Gas Cylinder Scales and the Pressure Transducer

The semiconductor industry is often thought of as being at the leading edge of technology. That is certainly the case in many of the operations necessary for producing today’s microprocessor and memory devices. But in fact, most of the processes required for fabricating integrated circuits are much more standard levels of complexity. For example, a number of different types of gasses are used during the manufacturing procedure. These gasses are used when applying or etching layers off of a silicon substrate. It is important that the proper amount of gas be dispensed out of a gas cylinder, and most importantly, that there is enough gas available in the cylinder to finish the process. There are many other industries also need to measure the amount of gas in a cylinder. This is the case throughout the chemical industry. It is also found within the food industry. Gas cylinders are used for welding operations. Even paper and pulp processing requires a variety of different gasses. The least expensive way to determine the amount of gas within a cylinder would be to use a pressure transducer. The higher the pressure is, the greater the amount of gas that is still in the cylinder. As the pressure falls, the amount of material remaining also falls. This method only works in the case of a standard gas. But many gasses are supplied in a liquefied form. The major advantage is that a much larger amount of material may be stored in a cylinder. As gas is used, some more of the liquid will boil off into a gaseous state. This will continue, until all of the liquid has been used. A pressure transducer will have no value in this case. The gas pressure will always be the same, as the liquefied gas boils off. When the liquid is completely finished, the gas pressure will very quickly fall to nearly zero. Therefore, the cylinder must be weighed in order to determine the amount of liquefied gas remaining. Gas cylinder scales are designed to provide an easy way to determine this weight. The size of the scale platform is chosen to match the diameter of the cylinder. The scale capacity must exceed the combined weight of the liquefied gas and the cylinder itself, when the cylinder is full. One of the most standard industrial scales of this type have a 9 x 9 platform. The capacity is 300 lb, which is more than a typical, full gas cylinder. The weight output may be shown on a digital indicator attached to the electronic scale, or may be transmitted from the scale to other process equipment. Placing full gas cylinders on the scale platform can be a difficult process. Often this is done by hand, or with a manual cart. Therefore, it is important the cylinder scale offer a very low profile, so that the cylinder does not have to be lifted by a significant amount. Some of the lowest profile scales, such as those manufactured by Arlyn Scales, are only 1 3/8 high. Even with such a thin profile, it is still easy to damage the scale during the cylinder loading process. The most critical component within the scale is the load cell. Typically, these are made from aluminum, or plated steel. To provide the best shock load and overload protection, it is preferable to specify an industrial scale with stainless steel load cells, such as the Arlyn cylinder scales. Other reasons for choosing stainless steel include superior chemical resistance, and the ability to design a lower profile on the load cell, and therefore a lower overall profile of the scale. As the scales are used to monitor the remaining weight in the cylinder, it is beneficial to provide some method of warning the user when the gas level is getting too low. Arlyn Scales provides a Setpoint Controller on their digital scales. This allows the user to set a number of target weights. One value may be a caution level, indicating the material is being depleted. A second value may be a warning level to note that the gas has been finished. The electronic scale has warning indicators that correspond to these programmable values. There are also electronic signals output from the scale which may operate solid state relays. In turn, the relay may be used to automatically close or open valves to connect a backup cylinder. Another common cylinder scale output is a 4 Ð 20 ma analog signal. This is often connected to other process equipment. The current signal will be proportional to the amount of weight on the scale platform.