A Guide to Weighing Scale Calibration Weights: Terminology, Approvals and Accuracy

You have a scale or balance that requires regular calibration, so you need weighing scale calibration weights. Calibration weights can be complicated, so this guide is here to help you understand them better and make an informed purchase.

What is Calibration?

Calibration is a procedure that forces the scale to display a certain value for a known standard mass. Then, you can determine any other mass by using the standard as a reference. Even though your brand-new scales and balances are fully calibrated before they're delivered to your facility, you'll need to re-calibrate them regularly to maintain their accuracy and precision over time.

What is a Calibration Weight?

A calibration weight is a block of metal, typically stainless steel though commonly cast iron or brass, that has been carefully constructed to weigh the same as a known standard mass such as one kilogram, one gram, etc. You can then use it as a reliable reference for your calibration procedures.

Why Are Calibration Weights Needed?

Weighing scale calibration weights are an extremely critical tool for keeping your scales and balances precise and accurate. When in calibration mode, your scale will typically request a certain reference weight when in calibration mode, such as 200g. If you place down an object that’s 205g, your scale will accept that as its new standard and all your future results will be off by as many as five grams from then on. This can be devastating for operations that require absolute accuracy from a weighing scale – and highlights the need to use professional calibration weights for checking calibration regularly.  

How Often Do You Need to Use Calibration Weights?

The more precise and accurate you need your results, the more often you should calibrate. For laboratories using semi-micro balances, this could be before every major weighing session, while for a warehouse bench scale, this could be every few months, or even once a year. Trade approved scales are a slightly different story. If you use a trade approved scale for commercial transactions and it's revealed that it provides inaccurate results, you could get in significant trouble. Frequent calibration helps protect you by guaranteeing each transaction is safe and sound. Learn more about calibration frequency in this blog.

Understanding Calibration Weight Terminology

Calibration weights have their own terminology. Learning it will help you choose the best calibration weight for your scale or balance.

Nominal Value

A calibration weight’s nominal value is which standard mass it reflects, such as 1kg, 1g, 20g, 5mg, etc. If you’re looking to purchase a calibration weight for your scale or balance, you should look in your manual to see if it recommends a specific weight for calibration.

Tolerance

It’s extremely difficult to manufacture a test weight to represent its nominal value exactly, though modern techniques are getting closer and closer. The tolerance of a weighing scale calibration weight is how much it could deviate from the nominal value it represents while still being accurate enough for use. For example, the tolerance on Kern’s 2mg E1 weight is 0.003mg. When the E1 weights underwent quality control, weights were deemed acceptable for commercial sale if they were within a range of 1.997 – 2.003mg, while the rest were discarded as inaccurate.

As a consumer, the goal is to find calibration weights that have the narrowest tolerance for their class – ideally, without breaking the bank.

Calibration Weight Accuracy Classes

Let’s explain what we mean by “for their class”.

What is a Weight Class?

When we refer to a calibration weight’s class, we’re talking about their accuracy class, as assigned by the International Organization of Legal Metrology (OIML). It’s a system that groups calibration weights according to how accurate they’re considered, with different groups being ideal for use with different scales and balances.

At Inscale, we offer calibration weights in five different accuracy classes:

• E1
• E2
• F1
• F2
• M1

Though there’s also M1-2, M2, M2-3 and M3.

OIML Weight Classes

OIML thoroughly spells out their guidelines for each weight in this PDF here, but we’ve summarised what the different classes are for in the chart below.

Choosing the most precise calibration weight isn’t necessarily in your best interest. The higher the class, the more expensive they are, which means you’ll need to be way more careful about upkeep and use. If you have a precision balance, you don’t need an E1 weight, an F1 or F2 equivalent is perfectly fine.  

How to Choose the Right Weighing Scale Calibration Weight

Inscale offers a variety of individual weights as well as calibration weight sets.  So, which weighing scale calibration weight is right for you?

Consider Your Weighing Instrument

At Inscale, our calibration weights are grouped by class to make it easier to search. Which class you choose will depend on your weighing instrument. If you’re using a balance like Adam’s Polaris Semi-Micro with a readability of 0.00001g, you’ll want to use either E1 or E2 calibration weights. If you’re using a Luna LTB Top-Pan Precision balance with a readability of 0.01g, an F2 weight will work.

Once you know what class of weight you need, you’ll want to choose from the different nominal values available. As mentioned earlier, you can discover which weight is recommended in your balance’s manual. Some scales and balances will also require certain weights in calibration mode. Always check before purchasing.

Consider Your Purpose

Calibration weights aren’t only used for calibrating. They can also be used as general tools for verifying that your balance is accurate in the range you’re requiring. For example, you might not need to calibrate a balance with a 1mg E1 test weight, but if you’re weighing items in the 1mg range, you can gently place it on the pan to ensure that it’s going to weigh your items precisely enough for your purposes.

Sets vs. Individual Weighing Scale Calibration Weights

A set of calibration weights is just a collection of weights in a variety of nominal values. If you know that you weigh items around 200g almost exclusively, you may decide to only purchase a 200g calibration weight. That’s fine – but if you weigh a range of weights or have multiple different types of scales, a set can be helpful. They’re also helpful for checking the accuracy of your scale through its capacity range, so you can guarantee it’s accurate from the lightest to the heaviest items it can handle.

How to Use Calibration Weights Correctly

Using calibration weights is simple. The first rule – don’t touch them with your bare hands. Believe it or not, the oil on your fingers can stick and add a miniscule amount of weight to your results. Or, worse, it can break down the weight over time. Always wear cotton gloves or use tweezers.

Before loading, examine the weight for any surface lint or hairs that could influence your calibration. If debris is discovered, you can remove it with a soft microfibre cloth or a soft bristled brush. If it’s really dirty, an ethanol based gentle cleanser can be used as a last resort. Be sure to let it fully dry before use.

When loading the weight onto the pan, set it down directly, without dragging. Not only can dragging lead to scratches on the surface of both your weighing pan and your test weight, but it also causes friction that can make it more likely static to generate. Same for lifting it up afterwards!

You can stack calibration weights to check all the way up to capacity and back down again, but always go largest to smallest and avoid dragging. Remove one at a time.

Storage

Calibration weights need to be stored carefully to prevent them from losing their precision over time.

Always ensure you store them in a padded box with individual compartments for each weight so they don’t touch. Keep the box in a temperature-controlled room away from direct sunlight.

Still need help choosing between weighing scale calibration weights? We can help. Contact Inscale today.