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Load Cell Meter Frequently Asked Technical Questions
Yes, they do. The Signal inputs of the load cell meter must receive the millivolt signal outputs of the load cell. At 10V excitation, these can range from -20 mV to +20 mV for a 2 mV/V load cell or -30 mV to +30 mV for a 3 mV/V load cell. The Signal is what is converted by the meter into a reading in engineering units like pounds or PSI. The Sense inputs of the meter must sense the excitation voltage, which is also used as the reference for the meter's analog-to-digital converter. With a 6-wire load cell connection, the excitation voltage should be sensed at the load cell after any voltage drop due to resistance of the excitation leads. With a 4-wire load cell connection, the excitation voltage can be sensed at the meter by connecting +Sense to +Excitation and -Sense to -Excitation, but the sensed voltage will then higher than the actual voltage applied to the load cell.
Load cell meter scaling is the process of converting input millivolts to a reading in engineering units. For example, meter scaling is the process of converting the 0-20.000 mV DC output of a load cell to a 0-5000 lb weight reading on the meter.
A count is a unit of resolution. For the Laureate ±20.000 mV and ±50.000 mV ranges, each input count is 0.001 mV (or 1 microvolt) of resolution. For example, 17.354 mV is 17354 input counts. Display counts are the values to be displayed as readings. For example, a reading of 2465.7 lbs is 24657 display counts. The decimal point is set separately as a decoration.
Simply enter data from the load cell label. For example, the label may state 10V excitation, a load rating of 5000 lbs, and 2.020 mV per volt sensitivity. This means 0 mV should be displayed as 0 lbs, and 20.20 mV should be displayed as 5000. To enter this data into the meter, go to the front panel menu item “SEtuP” and select “Coordinates of 2 points method.” Enter Lo In as 0.000 mV, Hi In as 20.200 mV, Lo Read as 0 lbs, and Hi Read as 5000 lbs.
First zero the scale and meter as a system by not applying any weight, then weigh a known, calibrated weight and enter that value into the meter. To do so, go to the front panel menu item “SEtuP” and select “Reading coordinates of 2 points method.” The meter will then prompt you to measure Lo In and High In, and to enter values for Lo Read and High Read.
The ordering process for a Laureate load cell / microvolt meter requires that min input, min reading and max input, max reading be specified at the time of order. This information allows Laurel to set the meter range, which can be ±20.000 mV, ±50.000 mV, ±100.00 mV, ±250.00 mV, or ±500.00 mV, and to scale the meter using the coordinates of 2 points method. This initial setup can easily be changed by the user by moving jumpers and changing scaling via the meter’s front panel, as explained in Section 19 of the Laureate DPM user manual.
The + and - signal inputs measure the millivolt output of the load cell, while the + and - sense inputs measure the excitation voltage that is applied to the load cell. The sensed voltage is used as the reference to the meter’s analog-to-digital converter. If possible, the excitation voltage should be sensed at the load cell, not at the meter, so that it is measured after any voltage drop through the excitation wires. If these wires are long, this voltage drop can be substantial. For example, if the excitation and excitation return wires each have a resistance of 1 ohm, the voltage drop will be V = IR = (10V / 350 ohm) * 2 ohm = 0.057V, so that the actual voltage applied to the load cell is 10V – 0.057V = 9.94V.
Laureate load cell meters are modular with slots for an analog output board, a choice of relay boards, and a choice of communication boards, as illustrated in our Laureates Overview web page. These boards can be ordered installed with a new meter, but they can also be purchased separately later and be simply plugged in. The presence of a new board and the type of board are automatically sensed by the meter’s firmware or by  Instrument Setup (IS) software. If you change boards, also change the model number on the meter label.
The Laureate load cell meter offers resolution down to 1 microvolt per count, while the Laureate process meter is limited to 10 microvolts per count. The load cell meter can also be set up for a 6-wire connection, while the process meter is limited to 4 wires. The extra 2 wires are used to eliminate errors due to voltage drops in long cable runs. Both types of meters can be used with the same types of load cells and can scale millivolt signals for display in engineering units. The slightly more expensive load cell meter is recommended for more demanding applications.
The Laureate load cell meter and microvolt meter are the same meter, but they are set up differently. In load cell mode, the meter works in a ratiometric bridge mode, where the applied voltage excitation is also used as the reference for the analog-to-digital converter. This automatically corrects for any variations in excitation voltage. In microvolt meter mode, the meter is calibrated to NIST standards for absolute readings in millivolts or microvolts.
The hardware of the load cell meter and scale meter is the same, but the meter firmware is slightly different, as detailed in the Scale Meter for Weighing Applications web page. Compared to the standard load cell meter, the scale meter offers five extra user selectable functions specifically for weighing applications:
1) Relay setpoint offset,
2) Count-by function,
3) Fixed right-hand dummy zero,
4) Auto-zero function,
5) Manual tare function.
Yes. A summing box is simply a clean way to connect the meter to the 6 wires of multiple load cells in parallel. Up to four linear load cells of the same type can be connected in parallel and be powered by the same 10 Vdc, 120 mA excitation output of a Laureate meter. For example, four load cells can be used to support the four corners of a weighing platform. Each 350 ohm load cell will draw an I=V/R current of 28.6 mA, so that the total current requiremet for four load cells is 114.4 mA. For scaling purposes, multiply the weight read by each load cell by the number of load cells, since each load cell only carries part of the load. The best way scale a system with muliple laod cells is to use the "Reading Coordinates of 2 Points" scaling method, which uses actual weights.
Input calibration is the process of adjusting the load cell /microvolt meter in software so that absolute readings in millivolts or microvolts are within specified tolerances of a recognized national standard. Input calibration is required for microvolt meter mode and for load cell meter setup using the scale and offset scaling method or the coordinates of 2 points scaling method. Input calibration is not required for the reading coordinates of 2 points scaling methods, which uses actual weights for calibration, not known electrical signals. Note that the Laureate load cell / microvolt meter comes with five factory calibrated voltage ranges with calibration factors stored in EEPROM on the load cell signal conditioner board: ±20.000 mV, ±50.000 mV, ±100.00 mV, ±250.00 mV, ±500.00 mV.
Annual calibration to NIST standards can be performed by Laurel and by some of its distributors as a service. It can also be performed by the customer using Laurel’s free Windows based  Instrument Setup (IS) software. Open the Main Menu of IS software, click on Calibration in the top menu bar, and follow the prompts. An external DC voltage standard is required. Also required in the meter is a communications board, like the Laureate P/N LUSB USB interface board. Following calibration, that board can be removed and be used in another meter.
Analog output scaling is the process of converting the meter reading to an analog output, which can be user selected as 4-20 mA, 0-20 mA, 0-10V or -10 to +10V. Simply enter the load cell meter readings for the bottom and tops of the selected analog output range, and the output will be interpolated linearly between these two readings. Please see Section 17 of the Laureate DPM user manual.
Chances are that your meter is set for the factory default analog output, which is 4-20 mA, and is applying around 12V to force a current into an open circuit. To set your meter to a 0-10V or -10 to +10V analog output, set jumpers for unipolar 0-10V or bipolar -10 to +10V operation, select the desired range and scaling, and connect to the correct analog output pins, as explained in Section 17 of the Laureate DPM user manual.
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