Laboratory Balances

by Frank Weithöner

Laboratory balances or micro-balances are used in hospital laboratories, pharmaceutical industry, research institutions and schools. They are designed to determine precisely very small weights. Resolutions of a thousandth of a gram are not unusual. Laboratory balances are sophisticated and sensitive electromechanical devices which need special care and knowledge in the usage, maintenance and repair.

Laboratory balances for different purposes and requirements.

In laboratories we usually find two types of balances: The precision balance and the analytical balance. The difference is the accuracy. Analytical balances are more precise and thus more expensive.
But progress in electronics in the last years have simplified the usage and made balances cheaper. Precision balances are available from 400 €, analytical balances from 1,500 €.

Balance or scale?
Both, the term balance and weighing scale are used when people talk about the weighing of masses. Both expressions can be used but in practice the following definition prevails: Scales are used for weighing larger masses like a bathroom scale or baby scale and balances are used for precise weighing of substances, as such are used in laboratories.

Weight or mass?
A bit more tricky is the difference between weight and mass. Both words are used interchangeably. Strictly speaking we must consider the following facts:

  The unit of the mass is the kilogram [kg]. It is equal to the mass of the international
    prototype kilogram and therefore fixed.
  The weight is a force which is expressed in the unit Newton [N]. It is the product of mass
    and the earth's gravity. Therefore, the value is not fixed but depending on the location.

This is important when we think about the different weighing principles. A spring scale mechanic of a bathroom scale for example measures the distance a spring deflects under its load. It is influenced by the altitude (force of gravity) and so it measures the weight.
A laboratory balance measures a mass by comparing a unknown mass with a reference mass.
Unfortunately we always say, that we measure a weight and use calibration weights which is strictly spoken not correct: A balance measures a mass.

Technical expressions
When we talk about the balance's specification and measurement features we will stumble over technical expressions which may not necessarily be clear.
Here are some terms we should know:

Accuracy   How close the balance display is to the actual weight of a weighing sample.
Adjustment   » Calibration
Calibration   The deviation between the measurement value showing in the display and the true value of the measurement. An adjustment with internal or external calibration weights should then be made.
Capacity   The largest weight the balance is capable of measuring.
Corner load   The deviation of the measurement value through off-center loading.
Digit   The smallest increment of weight that the digital display resolves.
Drift   The drift is the slow change of the readout over a long time and with a constant load on the balance.
Linearity   Deviation from the theoretical linear curve of two variables: The zero point and the calibrated point close to the maximum capacity.
Ppm   Parts per million = 10-6 (e. g. 1 mg of 1 kg)
Readability   The precision of a balance. Readability is the smallest difference in mass that can be displayed by the balance.
Repeatability   The ability of the balance to produce the same result with one and the same load under the same measurement conditions.
Reproducibility   » Repeatability
Resolution   There is no standardized definition. In general it is used to indicate the readability compare to the maximum capacity of the balance. The resolution is the quotient of maximum capacity and readability.
Example: An analytical balance with a weighing capacity of 200 g and a readability of 0.1 mg has a resolution of 2,000,000 digits.
Response time   » Stabilization time
Stabilization time   The time it takes from placing the sample on the balance and displaying the final result of the measurement.
Taring   This function sets the display to zero when a load (e.g. an empty container) is already on the weighing pan. The net weight will be displayed then, after the container has been filled.
Weighing range   » Capacity

Types of balances
In laboratories we usually find two different types of balances: The precision balance and analytical balance. The difference is the accuracy. Analytical balances are more precise and more expensive. Semi-micro balances and micro balances are even more precise but not very common in hospital laboratories.
Here the standard readabilities for various balance types:

Type Readability
Precision balance
1 mg
0.001 g
3 digit
Analytical balance
0.1 mg
0.0001 g
4 digit
Semi-micro balance
0.01 mg
0.00001 g
5 digit
Micro balance
1 µg
0.000001 g
6 digit

Precision balances
Precision balances are used to measure mass with a precision of up to 1 mg. They are widely used because their accuracy is high enough for the common measurements.

Precision balances are highly reliable, robust and compact.

Precision balances are nowadays all microprocessor controlled. That makes the usage easy and brings more functionality. Beside normal and tare weighing, additional features are: piece counting, recipe weighing, percentage weighing, change of weighing units and the connection with other equipment or with a computer via data interface.
In principle all balances run on low-voltage or battery which is important for laboratories with unreliable power supply.
Most of the precision balances have no internal calibration weights. For calibration certified calibration weights of class E2 or better E1 are needed.
Precision balances come in different capacities. Balances of 50 g capacity are available as well as for 10 kg capacity. The readability of a typical precision balance of 100 g is 1 mg. A 1-kg-balance has a typical readability of 10 mg and a balance for up to 10 kg 50 mg.
The price for a common precision balance is between 100 and 500 €.

Analytical balances
Analytical balances are found in laboratories for the manufacture of pharmaceutical products, analysis, quality assurance or final checking. They are more precise, sensitive and more expensive. With a readability of 0.1 mg they are ten times more precise than precision balances.

Analytical balances are covered with a glass enclosure in order to limit measurement inaccuracy caused by dust and air flow.

Due to the high resolution analytical balances are also more sensitive to outside influences like dust and air flow. To avoid these factors, the weighing pan of analytical balances is protected by a transparent enclosure with sliding doors.
The electronic control units of precision and analytical balances are in principle identical so that the application possibilities are the same. Also analytical balances offer piece counting, recipe weighing, percentage weighing, change of weighing units and a data interface.
The only difference is an additional function: The automated calibration function. That means, that no external calibration weights are needed to calibrate the balance before the measurement. A simple press of the calibration button is enough and a built-in motor-driven adjusting weight will do the calibration.
This function is an enormous advantage over common balances. The calibration procedure is easier and faster and external weights are not needed and so can not get damaged.
The price for analytical balances start from 800 €.

Using laboratory balances
Thanks to modern electronics, the handling of laboratory balances has become easier and faster. Nevertheless, the user manual should be read by both the operator and the technician. The usage varies from manufacturer to manufacturer and navigating through the menu and additional functions may not be clear immediately. Only who has read the manual knows about all functions and the right working procedures and can avoid mistakes.

Laboratory balances are sensitive instruments and a reliable measurement can only be expected when the balance is prepared correctly.

  Is the balance exactly levelled? Check the internal spirit level.
  Was the balance connected to the mains for at least 4 hours?
  Is the balance and the workplace absolutely clean?
  Do you know the maximum load? Never overload a balance.

Using procedures
This is just a short overview. By all means, read the user manual of your balance before.

  Do a calibration if needed.
  Press the tare key to zero the display.
  Weigh powders only on paper or small containers.
  Never touch samples or samples containers with your bare fingers. Wear gloves or use
    anti-magnetic forceps.
  Place the sample in the centre of the weighing pan.
  Close the enclosure before starting the measurement.
  Wait until the stability indicator is displayed.
  Have you spilled chemicals on the balance? Clean up immediately.

When a load is already on the balance, pressing the tare-button sets the display to zero. It is used when a samples comes in a container. First the empty container is measured, the tare-button is pressed and then the filled container is placed back. The result is the net weight.

Warm-up time
A laboratory balance should always be connected to the power. Don't disconnect the balance from the wall socket or power supply. Switch off the balance only with the key of the keypad. Even when it is switched off and the balance is in standby mode, the measurement unit is still powered and has the necessary operating temperature. The balance then can be used immediately after switching on.
If the balance was disconnected from the mains the balance has to warm up for at least 4 hours before the first weighing. Only then is the measurement accurate and reproducible.

After every usage and at the end of the day the balance has to be cleaned carefully.

  Keep the weighing pan and weighing chamber clean at all times.
  Use a fine paintbrush to remove sample residues. If possible remove the weighing pan for
  Use an absorbent cloth to remove spilled liquids.
  If there is stain use a damp cloth and a mild soap solution for cleaning. Do not make the
    cloth too wet. Make sure that no moisture enters the balance. Wipe the balance with a
    soft and dry cloth afterwards.
  The glass of the weighing chamber of an analytical balance can be cleaned with a
    common window cleaner.

Tip! Buy a dust cover for your balance. It is cheap and helps to protect your expensive balance against dust and dirt.

User manuals
User manuals are always delivered with the balance. It is wise to copy the manual and leave one issue close to the equipment and keep one in a file with other technical manuals in the office or storage room.
If the manual is missing, you are probably able to download it from the manufacturers website. If this is not possible, try it here:

The more precise the measurement result has to be, the more often a calibration has to be done. While a weekly calibration for a normal precision balance is enough, for analytical balances a calibration before every measurement is common.
In order to keep the the calibration simple for the operator, analytical balances usually contain an automated calibration function. Internal calibration weights are then automatically placed on the sensor. The user only has to press a button and the balance calibrates itself. For all other balances extra calibration weights have to be taken and the user has to place them on the weighing plate manually.
Calibration does not mean, that a technician does adjustments to the inside of the equipment. Calibration means checking the correctness of the display by comparing the read-out with a calibrated reference weight by the user.
In order to calibrate a balance, we have to control and adjust two calibration points: Zero and full load. For zero we only reset the display when the balance is unloaded. For determine the full load point we need a precise calibration weight which is close to the capacity of the balance. Once these two extremities are set, the balance will now be able to calculate all quantities between these two points.
While the laboratory assistance does the calibration when ever he needs it, the biomedical technician is responsible for calibration during the installation and after every repair. The calibration always has to be done at the workplace in the laboratory and after every relocation. It is also advisable to make a frequent function check and calibration after some time. The sensor and the electronics 'ages' and the measurement drifts over time.

Before calibrating a balance, we have to make sure, that all influences from the outside are excluded. Interfering factors are:

  Unleveled balance or uneven surface of lab bench
  Air flow
  Unstable lab temperature
  Steel lab bench or equipment which produce magnetic fields close to the balance
  Interrupted power supply

Do the calibration according the user manual. Every balance is different and a general instruction does not exist. Only there the correct handling is explained.
If you want to calibrate a balance without an automated calibration function make sure that you have an appropriate calibration weight. The accuracy of the weight must be much higher than of the balance and the mass must be close to the maximum weighing capacity of the balance.

Calibration with internal calibration weights
Modern analytical balances already have integrated calibration weights. Only a single key has to be pressed and the balance does the whole calibration process automatically.
First the balance is set to zero without any load on the sensor and then a mechanic places automatically an internal calibration weight on the sensor.

Calibration weights
The balance manufacturers provide sets of calibration weights in various sizes and different calibration weight classes. Which weight class is needed depends on the accuracy of the balance.
Calibration weights are extremely precise and have to be handled with absolute care. The smallest scratches makes them useless and even dust and fingerprints distort the calibration. Touch the weights only with clean gloves or with forceps and put them immediately back into the box when the calibration is finished.

Before doing the calibration, be sure that the balance is in warm operating condition. The balance has to be connected to the power supply for at least four hours, some manufacturers even suggest ten hours. Only then it is guaranteed that the sensor and the electronics has operation temperature and does not drift any more.
That does not mean, that the balance has to be switched on four several hours, but it has to be connected to mains and be in stand-by mode. This is to be noted specially in countries with unreliable power supply where the equipment is always disconnected from mains when not in use.

Weight classes
Calibration weights are available in different weight classes. The weight class describes the accuracy of the weight. For calibrating a precise analytical balance, a more accurate weight is needed than for calibrating a less accurate balance.
It should be also clear, that the tolerance (quality) of the calibration weights has to be much better than the balance which has to be calibrated.
These weights are our standard and with the calibration we set our balance to this standard. A calibration is only as good as the calibration weight.

Important! The tolerance of the weights must be much better than the resolution of the balance. Otherwise the calibration is useless!

Here are the specifications of the different classes of calibration weights:

Class E1
Maximum error at 100 g
± 0.1 mg
199.9999 g - 200.0001 g
Maximum error at 1 kg
± 0.5 mg
999.9995 g - 10.0005 g

Class E2
Maximum error at 100 g
± 0.3 mg
199.9997 g - 200.0003 g
Maximum error at 1 kg
± 1.5 mg
999.9985 g - 10.0015 g

Class F1
Maximum error at 100 g
± 1 mg
199.999 g - 200.001 g
Maximum error at 1 kg
± 5 mg
999.995 g - 10.005 g

Class F2
Maximum error at 100 g
± 3 mg
199.997 g - 200.003 g
Maximum error at 1 kg
± 15 mg
999.985 g - 10.015 g

Class M1
Maximum error at 100 g
± 10 mg
199.99 g - 200.01 g
Maximum error at 1 kg
± 50 mg
999.95 g - 10.05 g

Calibration without internal calibration weights
Laboratory balances without internal calibration weights need external weights. The calibration has to be done manually by the user. That takes longer and the user has to work very carefully. But this does not mean, that a manual calibration is less precise.

Hint! Some balances without internal weights have a 'Calibration' key. By pressing this key the balance is just set to zero. The calibration with a weight still has to be done manually.

  Check out the capacity of the balance. Take the information from the balance or the user
  Choose the appropriate calibration weight.
    Example: Is the capacity 200 g, the weight also should have 200 g.
  Check out the accuracy of the balance you want to test. Take the information from
    balance or the user manual. The numbers of digits of the display are meaningless.
  Take appropriate calibration weights which are much more precise than the balance.
    Example: Has the balance a accuracy of ± 1.0 mg, Class-E1-weights are fine (± 0.1 mg).
  Do the calibration according to the user manual.
  Calculate the permitted tolerances of the balance according the accuracy taken from the
    manual and check if the read out is within the calculated limits.
    Example: A 200 g-weight is displayed with 200.00178
    The accuracy of the balance is only 1.0 mg. We have to round the result to 200.002
    The calculated result must be between 199.999 g - 200.001 g (± 1.0 mg)
    The tolerance is too big. The balance has to be calibrated to 200.000 g.

Calibration weights and analytical balances
In the above mentioned example, the laboratory balance with an accuracy of ± 1.0 mg was not a very precise one. Standard analytical balances have at least a accuracy of ± 0.1 mg and better ones even ± 0.01 mg. If we now want to calibrate an analytical balance we have a problem: We need calibration weights with a much smaller tolerance than the accuracy of the analytical balance. But how is that possible, when the most precise calibration weight we can get, only has a tolerance of ± 0.1 mg (Class E1). The answer is easy: It is not possible. The calibration of an analytical balance has only to be made with internal calibration weights. All existing external calibration weights are not precise enough.

Important! Never use external calibration weights to calibrate analytical balances! An analytical balance is more precise than the weights.

Tip! Why not using an analytical balance to calibrate the calibration weights? Even a set of less accurate and cheaper low class weights will give a highly reference for future measurements on other balances after a calibration with an analytical balance.
Buy a (set of) calibration weight(s) together with a new balance, determine the calibration weights with the new balance and you have a nice reference for future calibrations.

The location of the balance has to be chosen very carefully. Only a proper set-up of the balance insures perfect readouts over a long period of time.
Before setting up the balance we have to make sure that the workplace for the balance is suitable. The following criteria have to be noted:

  The room must be dust free and absolute clean. Dirt and dust will influence the weighing
    result and also can causes problems with the mechanic of the balance.
  The lab table or bench where the balance is placed, has to be absolutely stable and must
    not wobble. No vibrations should influence the measurement. The weighing bench should
    be fixed either on the wall or on the floor (or better both). It is a good idea to place the
    balance directly over the legs of the lab bench in order to maximize the stability.
  The surface of the lab bench has to be flat and the balance has to be aligned absolute
    horizontally. Use the internal or an external spirit level.
  The surface of the lab bench must not made out of steel. Steel can get magnetize and will
    influence the weighing sensor (which is technically a magnetic sensor).
  The same goes for any items which produce magnetic fields, like transformer or
    loudspeakers. Keep them away from the balance.
  Use the weighing bench only for weighing. A centrifuge has no place around a balance. It
    creates vibrations and a magnetic field through the motor.
  The room temperature has to be kept as constant as possible. Even small temperature
    changes can cause drifts in the measurement results.
  Remember that also direct sunlight and strong light bulbs will heat up the balance.
  Check also if the humidity in the lab. It should not change much and has to be within the
    range of the manufacturer's limits. Specially in tropical countries this may lead to a
  Avoid a place of much air flow. Do not place the balance next to a door. Ideally the room
    should have sliding doors in order to avoid air flow while opening and closing.
  The power supply must be stable and free of power cuts. If this can not guaranteed an
    additional UPS (Uninterruptible Power Supply) has to be used.

A bad example:
The table is not very strong, neither fixed to wall nor to the floor, the table-top is much too thin, made out of metal and the balance is placed close to a motor-driven test equipment which creates vibrations and magnetic interference fields.

When the best location is found, we can place the balance. Therefore we have to make sure that the balance itself is levelled perfectly horizontal. For this purpose all balances have adjustable equipment feet and usually an integrated spirit level. This so called 'bull's eye level' is a small circular glass device. An air bubble floating in a liquid (spirit) has to be positioned in the centre of the level by turning the adjustable equipment feet of the balance.

The internal spirit level helps to level the balance on the workbench. The screw feet of the balance must be adjusted so that the air bubble is precisely in the centre of the sight glass of the bubble level.
It is the task of the user to control the correct levelling from time to time but always after the balance has been removed.

After levelling a new calibration has to be made.

Load cell balances often have stops which prevent the balance from overloading. Check if they are adjusted well, that there is space enough for a proper operation and not too much space so that the mechanic can get damaged.
Put the maximum permitted weight on the balance and check the distance.

Tip! Consider when buying an expensive balance also to buy a solid bench for the instrument.

Preventive maintenance
The maintenance of balances covers cleaning and some test procedures. Internal adjustments should not be done. If the test results differ much from the balance's specifications, it is advisable to ship the balance to the manufacturer.
The tests should be done on-site in the laboratory. Moving the balance to a workshop is anyway not a good idea. Any moving can cause additional problems.

Needed tools
Beside the calibration weights for balances without internal calibration weights, no special tools are needed. A proper set of screwdrivers, the technical manual for the balance and a clean and bright workplace is all we need.

Step 1: Inspection
Before starting any maintenance or repair, start with a short visually inspection:

  Is there any contamination or anything else unusual?
  Does the doors close/open smoothly?
  Do the switches work properly?
  Is the zero-point display stable (without a load)?

Make a note of all objections. We come later to these points. First we have to clean the balance.

Step 2: Cleaning
  Use a fine paintbrush to remove sample residues. Often the weighing pan can be removed
    for easier cleaning.
  If there is stain, use a damp cloth and a mild soap solution. Do not make the cloth too
    wet. Make sure that no moisture enters the balance.
  Wipe the balance with a soft and dry cloth.
  The glass of the weighing chamber of an analytical balance can be cleaned with a
    common window cleaner.

Step 3: Maintenance
If the sliding doors move sluggishly, take out the doors and clean the rails and the edges of the glass doors. Do not lubricate. Oil or grease absorb more dirt over time and the function gets worse.
This also applies for the interior of the balance: Do not lubricate anything. Just clean all components from dust and dirt. Any additional substance interfere with the balance's mechanic.

Important! Never lubricate a balance unless the manufacturer has expressly indicated it.

Step 4: Standard tests
Beside the calibration procedure some tests can be done, in order to verify the accuracy of the balance. The result of these tests will show the deviation from a perfect balance with no deviation at all. Since such a balance in practice do not exists, there can be deviations expected. The problem now is to decide, if these differences are within the tolerance of the manufacturer. Therefore we have to know the specifications of the balance, compare the results and make a decision: Yes, the test results are within the tolerance, the balance is OK; or no, out-off the tolerance, the balance has to be repaired.
Since the technician is not the operator of the balance and does not necessarily know for what kind of measurements the balance is used, it is recommended to do the tests, record the test results and deviations and let the operator decide, if the results can be accepted and the balance is precise enough for future measurements.

The following performance tests can be done:

  Reproducibility (Repeatability)

The procedures to perform these test are explained below. Also a test chart for each test can be downloaded. The test charts can be printed out and the test results can be filled in. Later they can handed over to the responsible person.
The tests can be performed semi-annual as part of the preventive maintenance of all the other laboratory equipment.
Are there are significant changes, check again if there are any influences from outside which could create the problem (static, air draft, warm-up, vibration, etc.) and repeat the test. If the balance still fails the check, the balance has to be send to the manufacturer for a recalibration.

Reproducibility (Repeatability)
Reproducibility means that a repeated weighing of the same object must deliver the same weight reading. A calibration weight is measured for ten times and the readings are put into the test chart. After each weighing cycle the balance has to be returned to zero.

The following Reproducibility Test Chart can be printed out and used as a test report:

Reproducibility Test Chart      

40 KB      Download

(My standard deviation calculator you find here.)

A cornerload test is performed in order to find out if the balance delivers the same weight reading for a given object regardless of its position on the weighing pan.
Position a calibration weight at various locations on the weighing pan. The reading should be the same, within a few digits, at all positions.

Note! A balance which already delivered large deviations during the reproducibility test will also deliver wrong results also here.

The following Cornerload Test Chart can be printed out and used as a test report:

Cornerload Test Chart      

21 KB      Download

Linearity testing verifies the accuracy of the balance at intermediate values of weight. Since a laboratory balance will often be used to weigh items much smaller than the maximum capacity of the balance, this is an important test.

The following Linearity Test Chart can be printed out and used as a test report:

Linearity Test Chart      

29 KB      Download

Regular maintenance
The balance is a sensitive equipment which must operate absolutely correct in order to ensure the quality of the measurement. Therefore the adjusting and calibration is very important for any precision balance. Only with a frequent calibration the accuracy of the balance can be guaranteed.
After every repair and maintenance a calibration has to be done.
A full calibration should be done frequently every few year. Therefore appropriate and traceable reference standards have to be used.
The performance has to be calculated for each measurement range. The end user finally can decide if the balance still meets his requirements.

Laboratory balances are mechanical balance coupled to a sensor and a processing electronics. The sensor converts the mechanical force into an electronic signal. This analogue signal is converted into a digital one, processed and displayed.
The mechanic and the sensor are usually designed as a compact mechanical unit which is adjusted to the following electronics.

Mode of operation
For electronic balances two different weight measuring technologies are used: Strain gauge and electromagnetic force restoration.
While the strain gauge principle is used for all scales with higher limits and less accuracy like patient and baby scales, high quality analytical balances are all based on the principle of electromagnetic force restoration.

Strain gauge scales
Also commonly called load cell.
The working principle of the strain gauge is used for patient scales or baby scales where higher weights are measured and the measurement result within milligrams is not needed.
The unknown weight is measured by using a strain gauge. This is a aluminium beam, which is milled out in the centre. To one end the weighing pan is attached, the other end is mounted to the base of the balance. A load on the pan creates as force and the aluminium beam gets deformed. Since at the thinnest part of the beam strain gauges are embedded, these length-sensitive resistors also get deformed and so change their resistance.
The following amplifier creates a voltage out of the current through the resistances and delivers a measurement voltage which corresponds to the weight.

A standard resistor bridge (also called Wheatstone bridge) with three fixed resistors and a sensor resistor. When the bridge is in balance the output voltage is zero.

In practice often all four resistors are length-sensitive. The output voltage and the dynamic is higher and the sensitivity to disturbances smaller.

Here the sensor of a platform balance in reality. On the left side the aluminium beam is mounted to the frame. On the right side of the beam the weighing platform is usually mounted (on the top where the four screw holes are). Note how massiv the whole construction is and how thin the centre part of the block is. Here the block will bend under load and here the length-sensitive resistors are mounted. The sensors themselves are embedded in the aluminium block and sealed with plastic.

Note that there is nothing to adjust. The calibration is done electronically by the control unit.
Is the working principle based on load cells, then there is not much you can do wrong. The whole unit is build out of one solid block. On the other hand, there is also nothing to adjust and to repair.

Electromagnetic force restoration balances
Balances based on the electromagnetic force restoration principle work completely different. Here not the force of the load weight is measured directly but a counteracting force is created which works against the weight. This force is measured when the weighing pan is in balance. The method allows more precise measurements than the load cell principle.
The weighing pan with the unknown weight is attached to a force coil. The coil is floating in a magnetic field which is created by an amplifier. The amplifier delivers always the right current to keep the lever in balance, regardless of the weight on the pan. The information when the lever is balanced is given by a light barrier.

Due to the fact that the needed current for this balancing is proportional to the weight on the pan, the current is also used to measure the weight. Therefore the current flows through a precision resistor which creates a measurement voltage. An A/D unit transforms the voltage into a digital signal. The following micro processor with an appropriate software delivers the measurement result which is finally shown in the display.
Additional sensors like a displacement detector and a temperature sensor are also controlling the amplifier and ensure the accuracy of the balance.

Here the force coil mechanic of an analytic balance. Usually the mechanic is protected by a heavy steel cover. The black pin in the centre carries the weighing plate (removed). The weighing mechanic is hidden under the metal disk. The green, small electronic board on the right contains the LED light barrier. A lever, connected to the weighing mechanic, moves in this sensor.
The left part of the mechanic shows the automated calibration unit. The motor on the left turns the big black gear wheel below when the calibration function is activated. Then the calibration weight is attached to the weighing mechanic. The calibration weight consists of the two brass wheels in the front.

Balances are often damaged by mishandling or contamination from water or chemicals. These problems require cleaning and testing as described in the Maintenance section. Rarely component have to be replaced.
Laboratory balances consists in principle of two compact and integrated units: The computer controlled electronic unit and the mechanic with the sensor, often designed as a solid block. The two units are adjusted to each other and thus leave no space for repairs or adjustments. Only touch or modify them when you are sure what you do. Otherwise take hands off the sensors. You probably create more problems than you solve.
Is the balance not working at all, you can have a closer look at the power supply.
In case of a more complex problem or inaccuracy the balance should be send to the manufacturer for repair.

Be careful with dismantling balances. In this case only the two marked screws are holding the metal cover in place and can be removed.
The four other screws are for adjustments and must not turned!

Note! Before dismantling a laboratory balance consult the service manual.

Common problems
Malfunctions caused by sample residues and spilled liquids are typical for laboratory equipment.
Also problems with the power supply can occur.
A typical problem with load cell scales is, that they are sensitive to over-loading. An excessive load can permanently bend the load cell. Therefore, great care needs to be taken when using a scale with a load cell. The user should have a rough idea of how much an object weighs before placing it on the weighing pan. Bent load cells are not repairable and have to be exchanged. The repair is easy to do but sometimes not economic. The spare part is often not much cheaper than a complete new scale.

Most of the laboratory balances contain small batteries. The batteries do not run the balance but backup the stored microprocessor settings. Their lifetime is limited and should be replaced every two years.

Needed tools
No special tools are needed. A proper set of screwdrivers, a multimeter maybe, the technical manual for the balance and a clean and bright workplace is all we need.

Service manuals
While user manuals are always delivered with equipment, service manuals are never provided by the manufacturer. Also on the websites of the companies you will not find any service manual. The policy is clear: The companies want to do all repairs by themselves. Only a certified technician who took part in a special training for a certain equipment gets a service manual and technical support.
If the service manual is needed, you can try it here:

Power supply
As mentioned above, the possibilities of a repair of a laboratory balance are limited. An exception is, as always, the power supply. Here is something to do for the technician. In case of malfunctions, we can check the fuse(s), the output voltage(s) and the electronics in between. It is quite a lot, because here often problems appear.
Laboratory balances almost always have an external power supply (switch mode) and an internal voltage stabilization.
Here the view inside the balance:

  1   The input socket with the plug from the external power supply. Plugs and sockets often
       have bad contact.
  2   The glass fuse.
       In switch mode power supplies, also search for small, black plastic fuses. Remember:
       Blown fuses often have a cause.
  3   A capacitor. Works together with the following coil as a filter for the DC input voltage.
  4   Looks like a transformer, but is just a coil for filtering out any AC components of the DC
  5   Three stabilizers for three different DC output voltages. In this case +12 V, -12 V and
       +5 V. They are easy to check. 7805 means Stabilizer for 5 V, 7812 for 12V. Left is input,
       centre ground, right 5 V or 12 V output. Negative stabilizers (79xx) are different: Left is
       ground, centre is in and right is voltage output. More about stabilizers here.
  6   Capacitors for stabilizing the output voltages.
  7   Flat ribbon cable, which leads to the main board. Be careful with these cables. Once
       they are broken, they can not be repaired. In such a case, exchange the cable against
       normal wire cables.

Important manufacturer of laboratory balances are:

  A&D, Japan
  Kern & Sohn, Germany
  Mettler Toledo, Switzerland/USA
  Ohaus, USA
  Radwag, Poland
  Sartorius, Germany
  Seca   Should be avoided. Prohibited the download of service manuals.

Links and sources
Wikipedia: Weighing scale
Wikipedia: Spring scale
GWP - Proper Weighing with Laboratory Balances
IES Corporation - Calibration Guide of Balances
IES Corporation - Testing Your Laboratory Balance
Mettler Toledo - Weights
Sartorius - Correct Use and Handling of Analytical and Microbalances
Urs Berli - Weighing Principles