician must be re-certified every five years. The qualities you should look for in a CWT are superior documentation practices, attention to detail, honesty and integrity and understanding of the various calibration processes.
Let Precision Solutions Calibrate Your Equipment
Our operations, based out of our headquarters in Quakertown, Pennsylvania, are ISO/IEC 17025 accredited and can accommodate large jobs or jobs that require fast turnaround times. We also understand the value of calibrating your equipment right in your facility, so we’re also fully equipped and accredited to provide on-site calibration services.
To learn more about the calibration services we provide, visit the services page on our website. Or, contact us if you have questions about how we can help you develop a customized calibration plan for your instruments.
