Blog

When using laboratory balance calibration weights, also referred to as mass standards, for laboratory or calibration applications, the proper care, handling, and usage are extremely critical in maintaining the traceability and accuracy of the weights. It is essential that the weight maintain its accuracy, especially if they are ASTM Class 0, ASTM Class 1, Ultra Class, or OIML Class E2 weights. In addition for critical weighing applications and balance calibrations, you need to maintain the weight calibration certificate value from the last calibration for that weight. If a calibration weight is accidently dropped, the calibration weight essentially is useless to the user until the weight can be calibrated again. So maintaining the calibration weight’s value and compliance to the respective tolerance for the duration of the calibration frequency is essential.

CARE AND HANDLING
It all starts with the handling of the weights. Precision calibration weights are stainless steel or sometimes aluminum for very small milligram weights (20mg and below). Very good tweezers, preferably stainless steel with fine carbon tips, are best for weights 100g down to 1mg. This will help prevent the calibration weight being dropped, damaged during handling, or even lost. For the larger calibration weights, 200g and up, a clean cotton glove, that is changed as needed, works best.

Never handle the weights with bare hands, abrasive equipment, or a glove that could be dirty or contaminated. The oils from the bare hand or other contaminents will cause a weight to change its value and if it is an Ultra Class, ASTM Class 0, ASTM Class 1, or OIML Class E2 weight might even cause the weight to go out of tolerance.

Abrasive handling equipment will scratch and damage the weight causing the weight to lose mass and have its weight calibration certificate value altered and eventually go out of tolerance.

The calibration weight should be handled and placed on the balance with great care. Most of out of tolerance conditions on calibration weights are caused by wear and tear occurred during regular use. The weight class tolerances on ASTM Class 1, ASTM Class 0, Ultra Class, and OIML E2 weights are extremely fine (and can go down to 0.010 mg, 0.006mg and 0.005mg respectively) and a continuously used weight can easily go out of tolerance. A calibration weight should be handled and placed on the balance platform as carefully as possible. This technique will help reduce wear and tare and out of tolerance conditions. Again, having stainless steel fine carbon tip tweezers will also help in reducing wear and tear.

Cleaning weights is usually not something that is done other than by the weight calibration vendor. But I do recommend, for the very precise and smaller calibration weights (10g down thru 1mg), if not a periodic cleaning then at the least a camel hair brush to dust the weight off each time prior to use on the balance. There is almost always a build up of dust and debris (either from the case itself or the balance and or environment). The build up can become considerable, changing the weights value and causing an out of tolerance condition for the precision calibration weights. Also, if a weight becomes contaminated (testing material, fingerprints, etc.) it could probably be cleaned without having to go back to the weight calibration company. When a cleaning is needed and desired, again just for the 100g calibration weights and smaller, the calibration may be easily cleaned with a very fine cheesecloth and ethyl alcohol. Great care must obviously be taken so as no to damage or alter the weight during cleaning, but a very light cleaning with the ethyl alcohol should do the trick and bring the weight back to its weight calibration certificate value.

The first thing that needs to be determined is what calibration weights are needed. This entails choosing the calibration weight class, weight range, and weight test points. Ideally, a weight should be more accurate than the weighing (test) instrument, though this is not always possible. Whenever possible, the calibration weight range should bracket the user range of the balance.

If an analytical balance that has a readability of 0.0001g (gram), and has a needed measurement range of 100g (grams) down to 100mg (milligrams), with a critical measurement point at 1g (gram), then the NIST Traceable calibration weights needed for this internal check would be at a minimum 100g, 1g, and 100mg. The accuracy for the calibration weights should be ASTM Class 1. ASTM Class 1 is a very precise calibration weight accuracy and the most common in domestic laboratory applications.

So, if we have those three weights incorporated into our internal check program, we have bracketed our measurement range for that particular analytical balance. In addition, with the very accurate ASTM Class 1 weights, we can feel very confident that when the weight is placed on the analytical balance and we see a very large error or discrepancy (Example: The 1g weight has a balance reading of 1.0030g) we know that there is an inaccuracy with the balance and it is time to contact the balance calibration vendor and more importantly that particular analytical balance needs to be put out of service until the inaccuracy is corrected.

The only time that ASTM Class 1 weights would not be sufficient would be in micro balance applications. Micro balances are extremely fine balances with readabilities of 0.000001g (gram) or even 0.0000001g (gram). For micro balance applications ASTM Class 0 or Ultra Class (Manufacturer Accuracy Class) weights would be needed. These are the most accurate weight classes and are better suited for the extremely fine readability of the micro balances.

The internal calibration weight balance check is not intended to duplicate the balance calibration performed by the balance calibration technicians during regularly scheduled preventative maintenances. It is a part of your quality assurance program to ensure that your weighing instruments are measuring accurately. The checks should be performed as per a standardized operating procedure (SOP) along with results being documented in the respective log book.

Also keep in mind, that the internal calibration weight checks alone are not sufficient. A regularly scheduled preventive maintenance and calibrations with a qualified vendor is still needed. And lastly, make sure your your internal calibration weights are calibrated on an appropriate interval (six months, annually, etc.) to ensure accuracy and NIST traceability.

For more info please see our “Internal Calibration Weight Verifications (Checks)” under Technical Articles in this site.

This is a quick guide and background on weight accuracy classifications for your balance calibration weight (i.e. mass standards) to ensure traceable and accurate weighing measurement & calibrations.

When choosing balance calibration weights for your weighing and measurement application, the first thing that needs to be addressed is what accuracy class will be needed. Accuracy classifications are predetermined accuracy designations at the time of manufacturer. Currently there are three major categories for classifying precision laboratory weights; ASTM, OIML, and Manufacturing (Ultra Class and Ulti Class).

WEIGHT CLASS BACKGROUND
The ASTM Classification is as per the document ASTM E 617. Most of the domestic (United States) weight classifications adhere to this specification. The Weight Classes are broken out numerically from ASTM Class 0 to 7, starting with the most accurate classes first. ASTM Class 0 would be the most accurate and tightest allowable tolerance of the weight classifications followed by ASTM Class 1 as the next most accurate (tightest allowable tolerance). In most precision laboratory and calibration applications ASTM Class 4 weights would be as far down as you would probably want to go.

The OIML Classification is used internationally (Europe, Africa, Asia, South America, etc.). OIML R 111-1 is the document for the OIML weight classifications. The classifications are broken out as per alpha-numeric designations, Class E1, Class E2, Class F1, Class F2, Class M1, Class M2, and Class M3. OIML Class E1 would be the most accurate and tightest allowable tolerance followed by OIML Class E2 and OIML Class F1 respectively. For precision laboratory and calibration applications OIML Class F2 should be used as a minimum accuracy for the weights.

Manufacturing weight classifications, Ultra Class and Ulti Class, are for very precise laboratory applications. These weight classifications are only recognized by the end users and the manufacturers themselves. The weighing regulating bodies of ASTM, NIST, and OIML do not formally recognize Ultra Class or Ulti Class. The classifications and tolerance values are very similar to the ASTM Class 0 weight classification.

ACCURACY CLASS APPLICATIONS AND USAGE
There aren’t too many standards or references in terms of what weights to use and when.

Speaking in metrological terms, a practical guide would be the following:

ASTM Class 0, Ultra Class, and OIML Class E1 should be used as at the highest level of precision i.e. mass standards (calibrating other weights), micro-balances testing and calibration, and critical weighing applications.

ASTM Class 1, 2 and OIML Class E2, F1 should be used in precision applications i.e analytical balance testing and calibration.

ASTM Class 3, 4 and OIML Class F1, F2 are best suited for Top Loading Balance calibrations and testing, and moderate precision applications (laboratory non-critical).

For more information and background please see our Calibration Weight Accuracies under the Technical Articles in this site.