How does Rate Of Change (ROC) work?

Most of the Hardy weight controllers and weigh processors have a rate of change option.  This is an explanation of the rate of change calculation in these units.  This rate of change value is designed for reporting only.  This ROC value should not be used for control of the feed rate as it is a heavily averages rate of change.

The Rate Of Change (ROC) indicates the weight change over a period of time.  If you are viewing the ROC and the weight is not changing on the scale, then it should read zero. If you then add or subtract weight to the scale, the ROC will indicate the rate at which the weight is changing. As soon as the weight stabilizes the ROC will again be zero. Rate of change is a weight change over time. 

Rate of Change is normally used to measure the rate at which material enters or leaves a weigh hopper. It may be used in either “gain in weight” or “loss in weight” applications. The operator selectable parameters are Time base and Units of Measure.

Time base is the time in seconds between the two weight readings that are to be subtracted.  When establishing the Timebase length, insure you have two weight changes within the time allowed.  A 100lb scale will provide a stable and repeatable weight change of 0.01lb. (1:10,000) If you were to monitor a rate of change of 35 lb/hr divided by 3600 = .00972 lb/sec.  It will take slightly longer than one second to see a weight change.  using a three second time base insures you will see your weight change and calculate a proper ROC.

100/10,000= 0.01lb
35/3600= 0.0097 lb/sec
0.01/0.0097= 1.03  saying it will take 1.03 seconds to see a proper weight change.  You can set the HI 4060 ROC time base to at least 2 seconds and assure you will see a weight change to properly calculate a ROC.

Units of Measure selects if the flow rate is to be displayed in engineering units per SEC, MIN, or HR.

To develop ROC data, a weight register is used that is equal to the number of updates per second in length (15, 20, 55, or 110 depending on the unit in question). New weight data is input into the register at a rate equal to the update rate for the Time base, and all old data is shifted down one register. The last register location contains weight data one Time base older than the 1st register location. The 1st register location data is subtracted from the last register location data to determine a single ROC value. This check runs after every update after the initial time base length and does not wait for a complete refresh of the current ROC data.

The ROC values are input to the ROC register. The ROC register is also the same entries in length. New data is input to the ROC register at the rate of every update to the Time base and old data is shifted down one register location.  The results of this value are in U-TIME BASE and converted (multiplied by 60 or 3600) as configured for display as U-SEC, U-MIN, or U-HOUR.

EXAMPLE: If the Time base is 10, and the Units of Measure was selected as MIN, the results of the division would be multiplied by 6 to determine the flow rate for 60 seconds or 1 minute. 

You would multiply by 6, as 10 seconds is and 1/6th of a minute.