1 weighing routine maintenance guide EN

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 alwa

Day-to-Day Routine Maintenance

of Laboratory Balances

Balance Operation Tips for Care System Check

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

Co

Micro, semi-micro, analytical and precision balances are measuring instruments of the highest precision

The following points will help you obtain reliable weighing results

1.1 Switching on

• Do not disconnect the balance from the power supply and always leave it switched

on This allows the balance to reach thermal equilibrium

• When you switch the balance off, use the display key (on older models the tare

key) The balance is now in standby mode The electronics are still energized and

no warm-up period is necessary

TIP: We recommend a different warm-up time for each balance when it is first connect

to the power supply These are:

• Up to 12 hours for micro balances

• Approximately 6 hours for semi-micro and analytical balances

• Approximately 3 hours for precision balances

Apart from these guidelines, always observe the minimum times specified in the operating instructions

1.2 Leveling

• 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 adjustments You must then adjust the sensitivity 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 GxP1) we recommend the Excellence Plus family of balances with the built-in automatic warning function

“LevelControl”

1.3 Adjustment

• Adjust the sensitivity of the balance regularly, especially

• when you operate the balance for the first time

• when you change the location of the balance

• after leveling the balance

• after major changes in temperature, humidity or air pressure

TIP: If a fully automatic calibration following, for example, a temperature change

is important to you, it would be worthwhile to acquire balances with “FACT,” which

calibrate automatically These models also allow you to extend the time interval for routine checks

[1] GxP Good Laboratory Practice (GLP) or Good Manufacturing Practice (GMP)

MinW eigh Adjust.int

Gross Tare

Weigh ing Steve M iller

1.00000 mg 1.00005 mg

• 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 balances

display 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, can become 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: TemTem-perature)

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)

MinW eigh Adjust.int

Gross Tare

Weigh ing Steve M iller

MinW eigh Adjust.int

Gross Tare

Weigh ing Steve M iller

Figure 1: The “ErgoClip Basket” taring container holder.

• Open the draft 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”.

• 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

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3.1 General

Basic Rules for Handling Balances

• Before using a balance, make sure the balance was left 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

5

1

2

5

1

2

5

1 2

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• 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 “Differences”

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)

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ck Deviation Control Limit 1)

• 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

3.4 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

• Mark the balance as “out of control limits”

• Contact METTLER TOLEDO service organization for advice

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)