Scale Meter for Weighing Applications

High accuracy weight, low cost, plus control outputs

Scale-Meter

Features

  • Digital panel meter optimized for weighing applications
  • 5-digit resolution with scaling from 0 to 99,999, plus selectable fixed zero
  • Auto-tare or manual tare, with tare value stored in non-volatile memory.
  • Auto-zero display function
  • Display toggle between gross or net weight
  • Count by 1, 2, 5, 10, 20, 50 or 100 with rounding
  • Isolated 10 Vdc excitation to power four 350-ohm load cells in parallel
  • 4- or 6-wire hookup to avoid lead resistance effects
  • Universal 85-264 Vac / 90-300 Vdc or 10-48 Vdc / 12-32 Vac power
  • Wide choice of options:
    - 2 or 4 relays, mechanical or solid state (isolated)
    - Analog output, 4-20 mA, 0-10V, or -10V to +10V (isolated)
    - Serial data I/O: Ethernet, USB, RS485, RS232 (isolated)
    - Custom curve linearization, rate from consecutive readings
Description
dc-panel-meter

The Scale Meter is compact, inexpensive, and extremely accurate digital panel meter with special firmware for weighing applications. It is available with the load cell or DC signal conditioner board.

PowerThe load cell signal conditioner board accepts full-scale ranges of 20, 50, 100, 250 and 500 mV with 4- or 6-wire load cell hookup to display up to 99,999 counts. With 4 wires, the meter operates in a ratiometric mode to eliminate errors due to supply variations. With 6 wires, it also compensates for variations in transducer lead resistance, thereby allowing long cable runs in changing temperature environments. The built-in, isolated excitation supply can power up to four 350-ohm load cells in parallel at 10 Vdc.

The DC signal conditioner board can be jumpered for ratiometric strain gauge voltage ranges ±200 mV, ±2V or ±20V. With this board, a 20 mV strain gauge output can be scaled to display up to 10,000 counts, while a 200 mV output can be scaled to display up to 99,999 counts. Meter accuracy is 0.01% of full scale ± 2 counts.

Display & Setpoint Functions for Weighing Applications

  • Setpoint offset. The ON/OFF setpoint control action can be programmed to occur with a specified offset. For instance, if bags are to be filled to 100 lbs and the material delivery spout is known to hold and dispense an additional 2.5 lbs following shut-off, an offset of -2.5 lbs can be programmed. The setpoint can then be set to 100 lbs, and the filling valve will be automatically shut off when the measured weight reaches 97.5 lbs.
  • Count-by function. The weight meter can be programmed so that the display is rounded off to multiples of 1, 2, 5, 10, 20, 50 or 100. For example, if count-by 10 is selected, the meter will display 20 for an internal count of 15 to 24.
  • Fixed zero. The display can be shifted to the left for a fixed zero to the right, allowing values up to 999,990 to be displayed.
  • Auto-zero function. An auto-zero limit from 0 to 9 counts can be programmed to compensate for load cell drift. Whenever the meter comes to rest within that limit from zero, it will auto-zero. Entering 0 disables auto-zero.
  • Two fare functions: auto-tare and manual tare. In auto-tare, an input line is grounded by an external pushbutton. This causes the current weight, which is normally the empty weight of the container to be stored in memory as an offset. In manual tare, the tare value can be entered manually via the front panel or a computer. For instance, the tare value may be the stated empty weight of a truck or rail car. Pressing the Reset button on the front panel toggles the display between gross weight (total weight on the scale) and net weight (gross weight with tare subtracted).

Easy scale calibration is achieved using a simple two-point calibration method. First, the desired LO IN reading is set to 0, and the desired HI IN reading is set to a desired value. With no weight on the scale, a button is pushed for LO IN. With a known weight on the scale, that button is pushed again for HI IN. The meter then automatically computes scale and offset for readout up to five digits in weight units.

Fast read rates to 60/sec are provided for weigh-in-motion systems, setpoint control, and computer interface. Concurrent Slope (Pat 5,262,780) is a method of analog-to-digital conversion which allows up to 60 conversions per second while integrating the input signal over a full AC line cycle.

An adaptive digital filter can be set for time constants from 17 ms to 9 s, yet responds rapidly to a change in input signal level exceeding a threshold value. The meter can also automatically select the best filter setting for maximum noise rejection and minimum response time. Peak and valley capture are standard.

Designed for flexibility. Plug-in isolated analog output, dual setpoint controller, and serial communications boards can upgrade s from a simple monitor to system interface and control. s may be powered from 85-264 Vac or optionally from 12-32 Vac or 10-48 Vdc. They are available with red or green LEDs. The 1/8 DIN case meets NEMA 4X (IP65) specifications from the front when panel mounted. Any setup functions and front panel keys can be locked out for simplified usage and security.

Electrical Connections & Setpoint Offset

4-wire connection to resistance signal condidioner In 4-wire DC or load cell connection, the excitation and sense lines are tied together. The meter can make ratiometric corrections for supply voltage variations, but not compensate for variations in lead resistance. This connection is often used with short cable runs.
3-wire connection to resistance signal condidioner In 6-wire load cell connection, the sense lines are separate from the excitation lines, thereby eliminating effects due to variations in lead resistance. This allows long cable runs in outdoor environments with temperature extremes.
2-wire connection to resistance signal condidioner The DC signal conditioner board can also be utilized to accept preconditioned 4-20 mA or 0-10 V scale signals. In two-wire 4-20 mA transmitter connection, the same two wires are used to apply voltage and carry the output current.
2-wire connection to resistance signal condidioner In a repetitive fill operation (as illustrated by example), a setpoint offset of -50 allows a shutoff setpoint to be set for 1000 and actual shutoff to occur at 950 if it is known that 50 units will still flow following shutoff. An offset can also be applied to emptying operations.
Specifications

With Load Cell Signal Conditioner

Full-Scale Input Input Impedance Zero Adjust Span Adjust Error at 25°C
±20.00 mV        
±50.00 mV   -99,999 0  
±100.00 mV 1 GΩ to to 0.01% FS
±250.00 mV   +99,999 ±99,999 ± 2 counts
±500.00 mV        

With DC Signal Conditioner

Full-Scale Input Input Impedance Zero Adjust Span Adjust Error at 25°C
± 200.00 mV 1 GΩ      
± 2.0000 V 1 GΩ -99,999 0 to 0.01% FS
± 20.000 V 10 MΩ to +99,999 ±99,999 ± 2 counts
± 20.000 mA 10 Ω      

With Load Cell or DC Signal Conditioner

Display
Readout 5 LED digits, 7-segment, 14.2 mm (.56"), red or green
Range -99999 to +99999
  -999990 to +999990 (with fixed selectable right-hand zero)
Rounding Count by 1, 2, 5, 10, 20, 50, 100
Indicators Four LED lamps
Accuracy
Error at 25°C 0.01% FS ± 2 counts
Span tempco 0.0015% of reading/°C
Zero tempco 0.1 µV/°C (use auto-zero when temperature changes)
Calibration Method 2 points using zero weight and known weight
Noise Rejection
CMR, DC to 60 Hz 130 dB
NMR at 50/60 Hz 90 dB with min filtering
A-to-D Conversion
Technique Concurrent Slope (Pat 5,262,780)
A-to-D rate 60/s at 60 Hz, 50/s at 50 Hz
Output update 56/s at 60 Hz, 47/s at 50 Hz
Display update 3.5/s at 60 Hz, 3/s at 50 Hz
Power
Voltage, standard 85-264 Vac or 90-300 Vdc
Voltage, optional 12-32 Vac or 10-48 Vdc
Frequency DC or 47-63 Hz
Power  consumption (typ. with four 350Ω load cells at 10V) 2.4W @ 120 Vac, 2.7W @ 240 Vac, 2.5W @ 10 Vdc, 2.6W @ 20 Vdc, 2.75W @ 30 Vdc, 3.0W @ 40 Vdc, 3.35W @ 48 Vdc
Power Isolation 250V rms working, 2.3 kV rms per 1 min test
Excitation Output (standard)
5 Vdc 5 Vdc ± 5%, 100 mA
10 Vdc 10 Vdc ± 5%, 120 mA
24 Vdc 24 Vdc ± 5%, 50 mA
Output Isolation 50 Vdc to meter ground
Analog Output (optional)
Output levels 4-20 mA, 0-20 mA, 0-10V
Current compliance 2 mA at 10V ( > 5 kΩ load)
Voltage compliance 12V at 20 mA (< 60Ω load)
Scaling Zero and full scale adjustable from -99999 to +99999
Resolution 16 bits (0.0015% of full scale)
Isolation 250V rms working, 2.3 kV rms per 1 min test
Relay Outputs (optional)
Relay types 2 Form C contact relays or 4 Form A contact relays (NO) 2 or 4 Form A, AC/DC solid state relays (NO)
Current Ratings 8A at 250 Vac or 24 Vdc for contact relays 120 mA at 140 Vac or 180 Vdc for solid state relays
Output common Isolated commons for dual relays or each pair of quad relays
Isolation 250V rms working, 2.3 kV rms per 1 min tes
Serial Data I/O (optional)
Board selections RS232, RS485 (dual RJ11 or RJ45 connectors), USB, Ethernet, USB-to-RS485 converter, Ethernet-to-RS485 converter
Protocols Modbus RTU, Modbus ASCII, Modbus TCP (Ethernet), TDM ASCII
Data rates 300 to 19200 baud
Digital addresses 247 (Modbus), 31 (TDM ASCII),
Isolation 250V rms working, 2.3 kV rms per 1 min test
Environmental
Operating temperature 0°C to 55°C
Storage temperature. -40°C to 85°C
Relative humidity 95% at 40°C, non-condensing
Protection NEMA-4X (IP-65) when panel mounted
Applications

Application Example: Weighing the Load on a Truck

Electronic Counters & Timers

The weight meter power all four 350 ohm load cells of the scale with its 10 V, 120 mA isolated excitation output. Six-wire connection eliminates the effects of lead resistance and allows long cable runs from the control room to the scale.

The five-digit meter can be scaled to display truck weight up to 99,999 lbs with 1 lb resolution or 999,990 lbs with 10 lb resolution. Accuracy is 0.01% of full scale at 25°C. To avoid the effects of system-level noise, different filter settings as well as "count by" of 1, 2, 5, 10, 20, 50 or 100 with rounding are selectable.

To read out net weight of the load, the truck can first be weighed empty, and this weight can be entered as tare at the push of a button in an auto-tare mode. Or the nominal tare value of the truck can be entered manually. Display of net weight or gross weight is at the push of a button. The gross weight and net weight can be alarmed, be transmitted to a computer via RS232 or RS485, or be transmitted via a 4-20 mA analog signal.

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