Safe Industrial Scales for Hazardous Environments

Accurate weighing is essential in the industrial world. Platform scales are used to weigh different size packages, or to check that the weight of a package is within the specified tolerance. Bench scales are used to mix ingredients to the correct ratios. Drum scales can monitor the weight of a liquid chemical being used in a critical process. Pallet scales are used to measure the weight of items that are being shipped. Modern industrial scales are operated by electrical power. The load cells are usually powered by a voltage supply that may range up to 15 volts. Depending on the type of digital display used, the voltage and power consumption can be fairly significant. While the total amount of power used is quite small when compared to other industrial processes, it may be enough to eliminate their use in certain dangerous environments. Certain types of industrial manufacturing may produce situations where a small spark could cause a major explosion. This can be the case where there are flammable liquids or gasses. Even the fumes from some types of chemicals can be ignited. An atmosphere where there is a large amount of suspended dust can sometimes be a problem. Explosive materials, such as those used in armaments, are also problematic. Propellants and similar materials can be equally difficult. Generally, these types of environments can be placed into a number of different classifications. Depending on the type of hazard, they may fall into universally recognized categories such as Class I, Class II and Class III, along with a number of Divisions and Groups known as A, B, C, etc. Any equipment that is used in these types of areas must have approval ratings for their use. This includes virtually anything that uses electricity, including lights, communication equipment, mixing equipment, and industrial scales. To obtain these ratings the devices are tested by a rating organization, such as FM Approvals. One type of rating is known as Intrinsically Safe. Equipment of this type must be designed so that it cannot provide enough energy into its environment to cause any type of problem. Even if it experiences a failure mode, it must still not be able to generate a spark. Electronic scales could have difficulty in meeting these requirements for a number of reasons. As noted previously, the load cells themselves use too much power. Some designs, such as those offered by Arlyn Scales, construct their load cells with components that have a much higher value of resistance. This can reduce the energy consumption by a factor of 15 times. The digital indicator must also be designed for very low power consumption. The Arlyn Explosive Environment Scales are powered by just a single, rechargeable C cell battery. Special electronic circuitry is used to provide the necessary voltage levels to the scale components. The battery holder itself is designed so that wires improperly shorted within the scale will not allow the battery to deliver its energy within a short time frame. The holder is machined from solid aluminum, providing a robust container that will not be damaged if it is dropped or hit. Of course, in order to be useful, the explosive environment scale must also provide the type of accuracy that would be expected from any standard industrial scale. Stainless steel load cells are ideal for the triple purpose of working well with very low power electronics, providing very accurate weight results, and offering excellent ruggedness form difficult industrial use. Low power microcontroller can still offer all of the processing power needed to display the weight results in different units as necessary, and to store values of weight in internal memory for later use. Large graphics digits on the display panel make it easy to read the weight values. Front panel touch controls offer the user the ability to subtract container tare weights from the display, or to change the weight units from pounds to kilograms or ounces or grams. Although the amount of energy used by an intrinsically safe scale is much lower than that of a standard digital scale, the battery will eventually run low. Scales can be supplied with a number of spare batteries. A used battery may be removed from the scale and a fresh replacement battery installed. The used battery should be recharged, so that it will be ready when the replacement battery has been depleted. This must take place in an area that is considered to be safe. A fresh battery should power the scale for more than 20 hours of constant use. Recharge can occur in four hours or less.