Freezing Temperatures Challenge Industrial Scales

Precision instruments are generally expected to be operated in a reasonably controlled environment. Industrial scales fall within this category. This would be the case for bench scales, drum scales, cylinder scales, platform scales and floor scales. It would be even more so for parts counting scales, which may have higher levels of sensitivity. Laboratory scales are within their own category, and should be even more limited than other electronic scales.

Ideally, an electronic scale should be placed on a firm, stable, level surface. But often, that type of surface is not available. The scale operator can use the leveling legs to adjust the platform for a non-level surface. When there is vibration and other types of movement, it may be necessary to adjust the filtering constants of the digital display to obtain a reading with the desired stability.

Digital scales should generally be protected from water and other corrosive chemicals. But in actuality, industrial scales are often used in very difficult situations. All load cells should be built from stainless steel. Drum scales may need to have complete stainless steel construction, with special corrosive epoxy coatings applied. The display indicators should be housed in a gasketed enclosure to avoid damage by water.

Often, though, the most significant problem is operating an industrial scale in an environment where the temperature deviates considerably from standard room temperature. Virtually all available platform scales, floor scales, cylinder scales and drum scales use a technology known as strain gage load cells to measure the amount of weight on a scale platform. Most bench scales also use this technology, although there are some much more expensive alternatives. A strain gage load cell consists of a carefully machined piece of specialty metal with four strain gages adhesively bonded to it. The strain gages are a special type of resistor. It usually consists of a thin, special alloy metal foil with a very thin plastic backing. The foil is formed into an intricate, long pattern. The resulting device is adjusted so that it has a very exact value of resistance. When the resistors are attached to the load cell, they will bend when the load cell bends as a weight is applied to the digital scale platform. When they are forced to bend, the overall length of the resistive pattern will stretch. This causes the resistance of the strain gage to increase. When the weight is removed from the scale platform, the load cell will return to its original shape, and the strain gage resistive pattern will also return to its own original length. This will cause the value of resistance to once again return to its initial value. By measuring the change in the value of resistance of the strain gage, the amount of weight on the scale platform can be determined.

The temperature of the digital scale will have a number of major effects on its accuracy. Changes in temperature cause any material to expand or contract. Of course this will also be the case with a load cell. But when a load cell expands, it will cause its strain gages to stretch. The resistance will increase, and the electronics can think that a weight has been placed on the platform. Arlyn Scales and other high quality manufacturers of industrial scales provide temperature compensating resistors on the load cell. They are designed so that their temperature coefficients will change in a way that is equal but opposite that of the load cell. Of course any resistor by itself has its own temperature coefficient. Again, high quality digital scales are made with load cells that have four strain gages. Using a bridge method, the change of resistance in two of the strain gages will balance out the change in the remaining two.

For very low temperature environments, such as industrial freezers, the accuracy of temperature compensation must be excellent. Furthermore, the strain gages on the load cell must be protected so that moisture does not migrate between the strain gage and the load cell itself. Silicone RTV materials are good for this purpose. Of course, it is important that rust cannot occur on the load cell material. Stainless steel is the best material for load cells in terms of strength and ruggedness, and it is also an excellent material for avoiding corrosion. Again, Arlyn Scales uses only a special alloy of stainless steel for this purpose.