TRIDYNE NETWEIGHING - THEORY OF OPERATION

Netweighing or Weighcounting is an excellent method of dispensing most dry  flowable products, whether powders, pellets or small parts. In order to determine the suitability for handling your product, you will need to understand the Theory of Operation.

Tridyne Netweighers feature either single or dual vibratory feed systems and provide two-stage feeding. The initial stage feeds at high rate until a preset weight just below your target weight has been reached. At this point, the bulk or fast feed ceases and the dribble or final topping off stage takes over. During the brief dribble stage, the feeder(s) slow, the bulk gate closes (on systems requiring a bulk gate), and a fine stream completes the fill. Because of Tridyne's two-stage feed system, the packager needing both speed and accuracy can be satisfied.

STATIC ACCURACY

The ACCURACY of the fill depends on a number of factors, the first being static accuracy of the system. Static accuracy refers to the amount of deviation possible between the actual load applied and what the scale senses when in a static, or motionless, condition. This is expressed as a percentage of the capacity. For example, a two kilogram capacity system with a static accuracy of 0.1% of its full capacity would have a static accuracy of within plus or minus two grams (.001 X 2kg = 2gr). If a 2 kg capacity system has a static accuracy of 1.0% of its capacity, this indicates that the reading would be plus or minus 20gr (.01 X 2kg = 20gr). This factor must be considered when deciding on the system that best fits your needs. If you package an inexpensive product where accuracy is not critical, the 1.0% static accuracy may serve your needs well. However, if your product is fairly expensive, you would most likely opt for the system having 0.1% static accuracy.

OTHER FACTORS WHICH DETERMINE FILL ACCURACY

As the product feeds onto the scale, whether it be into a weigh hopper or a carton sitting on a load cell, the scale only senses the material that has completed the drop, with its full weight resting on the scale.

Next, we have to take into consideration the product that may be in suspension (product has left the feeder and is in free-fall on its way to the scale). By reducing the final feed to a mere dribble, this amount is kept to a minimum. Also, with careful observation, you can anticipate the approximate amount in suspension when the scale senses the correct weight and compensates for this by using the "under" tolerance setting.

When weighcounting parts, the dribble tray is designed to encourage single-file feeding at the final dribble stage, keeping overages to a minimum. Again, the overage depends on the static accuracy and the quantity of parts in suspension when the scale senses the correct weight. It also depends greatly on the consistency of the piece weights. When counting by weight, you need to realize that variations in piece weights will cause counts to vary accordingly -- this would be true even if you were placing the parts by hand on any scale. Often packagers who have been "throwing a few extra" in the bag to compensate for piece weight variation during their manual packaging operation surprisingly expect that this won't be necessary when automating. This, of course, is not the case and is a factor that the weighcounting system has no control over. DON'T EXPECT THE IMPOSSIBLE!

Speed and accuracy are a trade-off when netweighing. Increased speeds may be achieved by allowing a wider accuracy tolerance. By the same token, accuracy may be improved by decreasing speed. As you become familiar with your system and how your particular product flows, you will discover the balance between speed and accuracy that best fits your needs. Operating manuals are provided with each system and unlimited telephone support is available to you to help you achieve the optimum performance from your Tridyne system.