1 weighing routine maintenance guide en

8/17/2019 1 Weighing Routine Maintenance Guide En

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Day-to-Day Routine Maintenance

of Laboratory Balances

M a i n t e n a n c e G

u i d e

Balance Operation

Tips for Care

System Check



Editorial

Dear Reader,

Weighing is one of the most common tasks in the laboratory. Advanced micro, semi-micro,

analytical and precision balances have now been perfected to such a degree that, in general,no special weighing rooms are needed.

Technological advances in electronics have considerably simplified the operation of

balances, drastically cut weighing times, and made the balances so adaptable that they can

now be integrated directly into a production process.

The risk inherent in this progress, however, is that not enough care will be taken to prevent

disturbing influences from the surrounding area.

The purpose of these instructions is to explain the most important points to be noted when

working with micro, semi-micro and analytical balances if high-quality results are required.

METTLER TOLEDO

E d i t o r i a l



3METTLER TOLEDO Balance Routine Maintenance

C o n t e n t Content

1 Operation of the Balance 4

2 Care of the Balance 7

4 An Easy Balance Performance Check 8

5 More Information 11




O p e r a t i o n o f t h e

B a l a n c e 1.4 Reading

• Check that the balance displays exactly zero at the start of each weighing. Tare, if

needed, to avoid zero errors.

• Read off the result only after the small round circle in the upper left of the balance

display has faded. The weighing result is released through this stability detector.

TIP: The Excellence Plus line offers an advanced stability detector. These balancesdisplay unstable measurement values in blue. Once stability is regained, the display

immediately changes to black and the circle in the upper left disappears. This allows

you to identify a stable weighing result faster, safer and more reliably.

1.5 Weighing pan

• Always place the weighing sample in the middle of the weighing pan. This will

prevent 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 deactivate the “initial weigh-

ing effect”.

1.6 Weighing vessel

• Use the smallest possible weighing vessel.

• Avoid weighing vessels made of plastic when atmospheric humidity is below

30-40 %. These conditions increase the risk of electrostatic charge.

Materials with a high degree of electrical insulation, such as glass and plastic, canbecome electrostatically charged. This can drastically distort the weighing result.

For this reason, make sure you take the appropriate corrective measures (for more,

see page 14: Electrostatics)

• The weighing vessel and the sample it contains should have the same temperature as the surroundings. Tem-

perature differences can lead to air currents that distort the weighing result (see page 7: Temperature).

After removing the weighing vessel from a drying oven or dishwasher, allow it time to cool before placing it on

the balance.

• If possible, do not use your hands to place the weighing vessel in the weighing chamber. You could alter the

temperature and atmospheric humidity of the weighing chamber and the weighing vessel, which would have

an adverse effect on the measurement process.

TIP: Different taring container holders offer optimal conditions for error-free and safe

weighing (see illustrations).

W e i g h i n g S t e v e M i l l e r

W e i g h i n g S t e v e M i l l e r

Figure 1: The “ErgoClip Basket”

taring container holder.




O p e r a t i o n o f t h e

B a l a n c e 1.7 Draft shield

• Open the draf t shield only as far as necessary. This will keep the climate in the

weighing chamber constant, and the weighing result will not be influenced.

• Adjust balances that have an automatic and configurable draft shield, such as the

instruments of the Excellence Plus family, so that the opening of the draft shield is

minimal.

TIP: To make weighing simpler and more precise, even under the most challenging

conditions, we recommend specific accessories for our Excellence and Excellence

Plus families of balances. These balances will yield the best results, even when you

are weighing extremely small samples with narrow tolerances under unfavorable

ambient conditions. Our special draft shield “MinWeigh Door,” for example, is

designed to be perfect for use in weighing cabinets. But it also offers advantages for “normal” weighing

conditions. It can improve the repeatability of the net reading by a factor of about two!

Using “SmartGrid,” a weighing pan with a special grid structure, stabilizes weighing so well that the draft shield

doors for 4-digit analytical balances can normally be left open during weighing.

Figure 2: Round bottomed

flasks on special taring con-

tainer holders “ErgoClip Flask”

and “MinWeigh Door”.




C a r e o f t h e b a l a n c e 2. Care of the balance

• Keep the weighing chamber and weighing pan clean.

• Use only clean vessels for weighing.

(see chapter 2.1)

• The balance can be cleaned with conventional win-

dow-cleaning fluid.• Do not use cloths that contain fusel oil.

• Do not brush contaminants into potential openings.

• Before cleaning, remove all removable parts, such as

the weighing pan.

TIP: On Excellence and Excellence Plus analytical

balances, each of the !draft shield panes can be

removed for cleaning in a dishwasher.

2.1 Cleaning of the Balance

Many factors contribute to obtaining reliable weighing results which always fall within process specifications.

One factor which is often overlooked, and which can have a major influence on the quality of results is simply

the cleanliness of the measuring instrument being used. Especially with balances, the risks of sample (cross)

contamination can be large whenever cleaning guidelines are not strictly adhered to.

But which steps need to be considered when cleaning a balance? And which cleaning agents can/should be

used? This reference paper aims to provide an overview of typical cleaning products which can be used to clean

the various parts of a typical lab balance, and also offers some important considerations when purchasing

a new balance.

General recommendations for cleaning

• Depending on your laboratory guidelines, the frequency for cleaning of equipment may differ. Make sure that

your SOP specifies the frequency for balance cleaning, and that these intervals are carried out for each instru-

ment according to instructions.

• Frequency: in many laboratories, the rule of thumb is to check the balance on sight. If it is obviously dirty,

clean the balance before weighing. Otherwise the minimum recommendation would be to clean the balance at

least once per week.

• Where toxic samples are being weighed, the recommendation would be to clean the balance immediately after

each weighing.

• The use of aluminum foil (or similar) placed around the weighing pan to protect the balance from becoming

dirty is not recommended; it typically makes it more difficult to clean the surfaces thoroughly, and more

difficult to perform a visual check.

Observe the following when cleaning

• The balance should be completely disconnected from the power supply

• Ensure that no liquid comes into direct contact with the balance and its AC adapter

• Never completely dismantle a balance for cleaning – there are no components inside the balance that require

cleaning. In case of repair, contact a service technician.

• Under no circumstances use cleaning agents containing inappropriate solvents or abrasives; the use of such

agents can result in permanent damage to the panel or the balance itself.

XPE205




A n E a s y B a l a n c e P e r f o r m a n

c e C h e c k 3. An Easy Balance Performance Check

3.1 General

Basic Rules for Handling Balances

• Before using a balance, make sure the balance was lef t on power for a sufficient period of time (mentioned in

the balance operating instructions).• Make sure the balance is leveled.

• Minimize environmental influences, e.g. open windows, direct sunlight or strong drafts.

• Do not enter the draft shield with hands. Use gloves or long tweezers.

• Place objects gently and in the center of the pan.

Basic Rules for Handling Weights

Test Weights

• Only an external test weight with calibration certificate can make a balance a “traceable” piece of equipment.

• Test weights should always be placed gently on a clean weighing pan and put back immediately in their

storage place after use.

• Test weights (since they are also part of measuring equipment) need to be re-calibrated at specified intervals

(ISO 9001).

• Any incident, which might have affected the value of the test weight, should trigger an immediate re-calibra-

tion. METTLER TOLEDO’s calibration services will give advice on this.

How to Store Test Weights

• Test weights should be stored in their original box.

• Test weights should be stored in the same room as the balance they are used with, since temperature differ-

ences between test weights and their surrounding lead to measurement errors.

• Test weights that have not been stored at the same temperature need acclimatization, which can take several

hours.

How to Move Weights

• Test weights should only be handled with appropriate tools such as tweezers, forks, handles or gloves

(see METTLER TOLEDO’s accessories for weights).

• These tools should be exclusively used for transferring test weights, to avoid possible contamination.

3.2 Eccentricity Test

Preparation

• Before the test is performed, the test weight must be acclimatized to the ambient

temperature of the balance.• Prepare a sheet to note the readings of the eccentricity test.

Test Procedure

• Place the test weight in the center (1) of the pan and tare (if required press zero)

• Place the test weight on position 2 at “Left front”

Note: Move the weights by lifting them; do not shift weights on the pan

• Read the stable value from the display and note it

• Repeat the measurements on positions 3, 4 and 5.

1

2

3 4

5

1

2

3 4

5





c e C h e c k Evaluation

• The largest reading (positive or negative) of any of the 4 corners (2 to 5) is the eccentric load deviation.

• Evaluate whether the eccentric load deviation exceeds the defined “Warning Limit” 1).

• Evaluate whether the eccentric load deviation exceeds the defined “Control Limit” 2).

Deviation

Warning Limit

1)

(where defined)• If the warning limit is exceeded, level the balance and repeat the test.

• If the warning limit is exceeded again, report that the warning limit was not met to the laboratory supervisor or

the person responsible of the balance. Optionally, contact METTLER TOLEDO’s service organization

Control Limit 2)

• If the control limit is exceeded, report the problem to the laboratory supervisor or the person responsible of the

balance.

• Mark the balance as “out of control limits”.

• Contact METTLER TOLEDO service organization for advice.

3.3 Repeatability Test

Preparation

• Before performing the test, the test weight must be acclimatized to the ambient temperature of the balance.

• Prepare a sheet where you can note the readings of the repeatability test points. Prepare 3 columns titled “w/o

Test Wght”, “With Test Wght.” and “Dif ferences”.

Test Procedure

• Empty the pan

• If required, place tare load on the weighing pan• Tare the balance (if required press zero)

• Read the stable value from the display and note it in the column “w/o Test Wght.”

• Place the test weight in the center of the weighing pan

• Read the stable value from the display and note it in the column “With Test Wght.”

• Remove the test weight

• Repeat the measurements from “tare the balance” to “remove test weight” until the customer defined number of

“w/o Test Wght.” and “With Test Wght.” readings is performed

Evaluation

• Subtract each “w/o Test Wght.” from the corresponding “With Test Wght.” to calculate the difference.

• Calculate the standard deviation of the differences.• Evaluate whether standard deviation exceeds the defined “Control Limit” 3).

1) – Values within the warning limit: No action is necessary.

– Values between the warning and control limit are within the tolerance but must be kept under surveillance.

Corrective action may be appropriate, depending on the direction in which the values are changing.

2) – Values within the control limit, see 1)

– Values beyond the control limit show that weighing process is no longer under control and immediate action is therefore

required.

3) – Values within the control limit: No action is necessary.

– Values beyond the control limit show that weighing process is no longer under control and immediate action is therefore

required.





c e C h e c k

Deviation

Control Limit 1)

• If the control limit is exceeded, report the problem to the laboratory supervisor or the person responsible of

the balance.



3.4 Sensitivity Test

Preparation

• Before the test is performed, the test weight must be acclimatized to the ambient temperature of the balance.

• Prepare a sheet where you can note the readings of the sensitivity test.

Test Procedure

• Empty the pan

• If required, place the tare load on the weighing pan

• Tare the balance (if required press zero)

• Place the test weight on the weighing pan• Read the stable value from the display and note it

Evaluation

• Evaluate whether the noted value exceeds the defined “Warning Limit” 2).

• Evaluate whether the noted value exceeds the defined “Control Limit” 3).

Deviation

Warning Limit 2) (where defined)

• If the warning limit is exceeded, repeat the test.

• If the warning limit is exceeded again, report that the warning limit was not met to the laboratory supervisor

or the person responsible for the balance.• Level the balance, perform adjustment with built-in or external adjustment weight and repeat the test.

• If the warning limit is still exceeded, report the problem to the laboratory supervisor or the person responsible

of the balance. Optionally, contact METTLER TOLEDO’s service organization for advice.

Control Limit 3)

• If the control limit is exceeded, report the problem to the laboratory supervisor or the person responsible of the

balance.



1) – Values within the control limit: No action is necessary.

– Values beyond the control limit show that weighing process is no longer under control and immediate action is therefore required.

2) – Values within the warning limit: No action is necessary.

– Values between the warning and control limit are within the tolerance but must be kept under surveillance. Corrective action may

be appropriate, depending on the direction in which the values are changing.

3) – Values within the control limit, see 2)

– Values beyond the control limit show that weighing process is no longer under control and immediate action is therefore required.




M o r e I n f o r m a t i o n 4. More Information

4.1 Webinars and e-Trainings

We provide web-based seminars (webinars) on different topics. You can participate in on-demand webinars

at any convenient time and place.

Live webinars offer the added benefit of allowing you to ask questions and discuss points of interest with

METTLER TOLEDO specialists and other participants.

www.mt.com/webinars

4.2 Useful Links

For more information about LabX Laboratory Software Solution, visit www.mt.com/labx

For more information about the effect of external influences, visit www.mt.com/lab-properweighing

For more information about laboratory balances, visit www.mt.com/balances

For more information about GWP and routine testing, visit www.mt.com/gwp

For more information about SmartPan weighing pans, visit www.mt.com/smartpan

For more information about proper weighing and cleaning with dedicated elearning courses, visit

www.mt.com/lab-elearning



Good Weighing Practice™ (GWP®) is the science based global weighing

standard for the efficient life cycle management of weighing systems. Therisk-based approach allows you to improve control of your whole measuring

process, which in turn helps to avoid costly out of specification results.

Benchmark Your Quality

Management System

GWP® Verification helps you assure accurate

weighing results as part of

your quality management system.

Applicable for all balances and scales,

it provides an optimized testing andcalibrating scheme which may translate

to sustainable time and cost savings.

Your Benefits

• The accuracy of your weighing instruments is matched to your process tolerances.

• A comprehensive summary report provides an overview of the status of all

weighing equipment.

• An optimized routine testing and calibration schedule results in sustainable

cost savings.

• Audit-proof and up-to-date documentation complements your quality management

system.

For your success: you focus on your process, we take care of the measurement.

www.mt.com/gwp

For more information

Mettler-Toledo International Inc.

Laboratory Division

CH-8606 Greifensee, Switzerland

Subject to technical changes

© 06/2015 Mettler-Toledo AG

Global MarCom Switzerland

www.mt.com

Accurate Measuring through GWP®

The Global Weighing Standard

1 weighing routine maintenance guide en

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