How load cell output is calculated
A strain-gauge load cell is a Wheatstone bridge. Its output is proportional to both the load and the excitation voltage, so datasheets specify sensitivity as rated output in mV/V: the signal per volt of excitation at full rated load. A 2 mV/V cell excited with 10 V gives 20 mV at its rated capacity.
For a single cell:
U = S × Vexc × W / C
where S is the rated output in mV/V, Vexc the excitation voltage, W the load and C the rated capacity.
Several cells in parallel
Platforms, tanks and hoppers usually stand on three, four or more cells wired in parallel through a junction box. The junction box averages the bridge outputs, so:
- The system keeps the rated output of a single cell (in mV/V)
- Its capacity becomes N × the cell capacity
- The output for a total load W is U = S × Vexc × W / (N × C)
The bridges also load the excitation supply in parallel. Four 350 Ω cells look like 87.5 Ω, and at 10 V they draw 114.3 mA. Check that the indicator or amplifier can supply that current; many are limited to four or eight 350 Ω cells.
Worked example
Four 1000 kg cells, 2 mV/V, 10 V excitation, 350 Ω each. The platform weighs 300 kg (dead load) and the maximum product weight is 1500 kg (live load).
- Total capacity: 4 × 1000 = 4000 kg
- Full-scale output: 2 × 10 = 20 mV at 4000 kg
- Signal at 300 + 1500 kg: 20 × 1800 / 4000 = 9.000 mV
- Live-load signal: 20 × 1500 / 4000 = 7.500 mV
- Signal per kg: 20 mV / 4000 kg = 5 µV/kg
- With a 0.5 kg division: 2.5 µV per division, 3000 divisions across the live load
- Capacity used: 45 %
With two cells instead of four, the same load uses 90 % of the capacity and the signal doubles to 18 mV.
Microvolts per division
Weighing indicators specify a minimum input signal per division, typically 0.5 to 1 µV/e for standard instruments, and more for legal-for-trade approvals. The example above gives 2.5 µV per division, which is comfortable. If the result is close to the instrument’s limit, use a higher excitation voltage, cells with a higher mV/V, or a larger division.
Sizing capacity
The calculation assumes an even load split. Real installations need headroom for:
- Uneven loading. Off-centre loads put more weight on one cell; a single corner can carry far more than a quarter.
- Shock and dynamic loads. Dropping material onto a scale can exceed the static weight several times over.
- Wind and seismic loads on outdoor silos and tanks.
A common rule is to keep the total of dead load plus maximum live load below 70 to 80 % of the total capacity, and to check each cell’s safe overload rating.
Common mistakes
Using the system capacity instead of the cell capacity. mV/V refers to one cell’s rated load; in a parallel system the output at a given load drops by the number of cells.
Ignoring dead load. The platform or vessel weight uses up signal range and capacity, even though it is tared away.
Forgetting sense lines. Six-wire cells and indicators correct for voltage drop in long cables. Without sense lines, cable resistance reduces the effective excitation and the signal.
Mixing cells. Cells in parallel should have matched outputs, or be trimmed in the junction box; otherwise corner loads read differently.
Frequently asked questions
Which units can I use?
kg, t, lb, N and kN. Capacity, loads and division use the same unit.
Does the calculator handle 4–20 mA load cell amplifiers?
It gives the bridge signal. To scale an amplifier’s 4–20 mA output, use the 4–20 mA converter.
What excitation voltage should I use?
Use the value the indicator supplies, typically 5 or 10 V. Higher excitation gives more signal but more self-heating and current draw.
Can I copy the results into a report?
Yes. “Copy result” copies all inputs and results as plain text with a link back to the calculation.