TYPES OF LAB SCALES & BALANCES

Laboratory balances are used to measure an object’s mass to a very high degree of precision. They consist of a beam with a friction-free fulcrum, a pointer which attaches to the beam and amplifies deviation from the balance position, and fractional weights which are applied along the beam’s measuring arm. Often, the weighing pan is sealed to prevent the ingress of dust or other contaminants. Samples are maintained at room temperature to prevent the formation of air currents inside the enclosure. Other sources of error for laboratory balances include buoyancy, friction, improper miscalibration, misalignment, condensation, evaporation, gravitational abnormalities, and seismic disturbances. For best results, laboratory balances provide high readability, a broad weighting range, and a high degree of accuracy.  

There are many types of laboratory balances. Examples include precision scales, beam balances, equal-arm balances, unequal-arm balances, spring balances, analytical balances, moisture balances, top-loading balances, and platform balances. An equal-arm balance is the simplest type of beam balance. A uniform bar or beam is suspended at its exact center. By contrast, an unequal-arm balance is suspended at a point a very short distance from one of its ends. A spring balance consists of a coiled spring fixed to a support at one end, with a hook at the other to which the body to be weighed is applied. An analytical balance is designed for quantitative chemical analysis. A moisture balance is used to measure the moisture content in a material sample. A top-loading balance uses a glass or plastic breeze-break atop the scale. A platform balance is a form of equal-arm balance in which two flat platforms are attached to the top side of the beam, one at each end.  

Specifications for laboratory balances include capacity, resolution, platform width and length, display, interface, and ratings or certifications. Some laboratory balances have an analog display such as a needle. Others have a digital display such as a liquid crystal display (LCD), light-emitting diode (LED), or vacuum fluorescent display (VFD). Laboratory balances with remote displays are also available. There are four choices for laboratory balance interface: serial, parallel, universal serial bus (USB), and wireless. Ratings for laboratory balances include Ingress Protection (IP) standards, ratings from the National Electronic Manufacturers Association (NEMA), and certification by the National Type Evaluation Program (NTEP). Laboratory scales that sold in Europe must meet the Waste Electrical and Electronics Equipment (WEEE) directive from the European Union (EU).

Most types of scales are available from a number of manufacturers - for example, Kern & Sohn, a German company which sells through Precisely in the UK.

WHAT IS AN ANALYTICAL BALANCE?

Analytical balance

An analytical balance is used to measure mass to a very high degree of precision and accuracy. The measuring pan(s) of a high precision (0.1 mg or better) analytical balance are inside a transparent enclosure with doors so that dust does not collect and so any air currents in the room do not affect the balance's operation. The use of a vented balance safety enclosure, which has uniquely designed acrylic airfoils, allows a smooth turbulence-free airflow that prevents balance fluctuation and the measure of mass down to 1 μg without fluctuations or loss of product. Also, the sample must be at room temperature to prevent natural convection from forming air currents inside the enclosure, affecting the measure of mass.

Analytical precision is achieved by maintaining a constant load on the balance beam, by subtracting mass on the same side of the beam to which the sample is added. The final balance is achieved by using a small spring force rather than subtracting fixed masses. This makes calling it an "analytical balance" a misnomer, because it should actually be called an "analytical scale", due to it measuring force, rather than gravitational mass.