How can I determine minimum feed rates for my Hardy Rate controller?

What formulas are used to determine the recommended minimum feed rates for my Hardy Rate controller?

The formula to determine the minimum feed rate control depends on the scale capacity, the load cell (L/C) Millivolt per volt (Mv/V) output and the rate controller model number. Hardy Process Solutions has developed some guideline numbers to help determine if an application target rate is achievable. Production accuracy and quantities required must be addressed and compromises made based on this calculation.  This should be one of the first questions during the feeder project design phase.

Hardy has four rate controllers.  The now obsolete HI 2160RC, HI 4060, and our PLC plug in modules the HI 1769FC and the HI 1756FC.  I will make note to the differences.

For example: Look on the load cell certificate or data sheet and determine the capacity and the output rating.  Load cells are tested to differing rated outputs. These are 1 Mv/V, 2 Mv/V, 3 Mv/V, 4 Mv/v and higher.

HI 2160RC testing has given us the following constants to help determine the minimum controllable flow rates at a given capacity of load cells.
One Mv/V:  min rate = L/C capacity/125
Two Mv/V:  min rate = L/C capacity/175 
Three Mv/V:  min rate = L/C capacity/250

The recommended minimum repeatable feed rate is determined by dividing the total load cell capacity of the system as shown in the chart above. This mathematical computation yields the minimum recommended control rate in engineering units per minute.
Example: on a scale system with a 150 kg load cell capacity.  (Not load capacity, but load cell capacity.)

2 Mv/V example: 150 Kg/175=0.85Kg per minute

This is the recommended minimum feed rate for an unknown ingredient in an unknown feeder. When the correct ingredients are used with a tuned parameters using a good feeder it is often possible to control flow rates down to one half of the value found in the above formula. This would be consistent materials and correctly sized feeder auger/belt or feed mechanism.  This is accomplished by dividing the above answer by 2.

2 Mv/V example: 150 Kg/175=0.85/2=0.425 Kg per minute

When required flow rates are lower than the recommended minimums, the dead load and hopper capacity should be reduced to allow a smaller total load cell capacity. This will increase the resolution and make a smaller control rate possible.

With the reverse action in mind, if you had a minimum control rate specification of 10 kg/min.  Multiple the rate times the load cell's Mv/V rating and this represents the maximum load cell capacity.  This would assist you to determine a feeder type and product hopper capacity.

10 kg/min rate x 175 (2 Mv/v) = 1750 KG scale capacity and with fine tuning the parameters and matching the feeder that scale capacity could be doubled. (3500 kg)

Now you have some basic scale parameters to design around.

The HI4060 and PLC FC modules.

The control offered by the HI 4060 and PLC module rate controllers can easily achieve the control by dividing the end results by two. 

This area is still in the noise area of the load cell and should be avoided if you can.  Increasing time base length and effective rate calibrations, some very low control rates are achievable.

The key is the ROC time base to insure you are seeing a LIW every time base update. To determine the time base divide the total scale capacity by 10,000.  This is considered a base weight level for a stable and repeatable weight reading. 
1000 lb scale/ 10,000 = 0.1lb minimum weight increments. 
Convert your lowest control rate to lb/sec. (Engineering units/second, I use pounds in my example.)
75 lb/hr = 75/3600 = 0.02 lb/sec

Divide the scale mini