Main keywords for this article are Weigh Systems Definitions – Instrumentation. Calibration Curve of a Load Cell. load cell output. Weight Module. Weighbeam.
Weigh Systems Definitions – Instrumentation
Auto Jog.
Turning on feeding device for short time delay to bring weighment to within tolerance on an underweight weighment.
Auto Preact.
Automatically adjusting material preact based on previous weighments to bring final weight as close to set point as possible on a repeatable basis.
Axial Load.
An axial load is a weight applied along or parallel to and concentric with the primary axis of the load cell.
Batch Log.
A printout identifying scale, material, delivered weight of each material, and deviation from setpoint.
Batch Ready.
All selected materials are weighted.
Calibration Curve of a Load Cell.
The graphical comparison of the load cell outputs against the standard test loads.
Combined Error.
The algebraic difference between the indicated and true value of the load being measured due to the nonlinearity and hysteresis. The combined error is expressed by the load cell manufacturers as the percent of rated output.
Control Sequence.
Broad definition of sequence of weighing and discharging. The most common control sequences are as follows.
a. Weigh-hold
b. Weigh-discharge-hold
c. Weigh-hold-discharge-hold
d. Discharge-weigh-hold
Creep.
The rate of change in load cell output with respect to time when the rated load is applied to the load cell under constant environmental and mechanical conditions. Usually, creep is expressed as a percent of rated output over a specific period of time.
Cycle Complete.
A scale empty condition ready to start a weight sequence.
Dead Load (Tare Weight).
The weight of the empty vessel or container plus all attached piping, agitators, vibrators, insulation, and vessel heating fluids.
Deflection.
The change in length along the primary axis of the load cell for the change in loading from no-load to rated load.
Digital Indicator.
Converts analog load cell output to drive digital display and control output.
Discharging.
Draining of the hopper or tank.
Downstream.
Material handling after the weighing system.
Dribble.
Point at which feed rate is changed from fast to slow in order to increase weighing accuracy or repeatability toward end of feed cycle.
Gross Weight.
The sum of dead load and live load, or the sum of tare weight and net weight.
Hysteresis.
The maximum difference in output readings when the same load is applied to the load cell; one reading is obtained by increasing the load from zero and other is by decreasing the load from rated load. Hysteresis is usually given by the load cell manufacturers as a percent of rated output.
Interlocking.
Coordination of material handling devices to maintain integrity of the bath formula and to provide the safe operating condition.
Inventory Control.
Printout or display of the amount of each material on hand. Incremented as raw material is received and decremented as material is weighted for product use.
Live Load (Net Weight).
The maximum weight of the vessel contents less than the deal load or tare weight. Load cell. A device, whether electric, hydraulic, or pneumatic, that produces a signal proportional to the applied weight. The following 3 major types of load cells are used in weighing systems.
a. Hydraulic load cell – constructed so that the applied weight is supported by fluid confined by a diaphragm. The pressure generated in the fluid is a measure of the applied weight.
b. Pneumatic load cell – constructed so that the applied weight is supported by the controlled air pressure and measured by the pneumatic control instrumentation.
c. Strain-gage load cell – consists of an elastic member in the form of a beam, canister, block, disc, or shear element to which the strain gages are bonded. The deflection of the elastic member changes the resistance of the strain gage, which is measured electronically.
Microcomputer.
Controls weighing and discharge of scales, auxiliary batching functions, interlocking of all devices. It also stores formulas and provides inventory control, batch log and alarm printouts.
Multiplexer.
Provides weight indicator or transmitter inputs to computer memory.
Nonlinearity.
The maximum deviation of the calibration curve of a load cell from a straight line generated between the no-load and rated load outputs, expressed as a percent of rated output and measured on increasing load only.
Poise.
A movable weight mounted upon or suspended from a weighbeam bar. It is used in combination with graduations, and frequently with notches, on the bar to indicate weight values.
Preact.
Compensation for scale lag or material in suspension at time feeding device is turned off.
Primary Axis.
The axis along which the cell is designed to be loaded.
Programmable controller.
Coordinates scale indicators or controllers to maintain batch integrity and controls batch functions.
Rated Load (rated capacity) of a Load Cell.
The maximum axial load that the load cell is designed to measure.
Rated Output.
Rated output for a load cell is defined as the algebraic difference between outputs at no-load and at rated load.
Repeatability.
The maximum difference between load cell output readings for repeated loadings under the same loading environmental conditions. Repeatability also is expressed as percent of rated output.
Scale Controller.
Stores formula, controls weighing and discharge devices, and providesinterlocking to programmable controller.
Summing Junction Box.
A multicell interface unit for weight signal conditioning units or transducer indicating instruments. It performs the multiple functions of distributing excitation voltage to the weight transducers, summing the transducer output signals for input to the weight indicator / controller, and providing protection for interconnection wiring of a multiple load cell weighing system.
Temperature Compensated Range.
The range of temperature over which the load cell is compensated to maintain zero balance and rated output within manufacturer’s specification for the specific operating temperature range.
Temperature Effect on Zero Balance or Rated Output.
The change in zero balance or rated output due to a change in ambient temperature.
Terminal.
Used for controller diagnostics, entry and modification of formula, and display of system information.
Tolerance Check.
Checking delivered weight to ensure that it is within acceptable range.
Upstream.
Material handling prior to the weighing system such as conveying and storage of raw materials.
Weighing.
Feeding material to a weight hopper or tank.
Weighbeam.
An element, comprised of one or more bars, equipped with movable poises or means for applying counterpoise weights or both.
Weight Module.
Constructed to integrate a strain gage load cell and suitable mounting hardware into a single unit that makes electronic weighing possible. These weight modules are used in conjunction with electronics to convert process equipment into electronic weight scales. Process equipment such as silo, hoppers, tank, reactors, and blenders can all be highly accurate weight scales when the properly designed weight modules are utilized to support the vessels.
Zero Cutoff.
Control point at scale zero as scale discharges. Usually a few increments ahead of true zero.