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Tips and Tricks for Weighing

Helper Booklet

Weighing

2

Dear Reader

First of all, thank you for your interest in this tips and tricks booklet for the chemical industry. We have assembled a set of tips and tricks about weighing. They will help you in achieving better weighing results, or refresh your knowledge about these topics.

Weighing performance and accuracy depends to a certain level on the surroundings and from the performance of the user.

The tips and tricks give insight into good weighing for repeatable and accurate results.

We hope you enjoy reading them as much as we did assembling them.

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Ensure the Weighing Table is stable and vibration-free The weighing table should be reserved for the balance. It must not sag, and must be stable enough that the balance display does not change when someone leans on the table or steps up to the weighing sta-tion. The weighing table should transfer as few vibrations as possible and be fixed to either the floor or the wall, but not both, as this transfers vibrations from wall and floor.

Tip 1 Weighing Table

Avoid Electrostatic Charges from the Weighing TableThe weighing bench should be antimag-netic, with no steel plate, and be protected against electrostatic charges by the elimi-nation of plastic and glass.

Tip 2 Electrostatic Charge

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Avoid Vibrations and DraftsIdeally, the balance room should be situ-ated in the most vibration-free area of the building, and accessed through a sliding door. Locate the balance away from doors and areas of high traffic to avoid drafts. Do not weigh near windows or near radiators, which can cause strong currents of air that may interfere with the balance. Avoid plac-ing the balance in the air flow of air condi-tioning units or devices with fans, such as computers.

Tip 3 Vibrations

Influences of Light, Temperature and Humidity Weighing results are influenced by tem-perature. Locate the balance on a window free wall, away from direct sunlight (heat) and from heat radiating light fixtures, such as light bulbs. Keep the temperature of the room as constant as possible. The relative humidity (RH) should be between 45 and 60 %. Never operate the balance outside the specified range of 20 to 80 % RH. Temperature gradients between the weighing sample and the surrounding environment can lead to air currents along the weighing vessel. Air flowing along the side of the ves-sel generates an upward or downward force that affects the weighing result, impacting on reproducibility and accuracy. Always acclimatize samples to the temperature of the laboratory or weighing chamber.

Tip 4 Light and Humidity

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Absorption / Evaporation Effects Hygroscopic samples absorb atmospheric moisture, gaining weight, while volatile samples evaporate and show a weight loss. Use clean, dry, stoppered weighing vessels with narrow necks. Keep samples in closed containers at all times.

Tip 5 Absorption and Evaporation

Electrostatic Charge from the Weighing Vessel Weighing vessels and samples may be-come electrostatically charged, particularly in dry air. Metal is the best material for weighing vessels, and glass is preferable to plastic as it is less susceptible to elec-trostatic charge. Reduce weighing errors by increasing atmospheric moisture and using antistatic kits.

Tip 6 Electrostatic Charge

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WeighingSteve Miller

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Protect Balance from Body Heat Heat radiating from an operator’s body may have an adverse effect on weighing measurements. Where possible, avoid us-ing the hands to place the weighing vessel in the weighing chamber, as this can alter temperature and atmospheric humidity. Wear a lab coat to reduce the effect of the body’s surface temperature.

Tip 7 Body Heat

Electrostatic Charge Originating from the Operator Clothing can induce a strong attractive charge on the material on the weighing pan, causing very erratic readings. Wear cotton rather than synthetic fibres lab coats.

Tip 8 Electrostatic Charge

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Leveling your Balance Align the balance: To do this, check that the air bubble is in the center of the level indicator. Use the leveling feet to make ad-justments. You must then adjust the sensi-tivity of the balance. The exact procedure is described in the operating instructions for the balance. TIP: To ensure and document that the balance is correctly leveled every time for example, to comply with GLP and GMP we recommend the Excellence Plus family of balances with the built-in automatic warning function “LevelControl”.

Tip 9 Leveling

Avoid Corner Load Errors – Eccentricity Always place the weighing sample in the middle of the weighing pan. This will pre-vent corner load errors. With micro and semi-micro balances, the weighing pan should first be loaded once briefly after a relatively long pause (>30 min) to deacti-vate the “initial weighing effect”.

Tip 10 Eccentricity

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For more informationMettler-Toledo International Inc.Lab Division MarketingCH-8606 Greifensee, SwitzerlandTel. +41-44-944 22 11Fax +41-44-944 30 60

Subject to technical changes© 02/2012 Mettler-Toledo International Inc.Printed in Switzerland

www.mt.com/ChemLab

Suggested Reading:

The interactive refinery guide has been created for the petrochemical industry. A good reference source to get a quick and comprehensive overview. www.mt.com/Refinery-Lab

For more informationwww.mt.com

Mettler-Toledo AG Laboratory&WeighingTechnologiesCH-8606Greifensee,SwitzerlandTel.+41449442211Fax+41449443060

Subject to technical changes©01/2012Mettler-ToledoAGPrinted in SwitzerlandGlobalMarComSwitzerland

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Good Measuring PracticesRisk-basedguidelinesforweigh-ing,titrationandpipettingfromMETTLERTOLEDOempoweryouto make the right decision when it really matters. www.mt.com/gp

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Good Measuring Practices

Risk-based guidelines for weigh- ing, titration and pipetting from METTLER TOLEDO empower you to make the right decision when it really matters. www.mt.com/gp

Interactive Refinery Guide