Weight Is A Critical Factor In Most Industries

By Author: Larry Tang
Total Articles: 512
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Weight measurement is one of the most common industrial processes, and is performed by a wide variety of industrial scales. Bench scales may be used to weigh ingredients for various types of formulation. Platform scales can weigh the contents of 55 gallon drums. Parts counting scales are used to weigh components, and calculate the number of components based on weight. Floor scales weigh incoming and outgoing shipments.

Another common industrial measurement is flow rate. Many chemical processes need to know, and control, the rate of material being fed into a process. Dyes or colorants may need to be added to a process at a specific rate. Filling devices must often fill containers at a controlled flow rate.

There are a number of different techniques that are used to measure flow rate. When measuring the flow rate of a liquid, there are a number of mechanical methods that may be appropriate. Piston meters measure positive displacement through rotational movement. This rotation can be converted into an electronic signal. Venturi measurement causes a restriction to the liquid flow. Measuring the difference of pressure ...
... across that restriction will be an indication of the flow rate. A simple paddle wheel device placed within a tube of flowing liquid will have its speed of rotation vary with the flow rate. There are other non mechanical methods to measure liquid flow. When the liquid includes particles, an optical device can measure the speed with which the particles are moving.

It can be much more difficult to measure the flow rate of a gas as it is dispensed from its liquefied state within a gas cylinder. Many of the mechanical methods will not be accurate as the output may change with changes in temperature and other ambient conditions. Generally, optical methods are not at all useful.

It is also possible to calculate the gas flow rate by measuring the change in weight of the gas cylinder over time. Outside conditions will not affect this type of a reading. While this principal is straightforward, it does place considerable demands on the accuracy and resolution of the scale being used. A liquefied gas cylinder can be quite heavy, in the range of 150 to 200 pounds. The actual contents, the liquefied gas, may be quite light in relation to the weight of the tank. And the rate of usage of the material may be quite small.

For example, there may be only one pound of material used within the period of one minute. If the scale had a sensitivity of 0.1 lb, and the measurements were taken at the beginning and end of the one minute time period, the flow rate could only be calculated to within 10%. If the time period used was 10 minutes, then 10 lb of material would have been used, and the accuracy could now be as close as 1%. But it is often not feasible to use such a long time period. It would certainly be very difficult to use these readings to control the flow rate with that time period.

Another means of increasing the measurement accuracy is to use a scale with higher resolution and accuracy. The Arlyn Surface Acoustic Wave (SAW) scales are ideal for this purpose. For example, the 200 lb capacity Arlyn scale has a resolution of 0.002 lb. Using the same example as above, in a one minute time frame, the scale would measure the gas consumption to within 0.002 lb, giving an accuracy as close as 1/5 if 1%. In fact, the accuracy could still be about 1% if readings were taken as quickly as 12 seconds. This can allow the flow rate to be controlled, either manually or automatically. The SAW scales are particularly good for this application because they are extremely rugged. A scale can easily be damaged when a gas cylinder is loaded on to the scale platform. The SAW scales boast a 500% overload capability, so they will withstand harsh treatment.

Another benefit of using this type of scale is that it can also detect, and give notice of low levels of material in the liquefied gas cylinder. The optional setpoint controller can be connected to a light, or buzzer system, to alert the operator to this condition.