Balances

What is a Laboratory balance?
Laboratory balances are used to measure an object’s mass to a very high degree of precision.

Applications
Used to measure solids, liquids, tissue, etc. They have a wide range of uses in virtually any laboratory. 

Other Names
Scale

Working Principle
Balances do not directly measure mass; they measure the force (weight) that acts downward on the balance pan. Most analytical balances are electromagnetic balances and so measure this weight using an electromagnet. Figure 2 shows the electromagnetic servomotor which generates a force counter the weight of the mass being measured. The electrical current required to generate this force is proportional to the weight and so can be used, with appropriate calibration, to calculate the mass. This mass is then displayed on the screen. To signal when the weight and electromagnetic forces are equal many balances have a “null detector” that uses a light source and detector.

Operating Procedure
The following is a basic procedure for using an analytical balance. Before using a balance it is a good idea to check the calibration with a check weight.

  • Check the balance is level using the level indicator (see Figure 1). If the bubble is not in the centre adjust the level, normally by twisting the feet, until the bubble is in the centre of the inner circle.
  • Check that the balance is on and that the door is closed. Press the “Tare” button and wait 5-10 secs for a ‘*’ or similar symbol to appear in the upper left/right hand corner of the display, and the mass to read 0.0000 g.
  • Open the door and place a weigh boat, weight paper, or other container on the centre of the balance pan (ideally with tweezers or similar).
  • Close the door and wait for the digital readout to stabilize (‘*’).
  • If you do not wish to include container mass in your measurement then press “TARE” to reset the mass to zero (see Step 2 above),
  • Remove the container from the balance and add the substance to be weighed. Avoid adding substances on the balance pan as this can result in contaminating the balance.
  • Return container to balance and wait 5-10 secs (may take up to a minute) for the mass reading to settle.
  • If the mass reading is unstable it may be due to static electricity build up or other issues– see trouble shooting section.

Trouble shooting
The accuracy and precision of an analytical balance must be guarded and checked at regular intervals. There are many factors that govern whether an analytical balance behaves itself:

  • Gravitational acceleration differences across the globe mean that the balances calibration may require local adjustment.
  • Temperature: Balances take time to equilibrate to laboratory temperature changes. Also, hot or cold objects can create convection currents in the air which can cause variation in mass measured. For these reasons it is important to let the balance and objects equilibrate to the same temperature.
  • Moisture: Objects or materials that absorb moisture can appear to gain weight. This may particularly be an issue for objects that have recently been removed from a desiccator. Other materials may evaporate or sublime during measurement.
  • Air flows: Air movement in the laboratory across the pan will cause variations in the measurement. A draft shield reduces this but it will take time for the air within the draft shield to stabilise once the door is closed. Changes in air temperature within the draft shield will also cause air movement. These changes can be due to the temperature of the mass, hands, etc. Reducing air flow incident on the balance in the laboratory is key to reducing this issue and ensuring all items entering the draft shield are equilibrated to ambient temperature – using tweezers, not your warm hands, to move items can help.
  • Static electricity: This can be one of the biggest causes of frustration when using a balance. If the mass you are measuring wonders up or down and will not stabilise then there is a good chance that you have a static issue. The static-electrical field interferes with the electromagnetic field of the balance. To prevent this you can use an anti-static device which will “fire” positive and negative ions at the weight boat, powder, etc. to neutralise the static charge. The good anti-static system are incredibly effective and can save hours of pain and frustration. Anti-static plastic weigh boats or metal weigh weight boats can also help.

To help diagnose any issues you are having with a balance the check weight is a vital tool. A check weight (or calibration weight) is simply an object that has been certified to have certain mass. They are simple to use but must be handled very carefully as they are expensive and sensitive to touch and rough handling so should only be handled with tweezers and must not be dropped as this may change the mass.

There are different grades of check weights. The main accuracy classes for weights are as follows.

  • Class E1 the highest accuracy class used as primary laboratory reference standard (1 kg is ±0.5 mg).
  • Class E2 Used as a high precision standard for calibration of weights and special precision analytical instruments such as Ultra Micro, Micro, Semi–Micro Balances and analytical balances. (1 kg is ±1.6 mg).
  • Class F1 For the calibration of some analytical balance and high accuracy top-pan balances. (1 kg is ±5 mg).
  • Class F2, M1, M2, and M3 are of lower standard and not suitable for analytical balances.
  • It is worth checking each section of a balance pan to ensure you do not have any eccentricity error as well (see figure below). This type of error is the reason why it is good practice to always place items in the middle of a balance pan.

Different Types of Balances

Industrial Floor Scales

 

Precision / Top-pan balance (from 0.1g to 0.001g)

Toploading balances are appropriate when the upper margin of error on a weight is 0.05 g or larger, as opposed to analytical balances, which have a margin of error of 0.5 mg. The toploading balance does not offer protection from air disturbances, and generally can weigh up to the low-kilogram range.

Portable Balances

A  Portable Balance may be chosen for field work or simply for convenience. Durability becomes a very important feature of a portable balance, since it is likely to be heavily used and possibly take some abuse. Many appreciate a quick response and user-friendly design in a portable balance, especially if there is a large number of samples to be weighed.

Although portable balances are not as fussy as their analytical balance cousins, they do offer many of the same features, such as piece counting, dynamic weighing, and unit conversion. Sealed keypads and stainless steel platforms are useful for cleanup.

Analytical balance (0.0001g)

Most life science laboratories need an analytical balance for precision weighing. Analytical balances should only be used by trained personnel, and require regular calibration and maintenance. Because of this, they are generally set up in a separate, interior room of the laboratory, out of the way of general foot traffic, chemical spills, flying objects, vibrations, and other disturbances. Additionally, locking the room can keep well-intentioned, but untrained, visitors from using them.

For very sensitive balances, an antistatic mat will help to improve accuracy. ISO 9000 and GLP regulations require documentation of the accuracy and calibration of an analytical balance. Some analytical scales have a feature that allows automatic recalibration at specific intervals or under certain circumstances, such as temperature fluctuation. Other balances must be calibrated by hand. Calibration can be internally motor-driven, or external.

While a balance need not be set up in a cleanroom, it should not be subjected to dusty and dirty environments. When dust cannot be avoided, the user should look for a scale with an ingress protection (IP) rating. Scales with high IP numbers are resistant to particulate and liquid contaminants. The two-digit rating gives the resistance of the scale to solid and liquid matter, respectively.

Weighing capacities of an analytical balance range from tens or hundreds of grams to as low as 0.1 or 0.01 mg. Practical features include a clock, mass unit conversion, piece counting, and data record-keeping.

Semi-Micro Balances (0.00001g)

Microbalance (0.000001g) and Ultra microbalance (0.0000001g)

Semimicrobalances can weigh to onehundredth of a milligram. They are generally used for extremely specialized applications, such as differential weighing of a sample before and after incineration, measurement of coatings, or weighing chemically sensitive samples inside a glove box.

Even greater accuracy is possible with the use of a quartz crystal microbalance, an instrument that falls into the gap between analytical balance and mass spectrometer


Basic Layout

 

 

Maintenance
For most laboratories an annual service and calibration is sufficient. However, accredited laboratories will often check that calibration every day to ensure no error has occurred.

In between calibrations the key maintenance is to keep the balance clean. Any material that works its way inside a balance can cause major problems; especially if it is a corrosive material. Clean the balance by dusting off the stage and surrounding area with a paint brush or similar and then gently wipe down the balance, glass panels, and counter top around the balance with a lint free tissue. Ethanol can sometimes also be used depending on manufacturer and model – always check manufacturers instructions.

Important Terms

  • Mass is a measure of the amount of matter an object possesses. Stays constant no matter where the measurement is taken. Measured usually in kilogram or gram.
  • Weight is the force due to gravity acting on a mass. The weight of an object on the top of a mountain will be less than the weight of the same object at the bottom due to gravity variations. Weight is measured in newtons (N).
  • Accuracy: How close to the “true” value a balance measures. The “true” value is usually taken as the mass of a calibration standard (check weight).
  • Calibration: The process of checking the accuracy of a balance using a check weight or similar standard material
  • Capacity: The heaviest load that can be measured on the instrument.
  • Precision: The amount of variation between repeated measurements of the same mass.
    Readability The smallest division at which the scale can be read – number of decimal places. For example, 0.1g or 0.0000001g.
  • Tare: The process of resetting the balance reading to zero and so removing a known weight of an object
  • Animal Weighing: Some balances and scales feature a mode to allow accurate measurement of living animals that might move during weighing. The scale works out an average weight based on the force exerted by the animal over an extended period of time. Once the internal calculation is completed by the balance, it will hold the value.
  • Below Balance Weighing: To determine the specific gravity of a sample, below balance weighing is needed. The sample is tied to a wire attached to the under-hook of a balance and immersed into the liquid to know the weight of the sample in that liquid.
  • Capacity: The maximum weight that can be placed on the scale or balance.
  • Checkweighing: To compare a weight against limits to determine if the weight is within preset limits.
  • Density Determination: Using a balance to determine the density of material, either solids or liquids. (Density is the grams/cubic centimetre). If a solid material is used it is weighed in the air and when suspended in a liquid. The density can be determined either manually or in some balances by using special software. To determine the density of a liquid a special float of known volume is weighed in air and in the liquid.
  • Digital Filters: Filters can help reduce the influence of draft or vibration, enabling the balance to display a stable reading more quickly.
  • Drift: Gradual change in the readout of a weighing instrument at a constant load.
  • Dynamic Weighing: A mode featured on some balances and scales for weighing moving animals or measuring fluid samples. The device works out an average weight based on the force exerted by the animal or sample over an extended period of time. Once the internal calculation is completed by the balance, it will hold the value. See Animal Weighing.
  • External Calibration: The calibration process of a balance using a known external weight often referred to as suggested calibration weight.
  • GLP: Good Laboratory Practice or GLP generally refers to a system of management controls for laboratories and research organisations to ensure the consistency and reliability of results. The laboratory balances facilitate GLP Print outs with traceability in terms of date, time, balance ID number, etc.
  • GLP/GMP Printout Capability: Program that provides a specific set of information on the balance and the sample weight, in conformance with Good Laboratory/Manufacturing Practices.
  • Ingress Protection (IP) Rating: The IP rating classifies the degree of protection against the intrusion of solid objects such as dust and water. It provides specific information about a scale’s washdown ability, or if it can be used in a wet or dusty environment.
  • Internal Calibration: The calibration process using an internal weight available inside the balance for automatic calibration as on when desired by the user.
  • Leveling: The procedure to set the balance so the platform is horizontal. This gives the balance a position that is repeatable so the results will be the same if the balance is moved. This is done using levelling feet.
  • Linearity: The ability of a scale or balance to show the correct value throughout the weighing range. Usually tested by placing known weights on the balance from near zero to full capacity.
  • Load Cells: A load cell is a specialist transducer that is used to convert force (weight) exerted onto it into measurable electrical output. Load cells are used in many types of industrial machinery and equipment, however they are most often used in weighing scales. See also: Strain Gauge Load Cells
  • Readability: These terms are often used interchangeably to indicate how well a scale displays the correct results. Readability is the value of the finest division of the scale. See also: Accuracy, Increment, Resolution
  • Repeatability: The ability to display the same value when a weight is placed on a scale more than one time. Often expressed as a standard deviation of 5 to 10 tests. Also called reproducibility.
  • Reproducibility: The ability to display the same value when a weight is placed on a scale more than one time. Often expressed as a standard deviation of 5 to 10 tests. Also called reproducibility. See also: Repeatability
  • Resolution: These terms are often used interchangeably to indicate how well a scale displays the correct results. Resolution is the value of the finest division of the scale. See also: Accuracy, Increment, Readability
  • RS-232: A method of sending data over wires. Often used to communicate between balances and printers or computers.

 

Applicable Standards

Some laboratories require balance calibration to be in accordance with SANAS 17025. Laboratories that require this accrediation are generally pharmaceutical, SABS or other certified testing laboratories, pathology laboratories etc. SANAS accreditation certificates are a formal recognition by the Government of South Africa that an organisation is competent to perform specific tasks. SANAS provides formal recognition to. Laboratories (testing and calibration) to ISO/IEC 17025 and medical laboratories to ISO/IEC 17025 and/or ISO 15189. There are several companies across South Africa that are accredited to perform these calibrations. If our customer required our balance to be supplied with a SANAS certificate then we generally outsrouce the calibration to an accredited company.

Information Required to Quote on a Laboratory Balance

  1. What is your weighing range (lightest to heaviest)?
  2. What Readability / Accuracy do you require
  3. Do you need internal calibration?
  4. Do you require SANAS accredited onsite calibration on delivery?

Further reading

 

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