Analytical Balance

Introduction to Analytical Balances

This introduction to analytical balances describes terminology, how they are used, features to look for when specifying a unit to meet your requirements, and how to position and care for your analytical balance .

Precision and accuracy are two key features demanded by researchers when specifying an analytical balance.  These instruments are characterized by their ability to precisely and accurately measure extremely small samples. 

More to the point, analytical balances are capable of weighing to within the sub-milligram range such as 0.1mg or in the case of semi-micro balances within 0.01mg readability (readout), which is the number you see on the display.

Upper weighing capacity is limited for analytical balances, depending on the models. An example is 320g for a 0.1mg balance. Weighing pans are enclosed by a housing with sliding doors and a top to protect them from dust and stray air currents that can affect accuracy.

Analytical Balance Terminology

Accuracy and Precision: There is a difference and it is critical to know the difference.  As we explain to our clients an accurate analytical balance is always right in that it shows the correct weight.  This is further assured when the scale is calibrated on a regular basis (explained below).

Precision means the result is always the same.  Unfortunately a scale with excellent precision could also be wrong and therefore precisely inaccurate.  

A major contributor to both precision and accuracy in an analytical balance is called single cell weighing technology.

The weighing cell is constructed of a single block of material, which results in stable temperature behavior, shorter stabilization times, shock-proof construction and accuracy even when the material being weighed is positioned at the edge of the weighing pan.

The latter point is important because operators on occasion may initiate an eccentric loading error or corner load error that changes the readout when the same object is placed in various positions on the weighing pan of other types of balances.

Single cell weighing technology is found on several Kern analytical balances, one example being  ABS 320-4N  

Linearity: This is the maximum positive or negative deviation of the readout (see above) from the actual load of a calibrated balance throughout its weighing range. 

For example a 10-gram test weight on the balance should show 10 grams.  A 20-gram test weight should read 20 grams.  Both on the balance should display 30 grams. But nothing is perfect.  High-end analytical balances with 0.1 mg readout generally have a linearity of ±0.2 or ±0.3 mg.

Repeatability: This is the ability of a balance to display the same result when an object is repeatedly placed on the weighing pan and removed.   In general the difference between the largest and smallest result is used to specify repeatability.

Analytical Balance Features

Here's an explanation of terminology used by manufacturers when describing the specifications or features of their analytical balances.

• Internal calibration: Quick setting of the balance accuracy with an internal adjusting weight.
• External calibration:  Requires a correct calibration weight that may or may not be provided.  If the latter, see our post on calibration weights.
• Net-total weighing: Tare cup weight and weight of components memorized in two separate stores.
• Percent weighing: Displays sample weight as percentage of reference weight.
• Below balance weighing for bulky items.
• Units: switch from grams to carats to ounces to pounds.
• Data interface: RS232 serial interface for printer or computer.
• GLP/ISO record keeping capability.
• Piece counting: Reference quantities selectable. Display can be switched from piece to weight.

Analytical Balance Applications 

While most often associated with determining the weight of samples, analytical balances find much wider applications such as 

• tolerance weighing also called check weighing is a plus or minus value of the target weight or number of pieces that is acceptable for cost control or regulatory guidelines.
• recipe weighing documents the mixture of several components one by one and the total weight of all components can be called up for control purposes.
• dynamic weighing: The scale displays an average weight when environmental conditions are unstable.
• density determination of a solid or liquid is easier and more accurate with an analytical balance programmed to work in concert with a density determination kit.

Not all models offer all capabilities, which is why you should have a clear idea of what you expect your analytical balance to do, as well as typical sample weights.  Performance details are presented for the various models offered on the Tovatech website.  

This introduction to analytical balances also describes: 

How to Get the Best Performance from your Analytical Balance

While recommend procedures are generally spelled out in operations manuals we offer this brief as for new lab personnel and as a reference for the seasoned pro.

Start Right with Calibration

Gravitational pull varies from place to place throughout the world and can vary within a location.  Because of their extreme sensitivity to weight (affected by gravity) an analytical balance

• must be calibrated at the location where it will be used. 
• must be re-calibrated any time it is moved and
• should be re-calibrated on a regular basis.

Some balances are internally calibrated, others externally calibrated and still others automatically calibrate on programmed schedules, when there is a temperature fluctuation or other criteria.  User manuals provide instructions.  For more on this see our post on calibration .

Analytical Balances are Sensitive

It does not take much to throw a milligram analytical balance off so they must be positioned levelly on a vibration-free worktable.  Most balances are equipped with leveling bubbles; vibrations can come from a variety of sources but most likely by traffic in the lab. 

Protect them from temperature and humidity fluctuations, drafts, direct sunlight and away from HVAC vents.  Samples being weighed should be at room temperature to avoid air currents inside the enclosure. 

Avoid Sample-Induced Errors

Non-conductive samples such as plastics, china and glass may carry an electrostatic charge that can distort weighing results.  This situation can be avoided by an electrostatic discharge ionizer to neutralize electrostatic charge.

Wear Gloves

Use gloves or pan holders because oil on your fingers will lead to an incorrect reading.  This same caution applies to handing test weights for manually calibrated analytical balances. 

Keep it Clean

Operating manuals shipped with analytical balances provide instruction on cleaning these precision instruments.

Don't  Fix it Yourself

Servicing analytical balances should only be done by trained technicians authorized by the manufacturer.

Need more Analytical Balance Info?

For more information on selecting and using analytical balances please check our analytical balance blog posts.  For help in selecting and using a unit that meets your requirements contact the scientists at Tovatech.